Mystery 8085-related IC identification needed please

Tony Duell ard.p850ug1 at
Sat Dec 17 08:28:05 CST 2016

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 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.

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


> Back to the DMM then!

Given that these counters seem to be reset by the same signal that
resets the 8085, and that I assume the 8085 is not reset very often,
the counters will presumably count continuously. If you have a 'scope,
look at the input signals to the 2 sections of the counter. If you see
nice regular square waves there, then look at the outputs and see if
the frequencies fit the right division ratios for one of the 749x devices.

The ideal tool for this is a LogicDart which will display the waveform
and tell you the frequency (and will drop in your toolbox), but those
are not that easy to get now.


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