PC Serial Port Connections

Until the advent of the IBM/PC most RS-232 connections were by means of a 25 way D-type connector. The PC observed this convention but also gave rise to the alternative, and now popular, 9 way configuration.

The following table details the signals found on both the 9 way and the 25 way variants of the PC's serial ports.

Note: Both connectors are males at the back panel of the PC.

9 pin 25 pin Name Signal Flow at PC
1 8 DCD Data Carrier Detect Input
2 3 RxD Receive Data Input
3 2 TxD Transmit Data Output
4 20 DTR Data Terminal Ready Output
5 7 GND Signal Ground  
6 6 DSR Data Set Ready Input
7 4 RTS Request To Send Output
8 5 CTS Clear To Send Input
9 22 RI Ring Indicator Input


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