DB25 serial (RS232) loopback. Pinout and signals for building a serial port
loopback (Norton)
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 Norton Utilities: Norton Diagnostics from Symantec.