While legacy PCs (post IBM® PC/AT) typically use 9-pin RS-232 plugs, external modems are usually equipped with 25-pin RS-232 sockets.

This cable features a female DE-9 (DB-9) socket on one end and a male DB-25 plug on the other. The 9 pins of the DE-9 connector are wired to 9 of the pins on the DB-25 connector as shown in the following table:

DE-9 (DB-9) DB-25 Function

1 8 Data Carrier Detect (DCD)

2 3 Receive Data (RD or Rx)

3 2 Transmit Data (TD or Tx)

4 20 Data Terminal Ready (DTR)

5 7 Signal Ground (SGND or Gnd)

6 6 Data Set Read (DSR)

7 4 Request To Send (RTS)

8 5 Clear To Send (CTS)

9 22 Ring Indicator (RI)

The connector heads are molded onto the cable and each includes a pair of thumbscrews to secure the connector to the PC or modem.