by W.A. Steer PhD
This article explains the purpose of the various RS232 pins, and most
importantly, how to get two devices communicating over the link.
The original RS232 specification was designed for communication between
"data terminal equipment" (DTE) - end users of data, e.g. terminals and now
desktop computers, and "data communication equipment" (DCE) - a
forwarder of data, such as a modem. In theory a simple cable which
links pin 1 to pin 1, 2 to 2, 3 to 3, etc, should be all that is necessary
to connect DTE to DCE.
Frequently these days it is necessary to connect two items of DTE, such as
two desktop computers. While it should be as simple as crossing each
input/output pair, so that Transmitted Data from one machine goes to Received
Data of the other, and vice versa, etc., it's never that easy in practice!
Problems usually arise because each piece of equipment uses the various
handshaking signals in different ways. There can be a vast array of software
settings too. With so many permutations, a systematic approach is essential.
Let's start by looking at the signals:
- DTE = "Data terminal equipment" end user of data, e.g. desktop computers
- sends data on Transmitted Data (TxD)
- receives on Received Data (RxD)
- asserts Request to Send (RTS) when ready to receive
- waits for Clear To Send (CTS) to be asserted before sending
- asserts Data Terminal Ready (DTR) when ready and waiting (to receive)
- expects Data Set Ready (DSR) to be asserted before performing (sending)
- expects Data Carrier Detect (DCD) to be asserted before receiving
- DCE = "Data communication equipment" forwarder of data: a modem
- As DCE forwards data, its Transmitted Data connection is actually an
input for data to be transmitted down the (telephone) line.
Similarly, the input/output status of all the lines is the inverse of those
DTE to DTE connection (usual computer to computer)
Link gnd to gnd, cross the TxD/RxD pairs, cross RTS/CTS pairs.
Link DSR to DCD locally, and connect to either the local or remote DTR
(use local if the other side does not feature such connections).
In severe problem cases RTS can be linked to the local CTS, but then software
handshaking "XON/XOFF" will be required to prevent possible data loss
due to buffer overflow.
prot. GND 1 ------------- 1 GND prot. << NOT ALWAYS INTERNALLY CONNECTED! >>
TxD 2 -->--\ /--<-- 2 TxD cross
RxD 3 --<--/ \-->-- 3 RxD two
RTS 4 -->--\ /--<-- 4 RTS cross
CTS 5 --<--/ \-->-- 5 CTS two
DSR 6 -<- ->- 6 DSR link pins 6 and 8 locally
sig. GND 7 ------------- 7 GND sig. (always connect the signal ground)
DCD 8 -<-+-\ /-+->- 8 DCD cross
DTR 20 -->--/ \--<-- 20 DTR two
25-pin D 1 2 3 4 5 6 7 8 20 22
GND TxD RxD RTS CTS DSR GND DCD DTR RI
9-pin D - 3 2 7 8 6 5 1 4 9
See Horowitz & Hill "The Art of Electronics" (2nd edition)
chapter 10.19, pg 725 for more comprehensive coverage.
Other known abnormalities
BBC Master computer RS423 (5 volt system) inputs get overloaded by RS232 (12/15 volt)
drive & fail to work. Use 4k7 (top)/ 2k2 (bottom) resistive potential divider
on inputs (RxD & CTS) to BBC Master.
[Also my BBC Master at home transmits/receives JUNK if it gets too hot!]
Acorn 32-bit micros (Archimedes, Power PC...) use CTS as DSR and vice-versa.
On a plug for an Acorn 32-bit micro therefore, read CTS in place of DSR
and DSR for CTS.
RTS (o/p) == o o == data in BBC Computer, 5-pin DIN
/ \ RS423 connections.
notch ] o ===== GND (view of socket, or
\ / plugs' wiring-side)
CTS (i/p) == o o == data out
The most common software configuration is:
and 9600 baud is often used for relatively short data links, though
most PC's will operate at 19200 baud without problem.
- no parity, 8 data bits, 1 stop bit
From the DOS prompt, you can set the above configuration using the following
It is then possible to send a file out of the RS232 port COM1 by using
the DOS command copy eg.
- mode COM1:9600,n,8,1
to send a copy of your autoexec.bat file to the port.
- copy autoexec.bat com1
Unfortunately it does not seem to be possible to read data in from the
COM ports to files (or the screen) using the copy command - the machine
tends to lock up.
The MS Windows Terminal program can be useful for setting up
"The Art of Electronics" (2nd edn.), P. Horowitz & W. Hill (Cambridge, 1989)
Created: July 1996
Last modified: 12 June 2002
©2002 William Andrew Steer