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RS-232 Pinouts: pin connections & connectors

RS-232 Pinouts: pin connections & connectors


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 NoCircuit nameAbbreviationSource
1Protective earth
2Transmitted dataTXDDTE
3Received dataRXDDCE
4Request to sendRTSDTE
5Clear to sendCTSDCE
6Data set readyDSRDCE
7Signal ground
8Data carrier detectDCDDCE
9Reserved for data set test
10Reserved for data set test
11Unassigned
12Secondary received signal detectorDCE
13Secondary clear to sendDCE
14Secondary transmitted dataDTE
15Transmission signal element timingDCE
16Secondary received dataDCE
17Receive signal element timingDCE
18Unassigned
19Secondary request to sendDTE
20Data terminal readyDTRDTE
21Signal quality detectorDCE
22Ring indicatorDCE
23Data signal rate selectorDCE/DTR
24Transmit signal element timingDTE
25Unassigned
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 NoCircuit nameAbbreviationSource
1Protective earth
2Received dataRXDDCE
3Transmitted dataTXDDTE
4Clear to sendCTSDCE
5Request to sendRTSDTE
6Data set readyDCE
7Signal ground
8Data carrier detectDCDDCE
9Reserved for data set test
10Reserved for data set test
11Unassigned
12Secondary received signal detectorDCE
13Secondary request to sendDTE
14Secondary received dataDCE
15Transmission signal element timingDCE
16Secondary transmitted dataDTE
17Receive signal element timingDCE
18Unassigned
19Secondary clear to sendDCE
20Data terminal readyDTRDTE
21Signal quality detectorDCE
22Ring indicatorDCE
23Data signal rate selectorDCE/DTR
24Transmit signal element timingDTE
25Unassigned

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 NoCircuit nameAbbreviationSource
1Received line signal detect
2Receive dataRXD
3Transmit dataTXD
4Data terminal readyDTR
5Signal ground
6Data set readyDSRDCE
7Request to sendRTS
8Clear to sendCTS
9Ring indicator

RS232 9pin signal definition for the DCE device - looking into the DCE connector
DCE device is often a modem
Pin NoCircuit nameAbbreviationSource
1Received line signal detect
2Transmit dataTXD
3Receive dataRXD
4Data terminal readyDTR
5Signal ground
6Data set readyDSRDCE
7Clear to sendCTS
8Request to sendRTS
9Ring 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.

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