Mystery 8085-related IC identification needed please

[Adrian Graham]

On 17/12/2016 14:28, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

> On Sat, Dec 17, 2016 at 2:02 PM, Adrian Graham
> <witchy at binarydinosaurs.co.uk> 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.
 
> 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...

Cheers!

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