Mystery IC: Allen Bradley 314B102

Brent Hilpert hilpert at
Wed Dec 16 01:01:26 CST 2015

On 2015-Dec-15, at 6:21 PM, Mike Ross wrote:
> On Wed, Dec 16, 2015 at 11:43 AM, Mike Stein <mhs.stein at> wrote:
> I have taken Brent up on that :-)
> I'll poke a bit more myself and see what we can work out together
> before I decide if the effort is worth it.

First crack can be picked up here:

There are a few areas and pins I couldn't discern from the photos.
Mostly around U1 & U6 as the lens angle and lighting is hiding some connections around those.
If you take another photo or check some of the connections marked in red I can update the schematic.

I've labeled the host interface connections as per the most likely scenario:
	D0-D6: in correspondence to the 'normal' PROM addressing, so D0 is likely the ASCII LSB.
	nSTB: this should be the print-strobe input, looks like active-low.
	BUSY/RDY: haven't examined the logic enough to say whether this active-high or -low for whichever way one chooses to interpret it - BUSY / READY / ACK.

So there are 10 sigs out of the proms heading off to the selectric mechanism for the tilt/rotate / operation select.
But there are a lot of other connections to the mechanism. I've never played with Selectrics internally so I'm not familiar with the details of operation, sequencing, etc.

Note that only half the prom address space is accessed from the 7-bit data, and it looks like the board is set up so if a trace is cut and/or a switch added one could have two character code-sets. Who knows whether the PROMS are programmed for such though.

I threw out a print-only selectric a few years ago that had been used for printing airline tickets. No electronics in it, just the solenoids and contacts for the electrical control. Regret it now, just because it would have been fun to figure it out. C'est la vie.

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