Mystery 8085-related IC identification needed please

Tony Duell ard.p850ug1 at
Sat Dec 17 12:49:16 CST 2016

On Sat, Dec 17, 2016 at 6:34 PM, Adrian Graham
<witchy at> wrote:
> On 17/12/2016 14:28, "Tony Duell" <ard.p850ug1 at> wrote:
>> On Sat, Dec 17, 2016 at 2:02 PM, Adrian Graham
>> <witchy at> wrote:
>>> Thanks all, the pinouts are matching the LS9x counters so I just need to
>>> trace more lines to hopefully narrow it down. Pins 6 and 7 are definitely
>>> inputs so you're right Tony, the reset must come from elsewhere. One of the
>>> outputs is confusing though since it appears to come FROM 5V via a resistor,
>> 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 is possible that whatever it is driving needs a swing to +5V,
>> rather than just
>> a TTL high level. Adding a pull-up resistor is a way to kludge this.
> I thought that but it doesn't appear to go anywhere else. I'll keep looking,
> its only a 2 layer board so there's nothing hidden.
>> The 7490 has 2 sets of reset inputs (4 pins total), one pair to reset
>> it to 0, the
>> other pair to reset it to 9. Since these inputs are active high, they
>> can't be left
>> floating, they must be conneccted to something (even if directly to ground).
>> So
>> you could see if those pins go anywhwere, if not, then you can eliminate the
>> 7490
> One problem I have is that I've already found a few chips with dead outputs
> so I've no idea if these will be any different. The pinouts I have match the
> 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.


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