

In a 25-pin serial loopback plug, you need to connect the following
pins:

pin 1 to pin 7
pin 2 to pin 3
pins 4, 5 and 8 together
pins 6, 11, 20 and 22 together
pins 15, 17 and 23 together,
pin 18 to pin 25.

That should do it.  Note that *a lot* of these pins are redundant
in many implementations....

Phil