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.

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