DB9 serial (RS232) loopback. Pinout and signals for building a serial port loopback (CheckIt)

The PC loopback plug is a useful diagnostic tool. The loopback plug connects serial inputs to serial outputs so that the port may be tested. There is more than one way to wire up a loopback plug - but this is the most common. Used to verify that a port is working. This one works with CheckIt.

DB9 pin D-SUB male (at the computer)

Name	Pin	Pin	Name
CD	1	6	DSR
CD	1	9	RI
RXD	2	3	TXD
DTR	4	6	DSR
RTS	7	8	CTS