DB25 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.

db25 pin d-sub male at the dte (computer) DB25 pin D-SUB male (at the computer)

Name Pin Pin Pin Pin
Jumpering 1 2 3
Jumpering 2 4 5
Jumpering 3 6 8 20 22