RS-232 Pinouts: pin connections & connectors

When setting up a data link using RS232 it is necessary to ensure the right connections are made otherwise the system will not operate.

RS232 Includes:
RS232 basics RS232 standard Software handshaking Signals & voltage levels Pinouts & connectors Loopback connections RS232 cables

Serial Data standards: Serial data standards RS422 RS449 RS485 20 mA current loop

The correct connection of the various pins used for an RS232 data link are key to the operation of the system.

Knowing the RS232 pinouts, or pin connections, and being able to make the required links enables the handshaking to operate correctly and the data to flow smoothly.

Although the RS232 standard does not define a connector, the 9 way and 25 way D-type connectors are very popular and provide an excellent format.

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.

When using RS232 it is essential to ensure that the correct connections are being used. It can be very difficult to fault-find when the wrong connections are made as nothing happens. Checking the voltages are correct and that the correct pins have been connected normally resolves the issues.

Sometimes confusion can be caused by defining which is the DTE and which is the DCE. Once this has been correctly done, and the correct pins connected, the system should operate well.