Non-standard parallel port on Selectric I/O?

Mike Ross tmfdmike at
Sun Dec 13 05:18:45 CST 2015

I have what was once an IBM 2970 Reservation Terminal. Some time in
the late 1970s. an outfit called 'Western I/O' got hold of a bunch of
these, including mine, ripped out all the IBM electronic guts (but
left their electromechanical bits - solenoids and contacts) and
installed their own boards, and sold them to the home-brew computer
folks of that day - presumably hung off contemporary Altair and Imsai
machines etc.

They made two versions. One was a fancy full-blown terminal with a
6800 microprocessor, adjustable baud rates, standard RS232 port etc.
I'd really like to get hold of one of these if anyone has one
gathering dust btw...

The other one was a very dumb print-only versions. According to
contemporary ads, it has:

"• Printer-only model availability w/parallel ASCII interface."

"The Printer Terminal

IBM Selectrics are known for their well-defined,
high-quality printed characters and easy-to-
change elements. An ideal choice for text
processing, highly-legible source listings and
personal or business correspondence. And it's
easy to connect to home or business computer.

Just plug in 115 Vac, hook up the ASCII printer
port and let 'ergo."

That is ALL the documentation I have on this thing!

The 'parallel ASCII interface' is implemented as a DB25 female on the
rear of the Selectric. It *looks* exactly like a parallel port on a
PC. So I hoped it would use standard parallel port pinouts and a
straight-through ribbon cable would do it. No such luck.

I don't have a parallel port analyzer, but I have lots of serial port
analyzers, including ones with an LED per line, all 25 lines. So that
is an improvised window into what's happening. For starters. when I
operate the 'paper out' switch, I can see a line going high/low
corresponding to switch position - but it's pin 19 on the DB25
connector, which should be ground on a standard parallel port! 'Paper
out' should be on pin 12. So it's clearly not standard...

Now I  can start tracing pins on the interface back to lines on the
PCBs, and try to figure what they do. At least I can relatively
quickly eliminate those that aren't used, or are ground plane.

But, question: back in the day, was there *another* 'standard' for
parallel port pinouts? Used on S100 bus PIO etc. cards, the kind of
thing this product was intended to be used with? I've found Googling
on such data to be remarkably unproductive... any help out there?


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