Mystery IC: Allen Bradley 314B102

Mike Ross tmfdmike at
Mon Dec 14 16:02:06 CST 2015

On Tue, Dec 15, 2015 at 4:59 AM, tony duell <ard at> wrote:
> Some more random thoughts....
>> Of the above 10 pins, 11, 13, 22 & 23 are high at power-up (printer
>> NOT connected to any interface). The only pin with known function is
>> 19, which is 'paper out'; if I toggle the paper out switch I can see
>> it going high and low.
> Well, 19 could be a general printer-ready pin in that if the paper runs
> out it would say the printer is not ready for another character but
> it might well also be put to the not-ready state when the printer
> was printing the current character. Seen that before.

Maybe, but Selectrics aren't exactly fast devices; there's a whole lot
of potential 'no, wait, I'm not ready!' conditions. Would they all be
ORed onto one pin?

> Also, having now looked at the photos (sorry I was rushed before) I
> am pretty sure that Allen Bradley package is a pull-up resistor
> array. Half the pins seem to go to the +5V trace. The others go
> to signals

Confirmed off-list; it's a resistor network; 14 pins, seven resistors.

> Remember that a pulled-up logic input will test as a high level. So some
> or all of the 'high' pins might be inputs.

True and possible. But even if they're ALL inputs, only seven of them?

> I notice the 3 chips with the Pn lables. I think I can make out a Harris
> logo on one of them. I would guess these are programmed PROMs
> to convert between ASCII (I hope) and the solenoid codes for the
> Selectric.

That was my conclusion too. The old Western I/O ads I've seen
definitely refer to it as having an 'ASCII' or 'parallel' interface.
Assuming they only ever made the two models; I suppose it *could* be
some variant of RS232, with very non-standard pinouts - but the ads
are specific; they made a smart terminal with 6800 CPU & serial
interface, and a dumb printer with an 'ASCII parallel' interface. And
that's all I have to go on, beyond prodding the hardware.

> The next thing of interest to me is the pair of 7475 latches at the bottom
> of the board. 4 bits each. Maybe hold the 8 bit input character. I would
> trace where the D's and Q's of those go first.
> Perhaps you load the character a nybble at a time???

That would be well weird. Still trying to work out what exactly it was
intended to hook up to; a standard parallel port with a special cable,
using only four of the data lines, and a driver, to drive it
nybble-wise? Maybe I could use business records to try to track down
the former owner of 'Western I/O' and just ASK him, if he's still
alive, but that would be cheating :-)

I've gone over the connector again and we have ten signal pins plus a
ground plane... that's *just* enough for 8 data bits, a strobe, a
ready/wait line... but that Allen Bradley pull-up pack is only 14
leads, 7 lines... I'd expect to see 8 data lines all going to the same
place if it was anything resembling standard Centronics but with a
weird pinout. So I'm scratching my head still over just exactly what
it was supposed to hook up to.

> This board does not look that complicated and all the ICs have known
> numbers on them (mostly TTL logic). If it were mine I'd trace out the schematic.

That's true and possible. I'm in two minds on this thing:

- intention was to rip all this out and convert it to a full I/O
serial terminal, using an Arduino-based setup that Lawrence Wilkinson
has already built and tested:
- all the keyboard contacts are already in there, Western I/O just cut
the IBM wires off when they ripped the IBM guts out and converted it
printer-only. I'd like to figure out the interface that's presently in
it, just to check out the mechanism, and for that 'ah ha!' moment :)
- but I don't want to spend any significant time on it if I'm just
going to rip it all out.

- but, although the Western I/O conversion 'butchered' a perfectly
good IBM 2970, it IS a rare representative of that era, when all kinds
of Selectric conversions were commonplace. So perhaps, as a nod to
that era, it should be left as-is, as a preserved example? What say
people? I've seen posts on old lists where people have referred to
buying these back in the day - converted Selectrics I mean - and
seeing 'mountains' of them in warehouses. They were once common. Where
have they all gone? Is mine the *only* survivor from those mountains
of 3rd-party backstreet conversions? Does anyone else have any?

I've just spent a few hundred bucks with one of the few mechanical
Selectric gurus left standing - a local guy here in NZ who did an
amazing job, several broken and seized bits fixed, the mechanism is
now like new and works perfectly in typewriter mode - so it's going to
end up working, one way or another!

'No greater love hath a man than he lay down his life for his brother.
Not for millions, not for glory, not for fame.
For one person, in the dark, where no one will ever know or see.'

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