I also found this document while looking for accelerator cards for the trs-80 computers and found it may contain some information you may also need.
About incompatibilites with Tandy printers interface
We hear all kinds of horror stories about Tandy printers
not working with generic IBM/Epson cables. The problem is that
the Tandy version of the Centronics printer port and the IBM
version are not wired identically. Some of the signals are on
different pins, so each printer needs its own cable. My
experience from a few months back should explain this.
I bought a 26-227C Tandy cable last night, and it works
fine with the Sharp Laptop and the DMP132. The IBM Parallel
generic cable I had bought from Egghead didn't work. So I got
out the continuity tester and went at it. Some people have
suggested mismatched capacitance (give me a break!) and other
magic reasons for the problem, but the plain fact is that the
Tandy and IBM/Epson ports are wired differently, and need
different cables as follows:
Look at pins 13, 14, and 16, from the 25 pin side, and
you'll find the culprits. These three signals go to different
places in the 36 pin connector. All of the signals that are
tied together in the IBM cable are grounds, and the grounds in
the Tandy cable are separate, so that grounding takes place
after the cable is hooked up. But pin 33 in the Tandy printers
is the initialize printer signal, and with the IBM cable, it's
hooked to ground, and knocks the printer off line.
Chances are that swapping pins 31 and 33 (at the printer
end of the cable), plus disconnecting pin 14 will probably do
the job. If necessary, also swap pins 13 & 18. It's probably
less bother just to buy a Tandy cable.
So, it would seem that any printer cable is peculiar to
its printer, and should be treated as a matched set. By the
way, this newest Tandy cable has thumb screws too.
