RS232 Glossary

DCE  =  Data Communications Equipment DCE. This is another word for modem. This is a device which transforms the data in a way that they can be transmitted via a telephone line. Data Communication Equipment (i.e. modems and level converters) are devices that transmit and receive signals and pass them on to other equipment without any processing.

DTE  =  Data Terminal Equipment. This can be e.g. a computer terminal of a mainframe system, which in fact is the origin of the expression. It can be a printer, plotter or PC, etc. Data Terminal Equipments are devices which transmit and/or receive data and process them without passing them on to other eqipment.

Null Modem  =  This expression stands for a RS232 cable connection between two DTEs, which use this transmission protocol, without using modems. This is e.g. the case if a serial printer is connected to a PC. The length of the cable, however, is limited to 10 meters. There are amplifiers available which can be connected in between and allow a much longer cable length. Since there are no modems used, it is necessary to connect the odem specific signals in a certain way to simulate the presence of a modem. The wireing details are described below.

RS232 Signal Description

RD/RX  =  Received data. On this wire the data bits from the data terminal (DTE) are received.

TD/TX  =  Transmitted data. On this wire the data bits are transmitted to the data terminal (DTE).

CHS GND  =  Chassis ground. The data terminal and the modem have to have a common ground, to prevent ground loops.

DSR  =  Data Set Ready. This signal is generated by the modem and means that the modem is active and ready to communicate with the data terminal.

DTR  =  Data Terminal Ready. This signal is generated by the data terminal and transmitted to the modem and means that the data terminal is active and ready to comunicate with the modem.

DCD/CD  =  Data Carrier Detect or Carrier Detect. This signal shows that the modems on both sides are connected via the telephone line and are ready to interchange data.

RTS  =  Request To Send. This signal is generated by the data terminal and means that the terminal wants to transmit data.

CTS  =  Clear To Send. This is the response signal from the modem to the data terminal (followng a RTS) and signals that the modem is ready to receive the data from the data terminal and transform them for the telephone line.

SIG GND  =  Signal ground. This ground is the reference ground for all signals. Depending on the equipment this can be a separate ground or it can be connected to the chassis ground.

RI  =  Ring Indicator. Is a signal from the modem to the datat terminal which indicates that an external call is comming in on the telephone line. I.e. the telephone is "ringing". Depending on the application the receiver is "liftet off". The number of rings until lifting off can be set in the modem.

RS232 Pin Assignment

View upon the D-SUB 25 pin female connector (socket)

+-------------------------------------+
 \  13 O O O O O O O O O O O O O 1   /
  \  25 O O O O O O O O O O O O 14  /
   +------------------------------+

View upon the D-SUB 25 pin male connector (plug)

+-------------------------------------+
 \   1 + + + + + + + + + + + + + 13  /
  \  14 + + + + + + + + + + + + 25  /
   +------------------------------+

View upon the D-SUB 9 pin female connector (socket)

+-------------------+
 \  5 O O O O O 1  /
  \  9 O O O O 6  /
   +------------+

View upon the D-SUB 9 pin male connector (plug)

+-------------------+
 \  1 + + + + + 5  /
  \  6 + + + + 9  /
   +------------+

The pin assignment in this table can be used as a wireing schematic for an adpter between the two types of connectors.

RS232 Data Transmission

 DTE                            DCE
+------------+                 +------------+
| TxD   |  2 |-----------------|  2 |   TxD |
+------------+                 +------------+
| RxD   |  3 |-----------------|  3 |   RxD |
+------------+                 +------------+
| RTS   |  4 |                 |  4 |   RTS |
+------------+                 +------------+
| CTS   |  5 |                 |  5 |   CTS |
+------------+                 +------------+
| DSR   |  6 |                 |  6 |   DSR |
+------------+                 +------------+
| GND   |  7 |-----------------|  7 |   GND |
+------------+                 +------------+
| DCD   |  8 |                 |  8 |   DCD |
+------------+                 +------------+
| DTR   | 20 |                 | 20 |   DTR |
+------------+                 +------------+

 DTE                            DTE
+------------+                 +------------+
| TxD   |  2 |-------\  /------|  2 |   TxD |
+------------+        X        +------------+
| RxD   |  3 |-------/  \------|  3 |   RxD |
+------------+                 +------------+
| RTS   |  4 |                 |  4 |   RTS |
+------------+                 +------------+
| CTS   |  5 |                 |  5 |   CTS |
+------------+                 +------------+
| DSR   |  6 |                 |  6 |   DSR |
+------------+                 +------------+
| GND   |  7 |-----------------|  7 |   GND |
+------------+                 +------------+
| DCD   |  8 |                 |  8 |   DCD |
+------------+                 +------------+
| DTR   | 20 |                 | 20 |   DTR |
+------------+                 +------------+

 DTE                            DCE
+------------+                 +------------+
| TxD   |  2 |-----------------|  2 |   TxD |
+------------+                 +------------+
| RxD   |  3 |-----------------|  3 |   RxD |
+------------+                 +------------+
| RTS   |  4 |-----------------|  4 |   RTS |
+------------+                 +------------+
| CTS   |  5 |-----------------|  5 |   CTS |
+------------+                 +------------+
| DSR   |  6 |-----------------|  6 |   DSR |
+------------+                 +------------+
| GND   |  7 |-----------------|  7 |   GND |
+------------+                 +------------+
| DCD   |  8 |-----------------|  8 |   DCD |
+------------+                 +------------+
| DTR   | 20 |-----------------| 20 |   DTR |
+------------+                 +------------+

RS232 Null Modem

This is a full connection (with Hardware-Hand-Shake) between DTE (e.g. PC) and DTE (e.g. plotter). All other pins are not used. This cable can also be used to connect two PCs to exchange data (interlink cable WIN98 / WIN95)

 DTE                            DTE
+------------+                 +------------+
| TxD   |  2 |-------\ /-------|  2 |   TxD |
+------------+        X        +------------+
| RxD   |  3 |-------/ \-------|  3 |   RxD |
+------------+                 +------------+
| RTS   |  4 |-------\ /-------|  4 |   RTS |
+------------+        X        +------------+
| CTS   |  5 |-------/ \-------|  5 |   CTS |
+------------+                 +------------+
| DSR   |  6 |-----\     /-----|  6 |   DSR |
+------------+      \   /      +------------+
| GND   |  7 |-----------------|  7 |   GND |
+------------+        X        +------------+
| DCD   |  8 |-----------------|  8 |   DCD |
+------------+      /   \      +------------+
| DTR   | 20 |-----/     \-----| 20 |   DTR |
+------------+                 +------------+

Here is another table using the 9pin connectors

RS232 Mainboard Connectors

The serial RS232 output of some mainboards (10pol. DIL) is wired with a flat cable to the connectors in the back of the housing. Two variants of this are known.

IBM Assignment:

Everex Assignment:

RS232 Loopback Diagnostics

Using the fllowing wireing a test adapter can be made to check the function of the RS232 interface e.g. in a PC.

25pin D-Sub female loopback connector (socket)

2 to 3 (TxD to RxD)
4 to 5 to 22 (RTS to CTS to RI)
6 to 8 to 20 (DSR to CD to DTR)

9pin D-Sub female loopback connector (socket)

2 to 3 (TxD to RxD)
7 to 8 to 9 (RTS to CTS to RI)
6 to 1 to 4 (DSR to CD to DTR)