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RS-232 Pin Outs

- an overview RS232 pin outs or pinouts configurations used on the different connector types including RS232 9pin and RS232 25 pin connectors.

This RS232 tutorial is split into several pages each of which addresses different aspects of RS 232 operation and technology:

[1] RS232 serial interface tutorial
[2] EIA and RS232 standards
[3] RS232 Handshaking
[4] RS232 signals and voltage levels
[5] RS232 pinouts
[6] RS232 loopback
[6] RS232 serial data cables

To ensure that communications are possible between a variety of RS232 devices it is necessary that standard RS232 pin outs are used. Although a variety of connectors are used, the pin outs for each type of connector has to be standardised so that it is possible to know how to connect to them, and to be able to use standard RS232 cables.

The most common connectors use the D-type often also called a sub-D connector and both 9 way and 25 way versions are used. In addition to this other types of connector may be used on occasions.

When using RS232, two main types of equipment are nominated. One is a data Terminal Equipment, DCE and the other is known as the Data Terminal Equipment, DTE. These abbreviations are widely used within RS232 and other communications terminology.

By using the pin outs as detailed below the RS232 cables used should be a "one to one" configuration. This considerably simplifies the manufacture of RS232 cables and often means that standard cables can be used.

RS232 25 pin connections and functions

The RS232 (EIA-232-A) specification provides a full set of details for a wide variety of connections that can be used with RS232. While many of them are not normally known, they may be used in some circumstances and therefore the details are provided here for reference.

RS232 25 pin signal definition for the DTE device - looking into the DTE connector
DTE device is often a PC.

DTE Pin No	Circuit name	Abbreviation	Source
1	Protective earth
2	Transmitted data	TXD	DTE
3	Received data	RXD	DCE
4	Request to send	RTS	DTE
5	Clear to send	CTS	DCE
6	Data set ready	DSR	DCE
7	Signal ground
8	Data carrier detect	DCD	DCE
9	Reserved for data set test
10	Reserved for data set test
11	Unassigned
12	Secondary received signal detector		DCE
13	Secondary clear to send		DCE
14	Secondary transmitted data		DTE
15	Transmission signal element timing		DCE
16	Secondary received data		DCE
17	Receive signal element timing		DCE
18	Unassigned
19	Secondary request to send		DTE
20	Data terminal ready	DTR	DTE
21	Signal quality detector		DCE
22	Ring indicator		DCE
23	Data signal rate selector		DCE/DTR
24	Transmit signal element timing		DTE
25	Unassigned

RS232 25 pin signal definition for the DCE device - looking into the DCE connector
DTE device is often a device such as a modem.

DCE Pin No	Circuit name	Abbreviation	Source
1	Protective earth
2	Received data	RXD	DCE
3	Transmitted data	TXD	DTE
4	Clear to send	CTS	DCE
5	Request to send	RTS	DTE
6	Data set ready		DCE
7	Signal ground
8	Data carrier detect	DCD	DCE
9	Reserved for data set test
10	Reserved for data set test
11	Unassigned
12	Secondary received signal detector		DCE
13	Secondary request to send		DTE
14	Secondary received data		DCE
15	Transmission signal element timing		DCE
16	Secondary transmitted data		DTE
17	Receive signal element timing		DCE
18	Unassigned
19	Secondary clear to send		DCE
20	Data terminal ready	DTR	DTE
21	Signal quality detector		DCE
22	Ring indicator		DCE
23	Data signal rate selector		DCE/DTR
24	Transmit signal element timing		DTE
25	Unassigned

By using these configurations it means that an RS232 cable connecting a DTE to a DCE will be wired in a one-to-one configuration, i.e. pin 1 to pin 1, etc.

RS232 9pin connections, functions and pin-outs

Although the RS232 25 pin configuration with a 25 way D-type connector is very widely used, in many applications the smaller 9 way D-type connector is used in many applications. It provides an obvious size and cost benefit. Also the RS232 9pin configuration is quite sufficient in most circumstances because many of the lines available for RS232 signalling are rarely used. This means that the 9 way connector is able to provide all the required connectivity for most applications.

RS232 9pin signal definition for the DTE device - looking into the DTE connector
DTE RS232 device is often a PC.

Pin No	Circuit name	Abbreviation	Source
1	Received line signal detect
2	Receive data	RXD
3	Transmit data	TXD
4	Data terminal ready	DTR
5	Signal ground
6	Data set ready	DSR	DCE
7	Request to send	RTS
8	Clear to send	CTS
9	Ring indicator

RS232 9pin signal definition for the DCE device - looking into the DCE connector
DCE device is often a modem.

Pin No	Circuit name	Abbreviation	Source
1	Received line signal detect
2	Transmit data	TXD
3	Receive data	RXD
4	Data terminal ready	DTR
5	Signal ground
6	Data set ready	DSR	DCE
7	Clear to send	CTS
8	Request to send	RTS
9	Ring indicator

By using these configurations it means that an RS232 cable connecting two RS232 devices, i.e. a DTE to a DCE will be wired in a one-to-one configuration, i.e. pin 1 to pin 1, etc.

RS232 Pin Out Summary

RS232 is still widely used throughout the electronics and telecommunications industries. When using this form of serial communications system it is essential to ensure that the correct connections are being used. Accordingly it is necessary to check the RS232 pin outs when making any leads or other connections. Both RS232 25 pin and RS232 9pin connectors are used when making connections, so checking the correct connector and pin connections is important to ensure that communications can take place.

Further pages from this tutorial
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