Date of release: 22 Feb 1995 Release 18
This is a summary on serial communication using the TTY protocol. It contains information on the TTY protocol and hardware and software implemen- tations for IBM PCs which has been derived from National Semiconductor data sheets and practical experience of the author and his supporters. Starting with release 5, some information on modems has been added.
If you want to contribute to this file in any way, please email me (probably just reply to this posting). My email address is: firstname.lastname@example.org or email@example.com. See the end for details.
It's the seventeenth publication of this file. Some errors have been corrected and some information has been added (which has surely brought other errors with it, see Murphy's Law).
 brackets often indicate comments to sneaked material; copied lines are indented. I've made great efforts to always mention who's to be credited. Please tell me if you find something that you've written that's not correctly associated with your name.
This compilation of information is (C) Copyright 1993 - 1995 by Christian Blum; all rights reserved. This file is not to be reproduced commercially, not even partially, without written permission. You are allowed to use it in any other way you like. I don't want any (monetary) profit being drawn out of it (neither by me nor by others! I don't mind if you have a look or two at it at work though... :-). Please feel free to provide this file to others for free or at your own expenses.
Changes since the last publication ==================================
Used more proper interrupt acknowledging in the examples (namely the method I suggested some chapters before :) in order to avoid lock-ups on MCA computers. (Thanks, Erik)
Added some info on the auto flow-control feature of the TL16C550C. (Thanks, naddy)
What I'm doing ==============
I'm no longer very much into DOS (though I still make some money with it :), so don't expect me reading all the groups regularly that I'm posting this to.
What others are doing =====================
There is a file available from ftp.phil.uni-sb.de, pub/staff/chris called The_Serial_Port.more05 that contains an article by Bob Niland covering serial communication under Windows. It is regularly posted to comp.sys.ibm.pc.hardware.comm and other groups; if you obtain it from there it's probably more up to date.
No more "Automatic File Delivery" (AFD) service ===============================================
The automatic mail reply service I've mentioned in earlier releases of this file is still available, but I won't do anything to keep it alive from now on. Please obtain the files via anonymous ftp from ftp.phil.uni-sb.de (188.8.131.52), pub/staff/chris.
If you don't have access to ftp, try using an ftp to email gateway (firstname.lastname@example.org or email@example.com, and probably a lot more; put 'help' in the body to obtain instructions). If everything fails, write to me.
One of the most universal parts of the PC (except for the CPU, of course :-) is its serial port. You can connect a mouse, a modem, a printer, a plotter, another PC, dongles :) ...
But its usage (both software and hardware) is one of the best-kept secrets for most users, besides that it is not difficult to understand how to connect (not plug in) devices to it and how to program it.
Regard this file as a manual for the serial port of your PC for both hardware and software.
Historical summary ------------------
In early days of telecommunication, errand-boys and optical signals (flags, lights, clouds of smoke) were the only methods of transmitting information across long distances. With increasing requirements on speed and growing amount of information, more practical methods were developed. One milestone was the first wire-bound transmission on May 24th, 1844 ("What hath God wrought", using the famous Morse alphabet). Well, technology improved a bit, and soon there were machines that could be used like typewriters, except that you typed not only on your own sheet of paper but also on somebody elses. The only thing that has changed on the step from the teletype to your PC regarding serial communications is speed.
The TTY (teletyping) protocol -----------------------------
Definition: A protocol is a clear description of the LOGICAL method of transmitting information. This does NOT include physical realization.
There is a difference between bits per second and baud (named after J. M. E. Baudot, one of those guys who gave a real push to teletyping): 'baud' means 'state changes of the line per second' while 'bits per second' ... well, bits per second means bits per second. You may find this a bit weird because the numbers are often the same; there's only a difference if the line has more than two states. Since this is not the case with the RS-232C (EIA-232) port of your PC, most people don't differentiate between 'baud' and 'bits per second', while I do. For your convenience, I've replaced baud with bps even in copied material without special notice. Where you still find baud, it should read bps in most cases (I didn't change labels in source codes, pin names in data sheet information etc.). To illustrate the difference I give you some figures: 2400 bps at 8n1 carry 1920 bits of information per second, and modems send them at 600 baud thru' the phone wires using eight line states, while 1200 bps at 7e1 carry 840 bits of information per second that modems send at 600 baud using four different line states. I know it's confusing... that's why I quote this from a letter I received from Brent Beach. He explained it more clearly than I did (I've added some information):
Perhaps a small diagram might help, showing the relationship among the players:
[bps] [baud] CPU Data Serial Phone Bus -- bytes --> Port -- bits --> Modem -- tones --> line -- | | CPU Data Serial | Bus <-- bytes -- Port <-- bits -- Modem <-- tones ---------- (1) (2) (3)
The serial port accepts bytes from the CPU data bus and passes bits to the modem. In doing this, the serial port can add or delete bits, depending on the coding scheme in use.
At (1) we are concerned with bytes per second. At (2) we are concerned with bits per second, and at (3) it's baud. We distinguish because the number of bits at (2) need not be equal to the number of bits (that is, bytes times 8) at (1), and the number of state changes at (3) is not necessarily the same as the number of bits before. Bits can be stripped going from (1) to (2): the serial port may transmit only 6 or 7 of the 8 bits in the byte. Bits can be added going from (1) to (2): the serial port can add a parity bit and stop bits. From (2) to (3), bits may be clustered to groups that are transmitted using different encoding schemes like 'Frequency Shift Keying' or 'Quadrature Amplitude Modulation', to name some.
You can determine the transfer rate in bytes per second depending on the serial port speed and the coding system. For example,
8n1: 1 start bit + 8 data bits + 1 stop bit = 10 bits per word. At 2400 bps, this is 240 bytes/characters per second. 2400 bps are normally transmitted using QAM ('Quadrature Amplitude Modulation') where 4 bits are clustered, and hence encoded to 600 baud.
7e1: 1 start bit, 7 data bits, 1 even parity bit, 1 stop bit = 10 bits per word. At 1200 bps, this is 120 bytes/characters per second. 1200 bps are encoded using DPSK ('Differential Phase Shift Keying', two bits are clustered), and this results again in 600 baud.
Now let's leave modems for a while and have a look at the serial port itself.
The TTY protocol uses two different line states called 'mark' and 'space'. (For the sake of clearness I name the line states 'high' (voltage) for positive and 'low' (voltage) for negative voltages). If no data is transmitted, the line is in its quiescent 'low' ('mark') state or in the 'break' state ('high'). Data looks like
space +---+ +---+ +---+ high '0' +12V | | | | | | mark ----------+ +-------+ +---+ +------- low '1' -12V
(1) --------(2)-------- (3)
(1) start bit (2) data bits (3) stop bit(s)
Steve Walz reported that in most (all?) books these kind of diagrams are drewn the other way round (I just copied what I saw on the oscilloscope) and that he'd use the labels 'high' and 'low' the other way round, corresponding to the signals on the TTL level (a matter of taste I guess); here is what he told me:
In American texts, we will expect to see the data frame for serial transfer of all kinds represented, despite the method of transfer (RS-232C, RS-422, and optical even), as being an interruption of a normally HI state, and we expect to see the diagram you drew in the older release 8, but with the labelling corrected as I have indicated:
mark ----------+ +-------+ +---+ +------- high '1' -12V logical 1 | S | 1 1 | 0 | 1 | 0 | Stop space +---+ +---+ +---+ low '0' +12V (1) --------(2)---------(3) (1) start bit (2) data bits (3) stop bit(s) Thus transmitting the bit stream 01011, which is LSB first, MSB last.
Indeed it seems to us that a zero SHOULD be the quiescent state, and the one an active state, but the first teletypes used a current loop to continuously monitor the state of the line, and thus current flow was regarded as a 1 and it is "MARK" -ing time, and a signal then left a "SPACE" in the graph of current flow designating a zero. Thus the bits following the start bit at level zero were true to their bit values, and a 11111 in 5 bit baudot looked like this, using three dashes per bit:
mark ------ ------------------------ 1 HI +5V TTL -12V RS-232C space --- 0 LO 0V TTL +12V RS-232C s 1 1 1 1 1 stop
and the baudot 10101 would appear thus:
mark ------ --- --- ------------ 1 HI +5V TTL -12V RS-232C space --- --- --- 0 LO 0V TTL +12V RS-232C s 1 0 1 0 1 stop
and the baudot 01010 would appear thus:
mark ------ --- --- --------- 1 HI +5V TTL -12V RS-232C space ------ --- --- 0 LO 0V TTL +12V RS-232C s 0 1 0 1 0 stop
and finally baudot 00000 would appear:
mark ------ --------- 1 HI +5V TTL -12V RS-232C space ------------------ 0 LO 0V TTL +12V RS-232C s 0 0 0 0 0 stop
Now I know that we don't send five bit baudot over RS-232C now, but I wasn't about to try 8 bits, if you don't mind! :)
I know that people get confused about the proper way to draw these, since we use inverted voltages to send them via RS-232C interface now, but they are still called logical "1" and "mark" when it is really -12 Volts DC, and it is called "0" and "space" when it is +12 Volts. And logical one or "mark" corresponds to +5 Volts, while logical zero is "space" and corresponds to 0 Volts. It is this way both within the parallel bus of the computer or the transmit output of a UART/USART, with the exception that this data frame is terminated by remaining logic "1" or "mark" as a stop bit and preface to the next data frame.
Both transmitter (TX) and receiver (RX) use the same data rate (measured in bps, see above), which is the reciprocal value of the smallest time interval between two changes of the line state. TX and RX know about the number of data bits (probably with a parity bit added), and both know about the (minimum!) size of the stop step (called the stop bit or the stop bits, depending on the size of the stop step; normally 1, 1.5 or 2 times the size of a data bit). Data is transmitted bit-synchronously and word-asynchronously, which means that the size of the bits, the length of the words etc.pp. is clearly defined while the time between two words is undefined.
The start bit indicates the beginning of a new data word (this means one single character). It is used to synchronize transmitter and receiver and is always a logical '0' (so the line goes 'high' or 'space').
Data is transmitted LSB to MSB, which means that the least significant bit (LSB, Bit 0) is transmitted first with 4 to 7 bits of data following, resulting in 5 to 8 bits of data. A logical '0' is transmitted by the 'space' state of the line (+12V), a logical '1' by 'mark' (-12V).
A parity bit can be added to the data bits to allow error detection. There are two (well, actually five) kinds of parity: odd and even (plus none, mark and space). Odd parity means that the number of 'low' or 'mark' steps in the data word (including an optional parity bit, but not the framing bits) is always odd, so the parity bit is set accordingly (I don't have to explain 'even' parity, must I?). It is also possible to set the parity bit to a fixed state or to omit it. See Registers section for details on types of parity.
The stop bit does not indicate the end of the word (as it could be derived >from its name); it rather separates two consecutive words by putting the line into the quiescent state for a minimum time (that means the stop bit is a logical '1' or 'mark') in order for the next start bit to be clearly visible.
The framing protocol is usually described by a sequence of numbers and letters, eg. 8n1 means 1 start bit (always the same, thus omitted), 8 bits of data, no parity bit, 1 stop bit. 7e2 would indicate 7 bits of data, even parity, 2 stop bits (but I've never seen this one...). The usual thing is 8n1 or 7e1.
Your PC is capable of serial transmission at up to 115,200 bps (step size of 8.68 microseconds!). Typical rates are 300 bps, 1200 bps, 2400 bps and 9600 bps, with 19200 bps, 38400 bps and 57600 bps becoming more and more popular with high speed modems. Note that some serial ports have difficulties with high speeds! I've seen PS/2's failing to operate at more than 38400 bps! How come that IBM machines are often the least IBM compatible? :-)
This is what John A. Limpert told me about teletypes:
Real (mechanical) teletypes used 1 start bit, 5 data bits and 1.42 stop bits. Support for 1.5 stop bits in UARTs was a compromise to make the UART timing simpler. Normal speeds were 60 WPM (word per minute), 66 WPM, 75 WPM and 100 WPM. A word was defined as 6.1 characters. The odd stop bit size was a result of the mechanical nature of the machine. It was the time that the printer needed to finish the current character and get ready for the next character. Most teletypes used a 60 mA loop with a 130 V battery. 20 mA loops and lower battery voltages became common when 8 level ASCII teletypes were introduced. The typical ASCII teletype ran at 110 bps with 2 stop bits (11 bits per character).
It's surely more exact than what I wrote in previous releases. I've just got to add that at least in Germany 50 bps was a familiar speed. And I think the lower battery voltage he's talking about was 24 volts.
The physical transmission -------------------------
Teletypes used a closed-loop line with a quiescent current of 20ma and a space current of 0ma (typically), which allows to detect a 'broken line' (hence the name of the 'break' flag, see the Registers section). The RS-232C port of your PC uses voltages rather than currents to indicate logical states: 'mark'/'low' is signaled by -3v to -15v (typically -12V) and represents a logical '1', 'space'/'high' is signaled by +3v to +15v (typically +12V) and represents a logical '0'. The typical output impedance of the serial port of a PC is 2 kiloohms (resulting in about 5ma @ 10v), the typical input impedance is about 4.3 kiloohms, so there should be a maximum fan-out of 5 (5 inputs can be connected to 1 output). Please don't rely on this, it may differ from PC to PC.
Three lines (RX, TX & ground) are at least needed to make up a bidirectional connection.
Q. Why does my PC have a 25pin/9pin connector if there are only 3 lines needed? A. There are several status lines that are only used with modems etc. See the Hardware section and the Registers section of this file.
Q. How can I easily connect two PCs by a three-wire lead? A. Connect RX1 to TX2 and vice versa, GND1 to GND2. In addition to this, connect RTS to CTS & DCD and connect DTR to DSR at each end (modem software often relies on that). See the hardware section for further details.
Please be aware that at 115,200 bps (ie. ca. 115 kHz, but we need the harmonics up to at least 806 kHz) lines can no longer be regarded as 'ideal' transmission lines. They are low-pass filters and tend to reflect and mutilate the signals, but some ten meters of twisted wire should always be OK (I use 3m of screened audio cable for file transfer purposes, and it works fine. Not that other kinds of wire wouldn't do; I took what I found). See a good book on transmission lines if you're interested in why long lines can be a problem.
This has been posted to comp.os.msdos.programmer by Andrew M. Langmead:
The RS-232C spec. has an official limit of 50 ft for RS-232C cables. Realistically they can be much longer. The book "Managing UUCP and Usenet" by O'Reilly and Associates has a table that they credit to "Technical Aspects of Data Communications", by McNamara (Digital Press, 1992). It lists the maximum distances for an RS-232C connection.
Baud Rate | max distance | max distance | shielded cable | unshielded cable ---------------------------------------------------------- 110 | 5000ft | 3000ft 300 | 5000ft | 3000ft 1200 | 3000ft | 3000ft 2400 | 1000ft | 500ft 4800 | 1000ft | 250ft 9600 | 250ft | 250ft
Please note that "baud" is correct in this case, because we're speaking of the transmission line itself.
This is what Torbjoern (sp?) Lindgren told me:
I have successfully transmitted at 115,200 with over 30m long cables! And it wasn't especially good wires. I had some old telecables with 20 individual wires, and used 7 of them for transfer, and left the others unconnected.
I don't remember the exact length, but I know it was something over 30m, and it probably was closer to 40m than 30m. The unused lines probably shielded the lines from each other or something like that. The computers used were two PC-compatibles with off-the-shelf com-ports. Nothing fancy.
Note that some serial ports are more critical with mutilated signals than others, so you just have to try and find out yourself what works.
The connectors --------------
PCs have 9pin/25pin male SUB-D connectors. The pin layout is as follows (seen from outside your PC):
1 13 1 5 _______________________________ _______________ \ . . . . . . . . . . . . . / \ . . . . . / \ . . . . . . . . . . . . / \ . . . . / --------------------------- ----------- 14 25 6 9
Name (V24) 25pin 9pin Dir Full name Remarks -------------------------------------------------------------------------- TxD 2 3 o Transmit Data Data RxD 3 2 i Receive Data Data RTS 4 7 o Request To Send Handshaking CTS 5 8 i Clear To Send Handshaking DTR 20 4 o Data Terminal Ready Status DSR 6 6 i Data Set Ready Status RI 22 9 i Ring Indicator Status DCD 8 1 i Data Carrier Detect Status GND 7 5 - Signal ground Reference level - 1 - - Protective ground Don't use this one as signal ground!
The most important lines are RxD, TxD, and GND. Others are used with modems, printers and plotters to indicate internal states.
'1' ('mark', 'low') means -3v to -15v, '0' ('space', 'high') means +3v to +15v. On status lines, 'high' is the active state: status lines go to the positive voltage level to signal events.
The lines are:
RxD, TxD: These lines carry the data; 1 is transmitted as 'mark' (what I call 'low') and 0 is transmitted as 'space' ('high').
RTS, CTS: Are used by the PC and the modem/printer/whatsoever (further on referred to as the data set, or DCE) to start/stop a communication. The PC sets RTS to 'high', and the data set responds with CTS 'high'. (always in this order). If the data set wants to stop/interrupt the communication (eg. imminent buffer overflow), it drops CTS to 'low'; the PC uses RTS to control the data flow.
DTR, DSR: Are used to establish a connection at the very beginning, ie. the PC and the data set 'shake hands' first to assure they are both present. The PC sets DTR to 'high', and the data set answers with DSR 'high'. Modems often indicate hang-up by resetting DSR to 'low' (and sometimes are hung up by dropping DTR).
(These six lines plus GND are often referred to as '7 wire'-connection or 'hand shake'-connection.)
DCD: The modem uses this line to indicate that it has detected the carrier of the modem on the other side of the phone line. The signal is rarely used by the software.
RI: The modem uses this line to signal that 'the phone rings' (even if there is neither a bell fitted to your modem nor a phone connected :-).
GND: The 'signal ground', ie. the reference level for all signals.
Protective ground: This line is connected to the power ground of the serial adapter. It should not be used as a signal ground, and it MUST NOT be connected to GND (even if your DMM [Digital MultiMeter] shows up an ohmic connection!). Connect this line to the screen of the lead (if there is one). Connecting protective ground on both sides makes sure that no large currents flow thru' GND in case of an insulation defect on one side (hence the name).
Technical data (typical values for PCs):
Signal level: -10.5v/+11v Short circuit current: 6.8ma Output impedance: ca 2 kiloohms (non-linear!) Input impedance: ca 4.3 kiloohms (non-linear!)
Other asynchronous hardware than RS-232C ----------------------------------------
There are several other standards that use the same chipset and protocol as RS-232C. RS-422 and the more robust (but compatible) version RS-485 (to name some) use two wires for every signal. The transmitters can usually be disabled and enabled by software, which makes it possible to use such equipment in a bus system (RX and TX part share the same lines). Despite >from the possibility to enable and disable the receiver/transmitter section of the port, they are fully compatible to existing RS-232C software if a compatible chipset is used.
It's not possible to connect eg. RS-232C to RS-485 without an appropriate interface.
Connecting devices (or computers) ------------------
When you connect a data set or DCE (eg. a modem), use this connection:
GND1 to GND2 RxD1 to RxD2 TxD1 to TxD2 DTR1 to DTR2 DSR1 to DSR2 RTS1 to RTS2 CTS1 to CTS2 RI1 to RI2 DCD1 to DCD2
In other words, simply connect each pin of the first plug with the corresponding pin of the other. This can easily be done using a 25-wire ribbon cable and two crimp connectors.
When you connect another computer (or any other DTE, like a terminal), this is the wiring you need (it is called a "null modem" connection):
GND1 to GND2 RxD1 to TxD2 TxD1 to RxD2 DTR1 to DSR2 DSR1 to DTR2 RTS1 to CTS2 CTS1 to RTS2
If software wants it, connect DCD1 to CTS1 and DCD2 to CTS2.
If hardware handshaking is not needed, you can omit the status lines. Connect:
GND1 to GND2 RxD1 to TxD2 TxD1 to RxD2
Additionally, connect (if software needs it):
RTS1 to CTS1 & DCD1 RTS2 to CTS2 & DCD2 DTR1 to DSR1 DTR2 to DSR2
You won't need long wires for these! :-)
Remember: the names DTR, DSR, CTS & RTS refer to the lines as seen from the DTE (your PC). This means that for your data set DTR & RTS are incoming signals and DSR & CTS are outputs! Modems, printers, plotters etc. are connected 1:1, ie. pin x to pin x.
Base addresses & interrupts ---------------------------
Normally, the following list is correct for your PC; note however that if the BIOS can't find a port, it won't leave spaces in its port table, so if there is no UART at 0x3E8, the port at 0x2E8 will be called COM3 by DOS. Compare the section on logical vs. phyical names.
Port Name Base address Int # Int level (IRQ)
COM1 0x3F8 0xC 4 COM2 0x2F8 0xB 3 COM3 0x3E8 0xC 4 COM4 0x2E8 0xB 3
In your programs, you should refer to the table in the BIOS data segment. This is an excerpt from Ralf Brown's interrupt list (the actual author of this section is Robin Walker):
Format of BIOS Data Segment at segment 40h: Offset Size Description 00h WORD Base I/O address of 1st serial I/O port, zero if none 02h WORD Base I/O address of 2nd serial I/O port, zero if none 04h WORD Base I/O address of 3rd serial I/O port, zero if none 06h WORD Base I/O address of 4th serial I/O port, zero if none Note: Above fields filled in turn by POST as it finds serial ports. POST never leaves gaps. DOS and BIOS serial device numbers may be redefined by re-assigning these fields.
Please note that this table is not the bible and that the BIOS is not an evangelist (and I'm rather sceptical anyway :-). Your BIOS might not tell you the pure truth; if you get a zero it does not necessarily mean that there are no more serial ports available. Your programs should nevertheless have a look at the usual places for comm ports. See the "Programming" section for an example program that checks if a UART is installed at a given base address. Compare the "logical vs. physical names" section below.
Another good idea is writing a small program that's then run in the AUTOEXEC.BAT and that fills the empty fields in the table with the correct values. My Award BIOS fails to recognize my fourth port at 0x2E8, so I typed a few bytes (14 altogether) in the debugger that write 0x2E8 to 0040:0006 and wrote them to a .COM file called in the AUTOEXEC.BAT.
Also see the Programming section for a routine that detects the interrupt level/number that a UART uses. It's not a good idea to hard-code level 4 and 3; make it at least user configurable.
See the chapter "Multi-Port Serial Adapters" for further information.
Logical vs. physical ports --------------------------
DOS users (like card manufacturers) tend to confuse logical and physical names. COM1, COM2, etc. are _logical_ names for the serial ports 0, 1, etc. found by the BIOS during POST (Power-On Self Test). The BIOS searches at four different I/O addresses for UARTS: 0x3F8, 0x2F8, 0x3E8, 0x2E8, in exactly this order. Every UART found has an entry in the comm port table at segment 0x40, offset 0. The BIOS manages up to four different UARTs, because the table has no more than four spaces. To make the confusion complete, Microsoft decided that DOS users wouldn't be comfortable with counting from zero, so they numbered the logical names of the comm ports from 1 to 4. Thus COM1 is the first UART found by the BIOS during POST, COM2 the second, and so on. Usually COM1 has 0x3F8 as base addresses, COM2 0x2F8 and so on, but that's not necessarily the case. Please do not use the logical DOS names when you really mean physical addresses. It is _not_ possible to 'jumper a UART as COM3', at least not directly.
The chipsets ------------
In PCs, serial communication is realized with a set of three chips (there are no further components needed! (I know of the need of address logic & interrupt logic ;-) )): a UART (Universal Asynchronous Receiver/Transmitter) and two line drivers. Normally, the 82450/16450/8250 does the 'brain work' while the 1488 and 1489 drive the lines (they are level shifting inverters; the 1488 drives the outputs).
These chips are produced by many manufacturers; it's of no importance which letters are printed in front of the numbers (mostly NS for National Semiconductor). Don't regard the letters behind the number also (if it's not the 16550A or the 82C50A); they just indicate special features and packaging (Advanced, New, MILitary, bug fixes [see below] etc.) or classification. Letters in between the numbers (eg. 16C450) indicate technology (C=CMOS).
You might have heard that it is possible to replace the 16450 by a 16550A to improve reliability and reduce software overhead. This is only useful if your software is able to use the FIFO (first in-first out) buffer feature. The chips are fully pin-compatible except for two pins that are not used by any serial adapter card known to the author: pin 24 (CSOUT, chip select out) and pin 29 (NC, no internal connection). With the 16550A, pin 24 is -TXRDY and pin 29 is -RXRDY, signals that aren't needed (except for DMA access - but not in the PC) and that even won't care if they are shorted to +5V or ground. Therefore it should always be possible to simply replace the 16450 by the 16550A - even if it's not always useful due to lacking software capabilities. IT IS DEFINITELY NOT NECESSARY FOR COMMUNICATION AT UP TO LOUSY 9600 BPS! These rates can easily be handled by any CPU, and the interrupt-driven communication won't slow down the computer substantially. But if you want to use high-speed transfer with or without using the interrupt features (ie. by 'polling'), or multitasking, or multiple channels 'firing' at the same time, or disk I/O during transmission, it is recommendable to use the 16550A in order to make transmission more reliable if your software supports it (see excursion some pages below).
There *are* differences between the 16550A, 16550AF, and 16550AFN. The 16550AF has one more timing parameter (t_RXI) specified that's concerned with the -RXRDY pin and that's of no importance in the PC. And the 16550AFN is the only one still believed to be free of bugs (see below). So the best choice for your PC is 16550AFN, but you are well off with the 16550AN, too. [Info from a posting of Jim Graham.]
Don't worry about the missing 'A' if you have chips named xxx16550 which are not from National Semiconductor (eg. UM16550). As long as the first example in the 'Programming' section tells you that it is a 16550A, everything is fine. I've never heard of non-NS 16550s with the FIFO bug (see below).
How to detect which chip is used --------------------------------
This is really not difficult. The 8250 normally has no scratch register (see data sheet info below), the 16450/82450 has no FIFO, the 16550 has no working FIFO :-) and the 16550A performs alright. See the Programming section for an example program that detects which one is used in your PC.
Note that there _are_ versions of the 8250 that _do_ have a scratch register! It's rather impossible to distinguish them from the 16450, but then it's not necessary either... I know of the SAB 82C50 from Siemens and the UM8250B (from UMC, a taiwanese company with a globe symbol; thanks, Alfred, for helping me out with that). You won't find 8250s in fast computers however, because their bus timing is too slow.
Data sheet information ----------------------
Some hardware information taken from the data sheet of National Semiconductor (shortened and commented):
Pin description of the 16450 (16550A) [Dual-In-Line package]:
+-----+ +-----+ D0 -| 1 +-+ 40|- VCC D1 -| 2 39|- -RI D2 -| 3 38|- -DCD D3 -| 4 37|- -DSR D4 -| 5 36|- -CTS D5 -| 6 35|- MR D6 -| 7 34|- -OUT1 D7 -| 8 33|- -DTR RCLK -| 9 32|- -RTS SIN -| 10 31|- -OUT2 SOUT -| 11 30|- INTR CS0 -| 12 29|- NC (-RXRDY) CS1 -| 13 28|- A0 -CS2 -| 14 27|- A1 -BAUDOUT -| 15 26|- A2 XIN -| 16 25|- -ADS XOUT -| 17 24|- CSOUT (-TXRDY) -WR -| 18 23|- DDIS WR -| 19 22|- RD VSS -| 20 21|- -RD +-------------+
Note: The status signals are negated compared to the port! If you write a '1' to the appropriate register bit, the pin goes 'low' (to ground level). On its way to the port, the signal is inverted again; this means that the status line at the port goes 'high' if you write a '1'. The same is true for inputs: you get a '1' from the register bit if the line at the port is 'high'. SIN and SOUT are inverted, too. (negative voltage at the port means +5v at the UART).
A0, A1, A2, Register Select, Pins 26-28: Address signals connected to these 3 inputs select a UART register for the CPU to read from or to write to during data transfer. A table of registers and their addresses is shown below. Note that the state of the Divisor Latch Access Bit (DLAB), which is the most significant bit of the Line Control Register, affects the selection of certain UART registers. The DLAB must be set high by the system software to access the Baud Generator Divisor Latches. [I'm sorry, but it's called that way even if it's a bps rate generator... :-)]. 'x' means don't care.
DLAB A2 A1 A0 Register 0 0 0 0 Receive Buffer (read) Transmitter Holding Reg. (write) 0 0 0 1 Interrupt Enable x 0 1 0 Interrupt Identification (read) x 0 1 0 FIFO Control (write) [undefined with the 16450. CB] x 0 1 1 Line Control x 1 0 0 Modem Control x 1 0 1 Line Status x 1 1 0 Modem Status x 1 1 1 Scratch [special use on some boards. CB] 1 0 0 0 Divisor Latch (LSB) 1 0 0 1 Divisor Latch (MSB)
-ADS, Address Strobe, Pin 25: The positive edge of an active Address Strobe (-ADS) signal latches the Register Select (A0, A1, A2) and Chip Select (CS0, CS1, -CS2) signals. Note: An active -ADS input is required when Register Select and Chip Select signals are not stable for the duration of a read or write operation. If not required, tie the -ADS input permanently low. [As it is done in your PC. CB]
-BAUDOUT, Baud Out, Pin 15: This is the 16x clock signal from the transmitter section of the UART. The clock rate is equal to the main reference oscillator frequency divided by the specified divisor in the Baud Generator Divisor Latches. The -BAUDOUT may also be used for the receiver section by tying this output to the RCLK input of the chip. [Yep, that's true for your PC. CB].
CS0, CS1, -CS2, Chip Select, Pins 12-14: When CS0 and CS1 are high and CS2 is low, the chip is selected. This enables communication between the UART and the CPU.
-CTS, Clear To Send, Pin 36: When low, this indicates that the modem or data set is ready to exchange data. This signal can be tested by reading bit 4 of the MSR. Bit 4 is the complement of this signal, and Bit 0 is '1' if -CTS has changed state since the previous reading (bit0=1 generates an interrupt if the modem status interrupt has been enabled).
D0-D7, Data Bus, Pins 1-8: Connected to the data bus of the CPU.
-DCD, Data Carrier Detect, Pin 38: blah blah blah, can be tested by reading bit 7 / bit 3 of the MSR. Same text as -CTS.
DDIS, Driver Disable, Pin 23: This goes low whenever the CPU is reading data from the UART. It can be used to control bus arbitrary logic.
-DSR, Data Set Ready, Pin 37: blah, blah, blah, bit 5 / bit 1 of MSR.
-DTR, Data Terminal Ready, Pin 33: can be set active low by programming bit 0 of the MCR '1'. Loop mode operation holds this signal in its inactive state.
INTR, Interrupt, Pin 30: goes high when an interrupt is requested by the UART. Reset low by the MR.
MR, Master Reset, Pin 35: Schmitt Trigger input, resets internal registers to their initial values (see below).
-OUT1, Out 1, Pin 34: user-designated output, can be set low by programming bit 2 of the MCR '1' and vice versa. Loop mode operation holds this signal inactive high. [Not used in the PC. CB]
-OUT2, Out 2, Pin 31: blah blah blah, bit 3, see above. [Used in your PC to connect the UART to the interrupt line of the slot when '1'. CB]
RCLK, Receiver Clock, Pin 9: This input is the 16x bps rate clock for the receiver section of the chip. [Normally connected to -BAUDOUT, as in your PC. CB]
RD, -RD, Read, Pins 22 and 21: When RD is high *or* -RD is low while the chip is selected, the CPU can read data from the UART. [One of these is normally tied. CB]
-RI, Ring Indicator, Pin 39: blah blah blah, Bit 6 / Bit 2 of the MSR. [Bit 2 only indicates change from active low to inactive high! Curious, isn't it? CB]
-RTS, Request To Send, Pin 32: blah blah blah, see DTR (Bit 1).
SIN, Serial Input, Pin 10.
SOUT, Serial Output, Pin 11.
-RXRDY, -TXRDY: refer to NS data sheet. These pins are used for DMA channeling. Since they are not connected in your PC, I won't describe them here.
VCC, Pin 40, +5v
VSS, Pin 20, GND
WR, -WR: same as RD, -RD for writing data.
XIN, XOUT, Pins 16 and 17: Connect a crystal here (1.5k betw. xtal & pin 17) and pin 16 with a capacitor of approx. 20p to GND and other xtal conn. 40p to GND. Resistor of approx. 1meg parallel to xtal. Or use pin 16 as an input and pin 17 as an output for an external clock signal of up to 8 MHz.
Absolute Maximum Ratings:
Temperature under bias: 0 C to +70 C Storage Temperature: -65 C to 150 C All input or output voltages with respect to VSS: -0.5v to +7.0v Power dissipation: 1W
Further electrical characteristics see the very good data sheet of NS.
UART Reset Configuration
Register/Signal Reset Control Reset State -------------------------------------------------------------------- IER MR 0000 0000 IIR MR 0000 0001 FCR MR 0000 0000 LCR MR 0000 0000 MCR MR 0000 0000 LSR MR 0110 0000 MSR MR xxxx 0000 (according to signals) SOUT MR high (neg. voltage at the port) INTR (RCVR errs) Read LSR/MR low INTR (data ready) Read RBR/MR low INTR (THRE) Rd IIR/Wr THR/MR low INTR (modem status) Read MSR/MR low -OUT2 MR high -RTS MR high -DTR MR high -OUT1 MR high RCVR FIFO MR/FCR1&FCR0/DFCR0 all bits low XMIT FIFO MR/FCR1&FCR0/DFCR0 all bits low
Known problems with several chips ---------------------------------
(From material Madis Kaal received from Dan Norstedt and stuff Erik Suurmaa sent me)
8250 and 8250-B:
* These UARTs pulse the INT line after each interrupt cause has been serviced (which none of the others do). [Generates interrupt overhead. CB]
* The start bit is about 1 us longer than it ought to be. [This shouldn't be a problem. CB]
* 5 data bits and 1.5 stop bits doesn't work.
* When a 1 is written to the bit 1 (Tx int enab) in the IER, a Tx interrupt is generated. This is an erroneous interrupt if the THRE bit is not set. [So don't set this bit as long as the THRE bit isn't set. CB]
* The first valid Tx interrupt after the Tx interrupt is enabled is probably missed. Suggested workaround: 1) Wait for the THRE bit to become set. 2) Disable CPU interrupts. [?] 3) Write Tx interrupt enable to the IER. 4) Write Tx interrupt enable to the IER again. [Don't ask me why. I don't think it's necessary. CB] 5) Enable CPU interrupts. [?]
* The TEMT (bit 6) doesn't work properly.
* If both the Rx and Tx interrupts are enabled, and a Rx interrupt occurs, the IIR indication of the Tx interrupt may be lost. Suggested workarounds: 1) Test THRE bit in the Rx routine, and either set IER bit 1 or call the Tx routine directly if it is set. 2) Test the THRE bit instead of using the IIR for Tx.
[If one of these chips vegetates in your PC, go get your solder iron heated... CB]
8250A, 82C50A, 16450 and 16C450:
* (Same problem as above:) If both the Rx and Tx interrupts are enabled, and a Rx interrupt occurs, the IIR indication may be lost; Suggested workarounds: 1) Test THRE bit in the Rx routine, and either set IER bit 1 or call the Tx routine directly if it is set. 2) Test the THRE bit instead of using the IIR. 3) [Don't enable both interrupts at the same time. CB] 4) [Replace the chip by a 16550AFN; it has this bug fixed. CB]
16550 (without the A):
* Rx FIFO bug: Sometimes the FIFO will get extra characters. [This seemed to be very embarrassing for NS; they've added a simple detection method for the 16550A (bit 6 of IIR). CB]
* When the TX FIFO is enabled, a character loss can appear if the CPU writes a byte into the THR while the last one is still in the shift register (not completely sent). [This is documented by National Semiconductor; I've never experienced that, but that might be because I've never seen a 16550 AF :) CB]
* Terence Edwards reports that his RS485 adapter with 16550 AF chips and a 16 MHz xtal gets parity bits wrong at 512 kbps; not very astonishing I'd say because the chip is only guaranteed to operate at 256kbps, with an 8 MHz xtal, and parity generators are rather slow circuits.
No 16550 AFN bugs reported (yet?)
[Same is true for the 16552, a two-in-one version of the 16550AFN, and the 16554, a quad-in-one version. CB]
You might call this a bug, though: in FIFO mode, THRE (bit 5 or LSR) is cleared when there is at least one character in the Tx FIFO, not if the FIFO can't take any more bytes; that's rather absurd, but that's the way it is.
A very solid method of handling the UART interrupts that avoids all possible int failures has been suggested by Richard Clayton, and I recommend it as well. Let your interrupt handler do the following: 1. Disarm the UART interrupts by masking them in the IMR of the ICU. 2. Send a specific or an unspecific EOI to the ICU (first slave, then master, if you're using channels above 7). 3. Enable CPU interrupts (STI) to allow high priority ints to be processed. 4. Read IIR and follow its contents until bit 0 is set. 5. Check if transmission is to be kicked (when XON received or if CTS goes high); if yes, call tx interrupt handler manually. 6. Disable CPU interrupts (CLI). 7. Rearm the UART interrupts by unmasking them in the IMR of the ICU. 8. Return from interrupt. This way you can arm all four UART ints at initialization time without having to worry about stuck interrupts. Start transmission by simply calling the tx interrupt handler after you've written characters to the tx fifo of your program.
If you need details about programming the ICU, refer to Chris Hall's document about the 8259 that's available from my archive.
First some tables; full descriptions follow. Base addresses as specified by IBM for a full-blown system; compare the section on logical & physical names.
1st 2nd 3rd 4th Offs. DLAB Register ------------------------------------------------------------------------------ 3F8h 2F8h 3E8h 2E8h +0 0 RBR Receive Buffer Register (read only) or THR Transmitter Holding Register (write only) 3F9h 2F9h 3E9h 2E9h +1 0 IER Interrupt Enable Register 3F8h 2F8h 3E8h 2E8h +0 1 DL Divisor Latch (LSB) These registers can 3F9h 2F9h 3E9h 2E9h +1 1 DL Divisor Latch (MSB) be accessed as word 3FAh 2FAh 3EAh 2EAh +2 x IIR Interrupt Identification Register (r/o) or FCR FIFO Control Register (w/o, 16550+ only) 3FBh 2FBh 3EBh 2EBh +3 x LCR Line Control Register 3FCh 2FCh 3ECh 2ECh +4 x MCR Modem Control Register 3FDh 2FDh 3EDh 2EDh +5 x LSR Line Status Register 3FEh 2FEh 3EEh 2EEh +6 x MSR Modem Status Register 3FFh 2FFh 3EFh 2EFh +7 x SCR Scratch Register (16450+ and some 8250s, special use with some boards)
80h 40h 20h 10h 08h 04h 02h 01h Register Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 ------------------------------------------------------------------------------- IER 0 0 0 0 EDSSI ELSI ETBEI ERBFI IIR (r/o) FIFO en FIFO en 0 0 IID2 IID1 IID0 pending FCR (w/o) - RX trigger - 0 0 DMA sel XFres RFres enable LCR DLAB SBR stick par even sel Par en stopbits - word length - MCR 0 0 AFE Loop OUT2 OUT1 RTS DTR LSR FIFOerr TEMT THRE Break FE PE OE RBF MSR DCD RI DSR CTS DDCD TERI DDSR DCTS
EDSSI: Enable Delta Status Signals Interrupt ELSI: Enable Line Status Interrupt ETBEI: Enable Transmitter Buffer Empty Interrupt ERBFI: Enable Receiver Buffer Full Interrupt FIFO en: FIFO enable IID#: Interrupt IDentification pending: an interrupt is pending if '0' RX trigger: RX FIFO trigger level select DMA sel: DMA mode select XFres: Transmitter FIFO reset RFres: Receiver FIFO reset DLAB: Divisor Latch Access Bit SBR: Set BReak stick par: Stick Parity select even sel: Even Parity select stopbits: Stop bit select word length: Word length select FIFOerr: At least one error is pending in the RX FIFO chain TEMT: Transmitter Empty (last word has been sent) THRE: Transmitter Holding Register Empty (new data can be written to THR) Break: Broken line detected FE: Framing Error PE: Parity Error OE: Overrun Error RBF: Receiver Buffer Full (Data Available) DCD: Data Carrier Detect RI: Ring Indicator DSR: Data Set Ready CTS: Clear To Send DDCD: Delta Data Carrier Detect TERI: Trailing Edge Ring Indicator DDSR: Delta Data Set Ready DCTS: Delta Clear To Send AFE: Automatic Flow control Enable
RBR (Receive Buffer Register) 3F8h 2F8h 3E8h 2E8h +0 r/o ------------------------------------------------------------------------------
This is where you get received characters from. This register is read-only.
THR (Transmitter Holding Register) 3F8h 2F8h 3E8h 2E8h +0 w/o ------------------------------------------------------------------------------
Send characters by writing them to this register. It is write-only.
IER (Interrupt Enable Register) 3F9h 2F9h 3E9h 2E9h +1 r/w ------------------------------------------------------------------------------
Enable several interrupts by setting these bits: Bit 0: If set, DR (Data Ready) interrupt is enabled. It is generated if data waits to be read by the CPU. Bit 1: If set, THRE (THR Empty) interrupt is enabled. This interrupt tells the CPU to write characters to the THR. Bit 2: If set, Status interrupt is enabled. It informs the CPU of occurred transmission errors during reception. Bit 3: If set, Modem status interrupt is enabled. It is triggered whenever one of the delta-bits is set (see MSR). Bits 4-7 are not used and should be set 0.
DL (Divisor Latch) 3F8h 2F8h 3E8h 2E8h +0 r/w ------------------------------------------------------------------------------
To access this *WORD*, set DLAB in the LCR to 1. Then write a word (16 bits) to this register or write the lower byte to base+0 and the higher byte to base+1 (the order is not important) to program the bps rate as follows:
xtal frequency in Hz / 16 / desired rate = divisor xtal frequency in Hz / 16 / divisor = obtained rate
Your PC uses an xtal frequency of 1.8432 MHz (that's 1843200 Hz :-).
Do *NOT* use 0 as a divisor (your maths teacher told you so)! It results in a rate of about 3500 bps, but it is not guaranteed to work with all chips in the same way.
An error of up to 3-5 percent is irrelevant.
Some values (1.8432 MHz quartz, as in the PC):
bps rate Divisor (hex) Divisor (dec) Percent Error 50 900 2304 0.0% 75 600 1536 0.0% 110 417 1047 0.026% 134.5 359 857 0.058% 150 300 768 0.0% 300 180 384 0.0% 600 C0 192 0.0% 1200 60 96 0.0% 1800 40 64 0.0% 2000 3A 58 0.69% 2400 30 48 0.0% 3600 20 32 0.0% 4800 18 24 0.0% 7200 10 16 0.0% 9600 C 12 0.0% 19200 6 6 0.0% 38400 3 3 0.0% 57600 2 2 0.0% 115200 1 1 0.0%
The 16450 is capable of up to 512 kbps according to NS.
NS specifies that the 16550A is capable of 256 kbps if you use a 4 MHz or an 8 MHz crystal. But a staff member of NS Germany (I know that this abbreviation is not well-chosen :-( ) told one of my friends on the phone that it runs correctly at 512 kbps as well; I don't know if the 1488/1489 manage this, though. This is true for the 16C552, too. See the "known problems" section.
BTW: Ever tried 1.76 bps, the slowest rate possible? Kindergarten kids write faster.
The Microsoft mouse uses 1200 bps, 7n1, the Mouse Systems mouse uses 1200 bps, 8n1. See the Mouse chapter for details.
IIR (Interrupt Identification Register) 3FAh 2FAh 3EAh 2EAh +2 r/o ------------------------------------------------------------------------------
This register allows you to detect the cause of an interrupt. Only one interrupt is reported at a time; they are priorized. If an interrupt occurs, Bit 0 tells you if the UART has triggered it. Follow the information in this register, then test bit 0 again. If it is still not set, there is another interrupt to be serviced. BTW: If you AND the value of this register with 06h, you get a pointer to a table of four words... ideal for near calls. Another hint: make sure your software reads this register just once and then follows the information it got before it is read again, otherwise your code won't work. (Turbo Pascal "programmers" beware! :)
The bits 6 and 7 allow you to detect if the FIFOs of the 16550+ have been activated.
Bit 3 Bit 2 Bit 1 Bit 0 Priority Source Description 0 0 0 1 none no interrupt pending 0 1 1 0 highest Status OE, PE, FE or BI of the LSR set. Serviced by reading the LSR. 0 1 0 0 second Receiver DR or trigger level rea- ched. Serviced by read- ing RBR 'til under level 1 1 0 0 second FIFO No Receiver FIFO action since 4 words' time (neither in nor out) but data in RX-FIFO. Serviced by reading RBR. 0 0 1 0 third Transm. THRE. Serviced by read- ing IIR (if source of int only!) or writing to THR. 0 0 0 0 lowest Modem One of the delta flags in the MSR set. Serviced by reading MSR. Bit 6 & 7: 16550A: set if FCR bit 0 set. 16550: bit 7 set, bit 6 cleared if FCR bit 0 set. others: clear Other bits: clear (but don't rely on it; this is subject to change).
In most software applications bits 3 to 7 should be masked when servicing the interrupt since they are not relevant. These bits cause trouble with old software relying on that they are cleared...
NOTE! Even if some of these interrupts are disabled, the service routine can be confronted with *all* states shown above when the IIR is loop-polled until bit 0 is set (don't ask me why; it's just that I encontered this, and it's not much more work to play it safe). Check examples in the Programming section.
FCR (FIFO Control Register) 3FAh 2FAh 3EAh 2EAh +2 w/o ------------------------------------------------------------------------------
This register allows you to control the FIFOs of the 16550+. It does not exist on the 8250/16450.
Bit 0: FIFO enable. Bit 1: Clear receiver FIFO. This bit is self-clearing. Bit 2: Clear transmitter FIFO. This bit is self-clearing. Bit 3: DMA mode (pins -RXRDY and -TXRDY), see below Bits 6-7: Trigger level of the DR-interrupt.
Bit 7 Bit 6 Receiver FIFO trigger level 0 0 1 0 1 4 1 0 8 1 1 14
Note: if bit 0 is cleared, all other bits are ignored.
DMA mode operation is not available with your PC, but for the sake of completeness... here we go.
If bit 3 is 0, DMA mode 0 is selected. The -RXRDY pin goes active-low whenever there is at least one character in the RX FIFO or in the RBR if the FIFO is disabled. -TXRDY goes active-low when the TX FIFO or the THR is empty. It goes high if one character is written to the THR (same as THRE, that's bit 5 of the LSR).
If this bit is 1, DMA mode 1 is selected. The -RXRDY pin goes low if the trigger level of the RX FIFO is reached or if reception timed out (no characters received for a time that would have allowed to receive 4 characters). -TXRDY goes low when the TX FIFO is empty. It goes high again if the FIFO is completely full. (Not that setting this bit to '1' would fix the weird behaviour of the THRE bit in FIFO mode operation, though). If the FIFOs are disabled, DMA mode 1 operates in the same way as DMA mode 0.
LCR (Line Control Register) 3FBh 2FBh 3EBh 2EBh +3 r/w ------------------------------------------------------------------------------
This register allows you to select the transmission protocol. It also contains the DLAB bit which switches the function of the addresses +0 and +1.
Bit 1 Bit 0 word length Bit 2 Stop bits 0 0 5 bits 0 1 0 1 6 bits 1 1.5/2 1 0 7 bits (1.5 if word length is 5) 1 1 8 bits (1.5 does not work with some chips, see above)
Bit 5 Bit 4 Bit 3 Parity type Bit 6 SOUT condition x x 0 no parity 0 normal operation 0 0 1 odd parity 1 forces TxD +12V (break) 0 1 1 even parity Bit 7 DLAB 1 0 1 mark parity 0 normal registers 1 1 1 space parity 1 divisor at reg 0, 1
Mark parity: The parity bit is always '1' (the line is 'low'). Space parity: The parity bit is always '0' (the line is 'high').
MCR (Modem Control Register) 3FCh 2FCh 3ECh 2ECh +4 r/w ------------------------------------------------------------------------------
This register allows to program some modem control lines and to switch to loopback mode.
Bit 0: Programs -DTR. If set, -DTR is low and the DTR pin of the port goes 'high'. Bit 1: Programs -RTS. dito. Bit 2: Programs -OUT1. Normally not used in a PC, but used with some multi-port serial adapters to enable or disable a port. Best thing is to write a '1' to this bit. Bit 3: Programs -OUT2. If set to 1, interrupts generated by the UART are transferred to the ICU (Interrupt Control Unit) while 0 sets the interrupt output of the card to high impedance. (This is PC-only). Bit 4: '1': local loopback. All outputs disabled. This is a means of testing the chip: you 'receive' all the data you send. Interrupts are fully operational in this mode. Bit 5: (Texas Instruments TL16C550C only, maybe some more; this is not a standard feature) '1': Enable automatic flow control. If RTS (bit 1) is '0', only auto-CTS is done, which means that no more characters are sent from the FIFO and no more Tx interrupts are generated as long as CTS is '0'. If RTS (bit 1) is '1', the RTS signal is dropped whenever the FIFO trigger level is reached. Note that if this bit is '1', delta CTS (see below) won't generate a modem status interrupt!
LSR (Line Status Register) 3FDh 2FDh 3EDh 2EDh +5 r/w ------------------------------------------------------------------------------
This register allows error detection and polled-mode operation.
Bit 0 Data Ready (DR). Reset by reading RBR (but only if the RX FIFO is empty, 16550+). Bit 1 Overrun Error (OE). Reset by reading LSR. Indicates loss of data. Bit 2 Parity Error (PE). Indicates transmission error. Reset by LSR. Bit 3 Framing Error (FE). Indicates missing stop bit. Reset by LSR. Bit 4 Break Indicator (BI). Set if RxD is 'space' for more than 1 word ('break'). Reset by reading LSR. Bit 5 Transmitter Holding Register Empty (THRE). Indicates that a new word can be written to THR. Reset by writing THR. Note that this bit works in a weird way when FIFOs are enabled: it goes 0 whenever there are characters in the TX-FIFO, not when the FIFO is full! Bit 6 Transmitter Empty (TEMT). Indicates that no transmission is running. Reset by reading LSR. Bit 7 (16550+ only) Set if at least one character in the RX FIFO has been received with an error. Cleared by reading LSR if there is no further error in the FIFO. Clear with all other chips.
MSR (Modem Status Register) 3FEh 2FEh 3EEh 2EEh +6 r/w ------------------------------------------------------------------------------
This register allows you to check several modem status lines. The delta bits are set if the corresponding signals have changed state since the last reading (except for TERI which is only set if -RI changed from active-low to inactive-high, that is if the RI line at the port changed from 'high' to 'low' and the phone stopped ringing).
Bit 0: Delta CTS. Set if CTS has changed state since last reading. Bit 1: Delta DSR. Set if DSR has changed state since last reading. Bit 2: TERI. Set if -RI has changed from low to high (ie. RI at port has changed from +12V to -12V). Bit 3: Delta DCD. Set if DCD has changed state since last reading. Bit 4: CTS. 1 if 'high' at port. Bit 5: DSR. dito. Bit 6: RI. dito. Bit 7: DCD.
In loopback mode (MCR bit 4 = 1), bit 4 shows the state of RTS (MCR bit 1), bit 5 shows the state of DTR (MCR bit 0), RI shows the state of OUT1 (MCR bit 2), and DCD shows the state of OUT2 (MCR bit 3). The delta registers act accordingly to the 'level transitions' of the data written to MCR. This is a good means of testing if a UART is present.
SCR (Scratch Register) 3FFh 2FFh 3EFh 2EFh +7 r/w ------------------------------------------------------------------------------
This is an all-purpose 8 bit store. NS recommends to store the value of the FCR (which is w/o) in this register for further use, but this is not mandatory and not recommended by me (see below). This register is only available with the 16450+; the standard 8250 doesn't have a scratch register (but then again some versions do).
On some boards (especially RS-422/RS-485 boards), this register has a special meaning (enable receiver/transmitter drivers etc.), and with multi-port serial adapters it is often used to select the interrupt levels of the several ports and to determine which port has triggered interrupt. So you shouldn't use it for anything else in your programs.
Excursion: Why and how to use the FIFOs (by Scott C. Sadow) -----------------------------------------------------------
Normally when transmitting or receiving, the UART generates one interrupt for every character sent or received. For 2400 bps, typically this is 240/second. For 115,200 bps, this means 11,520/second. With FIFOs enabled, the number of interrupts is greatly reduced.
A transmitter holding register empty interrupt is not generated until the FIFO is empty (last byte is being sent).
So if you know it's a 16550A and the FIFOs are enabled, your TX interrupt routine can write up to 16 characters to the THR. Monitoring bit 5 (THRE) of the LSR is _no_good_ because this bit will be cleared immediately after your routine has written the first character to the THR! The chip gives you no feedback at all.
Thus, the number of transmitter interrupts is reduced by a factor of 16. For 115,200 bps, this means only 720 interrupts per second. For receive data interrupts, the processing is similar to transmitter interrupts. The main difference is that the number of bytes in the FIFO (the trigger level) can be specified. When the trigger level is reached, a receive data interrupt is generated; any other data received is just put in the FIFO. The receive data interrupt is not cleared until the number of bytes in the FIFO is below the trigger level again.
To add 16550A support to existing code, there are 2 requirements to be met:
1) When reading the IIR to determine the interrupt source, only use the lower 3 bits.
2) After the existing UART initialization code, try to enable the FIFOs by writing to the FCR. (A value of C7 hex will enable FIFO mode, clear both FIFOs, and set the receive trigger level at 14 bytes). Next, read the IIR. If Bit 6 of the IIR is not set, the UART is not a 16550A, so write 0 to the FCR to disable FIFO mode.
Multi-Port Serial Adapters --------------------------
This is material I received from Mike Surikov.
I want to give you some information on Multi-Serial adapters that provide four or eight asynchronous serial communication ports.
Some of them have an Interrupt Vector (one for each four channels). The Interrupt Vector is used to enable/disable global interrupt and to detect which of the four channels is creating the interrupt (one IRQ is used for a group of four channels). Bit 7 of the Interrupt Vector is used to enable or disable ALL four channels by writing a logical 1 to enable or 0 to disable interrupts. At the same time, each channel can be enabled or disabled separately by programming the OUT2 (and/or OUT1) signal in the 16450 chip.
When you read the interrupt vector, you get an indication which port has triggered the interrupt, as it is shown below.
[Since this may be different with each board, check your manual for details.]
MSB LSB 7 6 5 4 3 2 1 0 <-- Interrupt Vector Register Channel 0 interrupt indicator (0-active) N/A Channel 1 interrupt indicator (0-active) Channel 2 interrupt indicator (0-active) Channel 3 interrupt indicator (0-active) Global interrupt: 1-enable; 0-disable
For example, an 8 PORT RS-232C CARD can have the following configuration:
Base IRQ Channel Interrupt Address Level Number Vector
2A0 7 0 2BF 2A8 7 1 2BF 2B0 7 2 2BF 2B8 7 3 2BF 1A0 5 0 1BF 1A8 5 1 1BF 1B0 5 2 1BF 1B8 5 3 1BF
[The base addresses should be configurable by jumpers or DIP switches.]
Note that the Interrupt Vector Registers overlap Scratch Registers, so the detect_UART routine must be changed for these boards. [See the Programming Section.]
Some words about timing -----------------------
The 8250 is a rather slow peripheral chip; it has a cycle delay for both reading and writing of 500nsec, which means that after every read or write access to any of the chip's registers the CPU has to wait at least 500nsec before reading or writing one of its registers again. Good thing that this chip is only used with some old XTs... the 8088/8086/V20/V30 family is slow enough for that.
The 16450 and 16550A are rather fast; they need a delay of 125nsec after read access and 150nsec after write access before any other transfer. This means we only have a problem with these fancy new machines that allow cycle times of 50nsec and less. Luckily they add wait states to I/O bus accesses (wait states are additional cycles during which the bus does not change its state) or use a slower clock speed for I/O transfers (8 or 12 MHz). So if you have 12 MHz I/O clock speed and one wait state for I/O transfers, you don't have to worry.
Some people believe in delaying I/O operations by adding NOPs or JMP $+2 to every I/O instruction (both do nothing but wasting time), but I don't think that's any good with a chip that needs stable data lines for at least 100nsec (so the CPU or the bus controller has to add a wait state anyway). You can always blame the hardware or the setup if your program doesn't work for timing reasons. :)
However, there may be a problem with block instructions, esp. OUTSB. This instruction allows you to fill the Tx fifo of the 16550A rather fast (just 5 cycles per transfer on the 286, others take longer), but even a 25MHz 286 takes 200nsec for each transfer, so this should be on the safe side, too. I don't use this instruction, but for other reasons than timing difficulties. It's just not very useful: it takes more time to make sure in advance that you don't overrun your buffer margins during an OUTSB than to check for the margins after every single transfer.
Please note that all this relates to ISA boards. I don't have any experience with EISA or other fancy things like VLB!
The method of exchanging signals for data flow control between computers and data sets is called handshaking. The most popular and most often used handshaking variant is called XON/XOFF; it's done by software, while other methods are hardware-based.
Two bytes that are not mapped to normal characters in the ASCII charset are called XON (DC1, Ctrl-Q, ASCII 17) and XOFF (DC3, Ctrl-S, ASCII 19). Whenever either one of the sides wants to interrupt the data flow from the other (eg. full buffers), it sends an XOFF ('Transmission Off'). When its buffers have been purged again, it sends an XON ('Transmission On') to signal that data can be sent again. (With some implementations, this can be any character).
XON/XOFF is of course limited to text transmission. It cannot be used with binary data since binary files tend to contain every single one of the 256 characters...
That's why hardware handshaking is normally used with modems, while XON/XOFF is often used with printers and plotters and terminals.
The 'Data Terminal Ready' and 'Data Set Ready' signals of the serial port can be used for handshaking purposes, too. Their names express what they do: the computer signals with DTR that it is ready to send and receive data, while the data set sets DSR. With most modems, the meaning of these signals is slightly different: DTR is ignored or causes the modem to hang up if it is dropped, while DSR signals that a connection has been established.
While DTR and DSR are mostly used to establish a connection, RTS and CTS have been specially designed for data flow control. The computer signals with RTS ('Request To Send') that it wishes to send data to the data set, while the data set (modem) sets CTS ('Clear To Send') when it is ready to do one part of its job: to send data thru' the phone wires.
A normal handshaking protocol between a computer and a modem looks like this:
(1)(2) (3)(4) (5) (6) (7)(8)(9)(10) (11)(12)(13)
(1) The computer sets DTR to indicate that it wants to make use of the modem. (2) The modem signals that it is ready and that a connection has been established. (3) The computer requests permission to send. (4) The modem informs the computer that it is now ready to receive data from the computer and send it through the phone wires. (5) The modem drops CTS to signal to the computer that its internal buffers are full; the computer stops sending characters to the modem. (6) The buffers of the modem have been purged, so the computer may continue to send data. (7) This situation is not clear; either the computer's buffers are full and it wants to inform the modem of this, or it doesn't have any more data to be send to the modem. Normally, modems are configured to stop any transmission between the computer and the modem when RTS is dropped. (8) The modem acknowledges RTS cleared by dropping CTS. (9) RTS is again raised by the computer to re-establish data transmission. (10) The modem shows that it is ready to do its job. (11) No more data is to be sent. (12) The modem acknowledges this. (13) DTR is dropped by the computer; this causes most modems to hang up. After hang-up, the modem acknowledges with DSR low. If the connection breaks, the modem also drops DSR to inform the computer about it.
BIOS API (Application Programs Interface) -----------------------------------------
PC programs are meant to use the BIOS routines to program the UARTs. Even though this is *NOT RECOMMENDED* by me (awfully slow, limited and complicated), I give you the BIOS calls as specified by Big Blue. Call INT 14h with:
AH=00h Serial port - Initialize
AL: see table DX: Port number (0-3; 0 equ. 0x3f8, 1 equ. 0x2f8, etc., see Hardware)
Bit 7 Bit 6 Bit 5 Rate [bps] Bit 4 Bit 3 Parity 1 1 1 9600 0 0 none 1 1 0 4800 1 0 none 1 0 1 2400 0 1 odd 1 0 0 1200 1 1 even 0 1 1 600 0 1 0 300 Bit 1 Bit 0 Data bits 0 0 1 150 0 0 5 0 0 0 110 0 1 6 1 0 7 Bit 2 0 -> 1 stop bit, 1 -> 2 stop bits 1 1 8
Returns: AH: RS-232C line status bits Bit 0: RBF - input data is available in buffer 1: OE - data has been lost 5: THRE - room is available in output buffer 6: TEMT - output buffer empty AL: Modem status bits 3: always 1 7: DCD - carrier detect
AH=01h Serial port - Write character
AL: character to be sent DX: Port
Returns: AH: Bit 7 clear if successful, set if not. Bits 0-6 see INT 14h AH=03h
AH=02h Serial port - Read character
Returns: AH: Line Status (see AH=03h) AL: Received character (if AH bit 7 is clear)
Note: This routine times out if DSR is not asserted, even if data is available! (That's why you need the short wires from the "Connecting devices" chapter with some programs).
AH=03h Serial port - Get port status
Returns: AH: Line Status Bit 7: Timeout Bit 6: TEMT Transmitter empty Bit 5: THRE Transmitter Holding Register Empty Bit 4: Break (broken line detected) Bit 3: FE Framing error Bit 2: PE Parity error Bit 1: OE Overrun error Bit 0: RDF Receiver buffer full (data available) AL: Modem Status Bit 7: DCD Carrier detect Bit 6: RI Ring indicator Bit 5: DSR Data set ready Bit 4: CTS Clear to send Bit 3: DDCD Delta carrier detect Bit 2: TERI Trailing edge of ring indicator Bit 1: DDSR Delta data set ready Bit 0: DCTS Delta Clear to send
BIOS variables in the Data Segment at segment 40h:
Offset Size Description 00h WORD Base I/O address of 1st serial I/O port, zero if none 02h WORD Base I/O address of 2nd serial I/O port, zero if none 04h WORD Base I/O address of 3rd serial I/O port, zero if none 06h WORD Base I/O address of 4th serial I/O port, zero if none Note: Above fields filled in turn by POST as it finds serial ports. POST never leaves gaps. DOS and BIOS serial device numbers may be redefined by re-assigning these fields. [POST: Power-On Self Test. CB] [Madis Kaal told me that there are BIOSes that leave gaps in the table, and I know of some that don't recognize COM4 correctly.]
This information is sneaked from Ralf Brown's famous interrupt list (hope he doesn't mind). If you want more detailed facts on this interrupt, refer to this list. It's available from lots of FTP sites (choose one in your vicinity; it is *huge*).
The Microsoft Serial Mouse (or compatibles) is the device that is most often used with the Serial Port of the PC; it's the one with the two buttons. Mouse Systems compatible mice have three buttons. Here's some information I received from Stephen Warner and Angelo Haritsis:
Pins Used: TxD, RTS and/or DTR are used as power sources for the mouse. RxD is used to receive data from the mouse.
Mouse reset: Set UART to 'broken line' state (set bit 6 of the LCR) and clear the bits 0-1 of the MCR; wait a while and reverse the bits again.
Serial transmission parameters: Microsoft Mouse 1200 bps, 7 data bits, 1 stop bit, no parity Mouse Systems Mouse 1200 bps, 8 data bits, 1 stop bit, no parity
Data packet format of the Microsoft mouse: The data packet consists of 3 bytes. It is sent to the computer every time the mouse changes state (ie. the mouse is moved or the buttons are released/ pressed).
D6 D5 D4 D3 D2 D1 D0
1st byte 1 LB RB Y7 Y6 X7 X6 2nd byte 0 X5 X4 X3 X2 X1 X0 3rd byte 0 Y5 Y4 Y3 Y2 Y1 Y0
The byte marked with 1 is sent first and then the others. The bit D6 in the first byte is used for synchronizing the software to the mouse packets if it goes out of sync.
LB is the state of the left button (1 being the LB is pressed) RB is the state of the right button (1 being the RB is pressed) X0-7 movement of the mouse in the X direction since last packet (+ right) Y0-7 movement of the mouse in the Y direction since last packet (+ down )
The Microsoft Mouse uses RTS as power source. Whenever RTS is set to '0' and reset to '1', the mouse performs an internal reset and sends the character 'M' to signal its presence. Three-button-mice send 'M3' if you drop and raise RTS (see above) in Microsoft mode; this is compatible with the Microsoft mouse driver and allows the firmware to check if it is really a three-button mouse. [Scott David Daniels received this info from Brian Onn]
Data packet format of the Mouse Systems mouse: The data packet consists of 5 bytes.
D7 D6 D5 D4 D3 D2 D1 D0
1st byte 1 0 0 0 0 LB MB RB 2nd byte X7 X6 X5 X4 X3 X2 X1 X0 3rd byte Y7 Y6 Y5 Y4 Y3 Y2 Y1 Y0 4th byte equal to 2nd byte 5th byte equal to 3rd byte
Bits 7-3 of the 1st byte are used for synchronization; it's rather improbable that they appear the same way in any of the other bytes.
LB is the state of the left button (1 being the LB is pressed) MB is the state of the middle button (1 being the MB is pressed) RB is the state of the right button (1 being the RB is pressed) X0-7 movement of the mouse in the X direction since last packet (+ right) Y0-7 movement of the mouse in the Y direction since last packet (+ up )
The mouse should rather be used with the mouse driver software; this ensures compatibility to future changes as well as bus mice and greatly reduces programming overhead. See Ralf Brown's interrupt list, interrupt 33h. It is available from lots of FTP sites (eg. garbo.uwasa.fi, /pc/programming), the files are called inter*.zip.
This chapter is rather brief for several reasons. I'm no modem expert at all and there exist better sources than this document if you want information on modems. Patrick Chen, the author of "The Joy of Telecomputing", has written such a file, and there's one available from Sergey Shulgin, too (I don't have their internet addresses). You can obtain these files from my archive; they are named "modem1" and "modem2".
A modem (MOdualtor-DEModulator) is an interface between the serial port of your computer and the public telephone network. Modern modems are small computers of their own: they accept commands, do the dialing for you, buffer incoming data, perform data compression and such things. Several standards have been established (Bell, CCITT), and several "command languages" are in use, with the Hayes and Microcom commands being the most popular ones.
Modems have two internal modes: the command mode and the data mode. After power-up, the modem is in the command mode, and this mode can be restalled by sending an 'escape sequence' (normally a pause of at least 1 second, then three '+' signs in one second, then a pause of at least 1 second).
All I know about modems is some commands and some encoding schemes; I share this knowledge with you - please share yours with me!
Encoding schemes ----------------
I've sneaked this table from the posting 'FAQ zu /Z-NETZ/TELECOM/ALLGEMEIN' of Kristian Koehntopp <firstname.lastname@example.org> in 'de.newusers.questions'. He has copyrighted his posting, so please contact him if you wish to reproduce this information in any commercial way.
These are the schemes recommended by CCITT (more than one speed means fallback/auto-retrain speeds):
Transmission speed in bps Baud Modulation duplex usage -------------------------------------------------------------------- V.17 14400 2400 TCM half FAX 12000, 9600, 7200 2400 TCM half FAX 4800 2400 QAM half FAX V.21 300 300 FSK full V.22 1200 600 DPSK full V.22bis 2400 600 QAM full V.23 1200/75 1200/75 FSK asymmetric BTX V.27ter 4800 1600 DPSK half FAX 2400 1200 DPSK half FAX V.29 9600 2400 QAM half FAX 7200 2400 QAM half FAX V.32 9600 2400 TCM/QAM full 4800 2400 QAM full V.32bis 14400 2400 TCM full 12000, 9600, 7200 2400 TCM full 4800 2400 QAM full
FSK Frequency Shift Keying DPSK Differential Phase Shift Keying QAM Quadrature Amplitude Modulation TCM Trellis Coded Modulation
Other V-Recommendations often heard of:
V.24 - Meaning of the signals at the serial port. V.28 - Electrical levels (V.24, V.28, and ISO 2110 are equivlaent to EIA RS232) V.42 - Data protection method, not dependening on the modulation scheme in use. V.42bis - Compression scheme, also called BTLZ.
Erich Smythe <email@example.com> posted a very informative and humorous article explaining different modulation schemes used with modems. You can find it in the FTP archive, named The_Serial_Port.more06.
Hayes commands --------------
Each command line starts with 'AT', then several commands, then carriage return.
The list is not comprehensive at all; most modems have several commands of their own, but these commands are available with most modems:
A/ Repeat last command (no prepending AT)
A Take over phone line (if you've already picked up the phone).
B Set communications standard. B0 - CCITT B1 - Bell
C Switch carrier on/off. C0 - carrier off C1 - carrier on
D Dial a number. Normally followed by T - tone dial P - pulse dial nothing - according to actual setting (see ATP/ATT) then a sequence of the follwing characters: 0-9 - the numbers to be dialed W - wait for dial tone , - wait 2 seconds @ - wait 5 seconds (?) ! - flash (put the phone on the hook for 1/2 second) > - earth key R - start connection right after dialing (eg. ATDPR equals ATA) If you just enter ATD, the modem takes over the line without dialing.
E Echo on/off in the command mode E0 - no echo E1 - echo
H Hang up
L Volume control; followed by 0-3 (0 equ. lowest, 3 equ. highest volume)
M Monitor M0 - Speaker off M1 - Speaker on while dialing and establishing a connection M2 - Speaker always on M3 - Speaker on while establishing a connection
O Switch to data mode O0 - promptly O1 - with retrain (reduction of the data rate)
P Pulse dial
Q Responses to commands on/off Q0 - on Q1 - off
S Set/read internal register, eg. S17=234 set reg. 17 to 234 S17? read reg. 17
T Tone dial
V Verbose mode on/off V0 - short responses V1 - full responses
X Phone tones recognition on/off X0 - Ignore busy sign, don't wait for dial tone, and just answer with "CONNECT" when a connection has been established (other settings produce more detailed messages) X1 - Ignore busy sign, don't wait for dial tone, but give full connect message X2 - Ignore busy sign but wait for dial tone X3 - Don't ignore busy sign, but don't wait for dial tone X4 - Don't ignore anything
Y Break setting Y0 - Don't hang up when break signal is detected Y1 - Hang up when break is detected (&D2, &M0)
Z Initialize modem Z - Default parameters Z0 - Setting 0 Z1 - Setting 1
&C DCD mode &C0 - always 1 &C1 - DCD according to carrier
&D DTR mode &D0 - ignore DTR &D1 - switch to command mode when DTR goes 0 &D2 - hang up if DTR goes 0 &D3 - initialize modem when DTR goes 0
&F Set operation mode &F0 - according to Hayes, no data protocol &F1 - according to Microcom; MNP1-4 or MNP5 as specified by %C &F2 - according to Sierra; MNP1-4 or MNP5 as specified by %C &F3 - according to Sierra, V.42 or V.42bis as specified by %C
These are the default settings: &F0 - B0, E1, L2, M1, P, Q0, V1, Y0, X1, &C1, &D0, &G0, &R0, &S0, S0=0, S1=0, S2=43, S3=13, S4=10, S5=8, S6=2, S7=30, S8=2, S9=6,S10=14, S11=75, S12=50, S14=AAh, S16=80h, S21=20h, S22=76h, S23=7, S25=5, S26=1, S27=40h &F1 - \A3, \C0, \E0, \G0, \K5, \N1, \Q0, \T0, \V0, \X0, %A0, %C1, %E1, %G0, &G1, S36=7h, S46=138h, S48=128h, S82=128h &F2 - \A3, \C2, \E0, \G1, \K5, \N3, \Q1, \T0, \V1, \X0, %A13, %C1, S36=7h, S46=138h, S48=128h, S82=128h &F3 - \A3, \C0, \E0, \G0, \K5, \N3, \O1, \T0, \V1, \X0, %A0, %C1, %E0, S36=7h, S46=138h, S48=7h, S82=128h
&G Guard tone &G0 - off &G1 - 550 Hz &G2 - 1800 Hz
&K Data flow control &K0 - none &K3 - bidirectional RTS/CTS handshaking &K4 - bidirectional XON/XOFF &K5 - unidirectional XON/XOFF
&M Synchronous/asynchronous operation &M0 - asynchronous (the usual thing) &M1 - command mode asynchronous, data mode synchronous. &M2 - switch to synchronous mode, start dialing after DTR 0->1 &M3 - switch to synchronous mode, don't dial
&Q Further specification of the communication &Q0 to &Q3 - no V.42bis &Q5 - V.42bis &Q6 - V.42bis off, buffer data
&R CTS mode &R0 - CTS follows RTS with the delay time of S26 &R1 - CTS is 1 if the modem is in the data mode
&S DSR mode &S0 - DSR always 1 &S1 - according to CCITT V.24
&T Test &T0 - normal operation (no test) &T1 - local analog loopback &T3 - local digital loopback &T4 - accept distant digital loopback &T5 - ignore distant digital loopback &T6 - start distant digital loopback &T7 - start distant digital loopback and self test &T8 - start distant analog loopback and self test
&V Show modem status
&Wn Save actual configuration (some modems only). Setting can be restored with ATZn. n normally ranges between 0 and 1. The following parameters are stored: B, C, E, L, M, P/T, Q, V, X, Y, &C, &D, &G, &R, &S, &T4/&T5, S0, S14, S18, S21, S22, S25, S26, S27
&X Specify clock source for synchronous operation &X0 - modem generates clock &X1 - modem synchronizes with local clock &X2 - modem synchronizes with distant clock
&Y Define default setting (see &W and Z) &Y0 - setting 0 is default &Y1 - setting 1 is default
&Z Store phone number in diary &Zn=XXXXXX stores phone number XXXXXX under index n, where XXXXXX can be up to 30 digits and n ranges between 0 and 3.
Microcom commands -----------------
\A Set block length for MNP \A0 - 64 characters \A1 - 128 characters \A2 - 192 characters \A3 - 256 characters
\Bn Send break signal for n times 100ms (MNP defaults to n=3).
\C Set buffering \C0 - none at all \C1 - buffer data for 4 seconds as long as 200 characters aren't reached or as long as no MNP block is found \C2 - don't buffer. Switch back to normal operation after reception of the control character (fall-back, see %C)
D/n Dial phone number n in the diary (see &Z)
DL Redial last number
\E Echo on/off in data mode \E0 - no echo \E1 - echo
\G Data flow on/off (see \Q) \G0 - off \G1 - on
\J Data rate adjust \J0 - the data rates computer-modem and modem-modem are independent \J1 - the data rate computer-modem follows the data rate modem-modem
\Kn Break setting (don't know anything about this, just that it exists ;-)
\N MNP select \N0 - standard mode, no MNP, data is buffered \N1 - direct mode, no MNP, no buffering \N2 - MNP, data is buffered \N3 - allow MNP on/off during connection, data is buffered
\O Switch on MNP during connection (the rest of the line is being ignored!)
\Pn Same as &Z
\Q Set handshake (compare &K) \Q0 - no handshaking \Q1 - XON/XOFF \Q2 - modem controls data flow with CTS \Q3 - data flow control with RTS/CTS
\S List complete configuration
\Tn Set idle timer \T0 - timer off \Tnn - break connection after nn minutes without data exchange (1-90)
\U Acknowledge MNP operation; rest of line is ignored!
\V Verbose mode \V0 - messages according to Hayes, even if MNP (no \REL) \V1 - messages according to Microcom (\REL appended if MNP)
\X Filter XON/XOFF characters \X0 - filter XOM/XOFF characters \X1 - don't filter them (the usual thing)
\Y Same as AT\O\U with the difference that it is not necessary to first send AT\O to one modem and then AT\U to the other; just send AT\Y to each modem within 5 seconds
%An Specify control character that provokes fallback from MNP to normal operation (see \C2). n=0..255 (ASCII code)
%C MNP5 %C0 - not allowed %C1 - allowed
%E auto-retrain %E0 - no auto-retrain allowed %E1 - auto-retrain allowed according to CCITT
%R Show all S registers
%V Same as I3 (but don't ask me what it is ;-) Gives info on the firmware version with some modems.
IRQ sharing - can it be done? (this applies to ISA bus systems only) -----------------------------
Yes and no. Yes, it can be done in principle, and no, it can't be done by just configuring two ports to use the same interrupt.
Let us first consider the hardware involved. PCs have ICUs (interrupt control units, or PICs - programmable interrupt controllers) of the 8259A type. They can be programmed to be triggered by a high signal level or a raising edge, which is already annoying because low level or falling edge would make add-on card design simpler. But to top this all off, they have internal pull-up resistors! Which means that if no card is using the interrupt, it is in the triggered state.
How would cards share interrupts? They'd only be allowed to have their IRQ output in two states: active high or 'floating'. 'Floating' means the line is not driven at all, neither high nor low, it 'floats'. If all sharers of an interrupt line in the PC would only drive the line high or let it 'float', we'd have a simple interrupt sharing scheme (that would allow for even simpler design if the active state of the line was low) - if there wasn't this nasty internal pull-up resistor in the 8259A. <sarcasm on> Sadly IBM didn't provide an external pull-down resistor on the main board of the very first PC, so later designs could not have one either for compatibility's sake. <sarcasm off> 1.5kOhms would be a fine value; the 8259A produces 300uA that have to be sunk below 0.8v (so 2.6kOhms would be enough in theory, but having some safety margin can't hurt).
So how can you have your ports sharing a common interrupt line? There are two approaches to this, each assuming you're familiar with using a soldering iron. What you must provide is a logical OR of all interrupt outputs that drive the line; while this can be done with an OR gate of course, it is far more practical to use some wired-OR facility. First you'll have to add the external pull-down resistor, either on the main board (where it really belongs) or on one of the cards. Use 1.5kOhms for this. Then cut the line between the card edge connector and the IRQ line driver (LS125) on each and every card. Do this carefully; if it's a multi-layer card, you'd better cut the pin of the LS125, or maybe you can just replace a jumper with a diode. Now solder a diode (1N4148 will do, slow power diodes won't) over the cut with the cathode (usually marked with a ring, but you'd better check that thoroughly if there are multiple rings; the 1N4148 normally has a yellow cathode ring) to the card edge connector. There you are! Now hardware will no longer be in the way of interrupt sharing. (A 'cleaner' solution would be to use a LS126 line driver instead of the diode with 'enable' connected to 'input', but that's only practical with from-scratch designs.)
Now let's face the software problems. In theory, interrupt sharing works fine between different pieces of hardware, but practically this is limited to real operating systems that do all interrupt processing by themselves; MSDOS doesn't do that, so it's not a good option for PCs (even Linux users boot DOS sometimes, if only to play games). Sharing interrupts even between UARTs becomes problematic if there are several programs involved, eg. the mouse driver and some comm application; they'd have to know of each other. 'Daisy chaining' the interrupt (a program 'hooks' the interrupt by placing its handler's address in the IRQ serivce table and letting the handler call the address it found in that table at install time when it exits; no interrupt acknowledging is done by the handlers themselves, just by the stub handler at the end of the chain) doesn't work because DOS doesn't even provide a stub interrupt handler! So one of the programs would have to issue EOI (end of interrupt) to the ICU, but which one? How would it know it's the last one in the chain? Better forget daisy chaining interrupts under DOS if you want your programs to work reliably.
The situation is much simpler if all UARTs sharing the same interrupt are used by the same program. This program has to be aware of the sharing mechanism, but programs that can make use of more than one serial port (especially libraries) usually are. Now there's only one problem to be solved: lock-up situations. As I already wrote, the ICUs in the PC are programmed to use raising edge trigger mode, and you can't change this without crashing the system. Now consider the following situation. Two UARTs share one IRQ line. UART #1 raises the line because it needs service; the service routine is called and detects that UART #1 needs service. Before it can perform the serivce, UART #2 raises the IRQ, too. Now UART #1 is serviced, the line should go to the 'low' state but it doesn't because of the other UART keeping it high; the handler checks the next UART in its table and sees that UART #2 needs service, too. Now UART #1 receives another character and keeps the line high while UART #2 is being serviced. How should the handler know that this has happened? If it just issued EOI and returned, the IRQ line would never have gone 'low' during the service, so there won't be any future raising edges to be detected, and thus no more interrupts!
What does the service routine do to avoid lock-ups? It has to mask the interrupt in the ICU; this resets the edge detector. If it unmasks the interrupt again at the end of the handler and the line is still 'high', this will trigger the edge detector and the interrupt will be scheduled again. See the 'known problems' section for a very solid method of handling interrupts suggested by Richard Clayton.
Windows allows for UARTs sharing interrupts; just make sure the COM ports are configured properly in the system setup.
A note to Linux users: Linux is fully capable of sharing interrupts between serial ports if the hardware problems described above are solved. Using the same interrupt for several UARTs even reduces CPU load, so it is definitely a Good Thing as long as there are not too many sharers. Having a well-designed and kernel-supported multi-port card is even better because these cards provide a mechanism for the handler to detect which UART has triggered interrupt without having to look at every single IIR, which reduces overhead