The serial port on IBM compatible PCs conform to the RS232 electrical standard and use a male DB-9 connector. (older PCs may use a DB-25 in which case a converter will be required). The DB-9 connector as the following 9 pins.
DB9 Pin |
EIA Description |
Description |
1 |
DCD |
Data carrier detect |
2 |
RXD |
Receive data |
3 |
TXD |
Transmit data |
4 |
DTR |
Data terminal ready |
5 |
GND |
Signal ground |
6 |
DSR |
Data set ready |
7 |
RTS |
Request to send |
8 |
CTS |
Clear to send |
9 |
RI |
Ring Indicator |
To make the loop back plug the following pins need to be connected together:
▪Receive and transmit (2 & 3)
▪Request to send and Clear to send (7 & 8)
▪Data carrier detect, Data set ready and Data terminal ready (1, 6 & 4)
This diagram shows the connections that need to be made. It's the rear view of the female DB-9 connector that's required for the plug. The red lines and gray dots show the connections that need to be made on the back of the plug.
Female DB-9 connector - Rear view