Mystery 8085-related IC identification needed please

[Adrian Graham]

On 17/12/2016 18:49, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

>>> What value resistor?
>> 
>> It's a 5-band red-red-black-black-violet so either 220R or 70k? Based on
>> what Pete said about the Z80 I'm going for 220R without pulling it out of
>> circuit.
>> 
> 
> Yes. I would agree with that.

It connects to some telephone-based doohickey that can be seen as the
right-most light-green module lower centre left of this pic, the one with
the red capacitor in the middle of it:

http://www.binarydinosaurs.co.uk/stcexecutelboard.jpg

Initially I figured the board was divided half and half into phone and
digital viewdata with the viewdata part on the right. Now I'm tracing more
lines that blur between the pair of them right down the centre, including
the source of the 8085's RESET which would seem to be the circuitry in the
top-middle of that pic where you can see the battery (now removed).

The chip directly below the battery is the RTC, to its left is an MC14081
(quad input AND) and left again is an ICL7611 op-amp. Those 3 have their own
5V supply that's separate from the rest of the board, yay. Not found the
source of that yet.
 
>> LS92 since pins 2/3/4/13 are NC. All testing so far has been done with a DMM
>> and cheap logic analyser. Since one of the possibly-LS92s is out of circuit
>> I'll build a little test circuit to see if it does actually count given a
>> clock source...
> 
> 
> It's not a 7493 as the resets don't match up. I would guess at 74x92 at least
> for the moment. See if the rest of the circuit makes any sense.

I'm 99% sure they're 7492s now, outputs and inputs match other surrounding
chips.

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?