Mystery 8085-related IC identification needed please

[Adrian Graham]

On 21/12/2016 19:37, "Brent Hilpert" <hilpert at cs.ubc.ca> wrote:

> On 2016-Dec-17, at 10:34 AM, Adrian Graham wrote:
>> 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...
> 
> The counters may be the beginning of a video timing divider chain or a clock
> divider for the LUCY chip.
> (e.g. a diagram in the document you mentioned shows a div-6 between a 6MHz
> crystal osc and the LUCY chip.
> The division factor may vary of course in the executel depending on the
> crystal.)

That's a good point so I've revisited that section of board - one of the
things that started me down this whole path was a missing video sync before
I realised that the CPU wasn't running because of RESET and have spent many
days since tracing and measuring trying to find the source. I'm pretty sure
the LUCY chip is only there as a modem since the teletext display side of
things is handled by a Plessey MR9735 and a pair of 2114 RAM chips as page
store.

The board's also fairly logically laid out, split 50/50 between phone and
teletext with the teletext side split between CPU/glue, ROMs, RAM and
display logic. LUCY is well over to the left on the phone side.

> Also to keep in mind as you trace it out, 749x counters were often used with
> gates between the outputs and reset inputs to change the count modulus /
> division factor.

Hopefully there's nothing complex here, the reset inputs are inverted via a
74LS14, input to that is from the CMOS MC14081B mentioned elsewhere through
two hops over pins 1-2 then 3-4 of a 74LS04N.

> 
> 
> On 2016-Dec-17, at 10:34 AM, Adrian Graham wrote:
>> 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.
> 
> 
> Looking at the photo in your earlier message
> http://www.binarydinosaurs.co.uk/stcexecuteltimingcircuits.jpg
> the wide red band on the resistors would be a 2% tolerance indicator.
> 
> As with Dwight's suggestion about colour confusion, if the resistors in
> question are those to the left of the two unmarked chips (or similar), they
> look like brown-black-black-red-wideRed, which would be standard 10K / 2%.

Those are pullups on an output from one of the '92s over to one of the 3
light green telephony bits that I've not looked up, marked 'NMC1515/6/7'

> You can try measuring resistors in circuits like this with a DMM.
> Switch the DMM leads to get both polarities through the resistor.
> It's not guaranteed as it depends on the connected circuitry and the R value,
> but one of the readings will often be your R value, or may confirm or give
> direction as to the value.

I normally pull up one end and test out of circuit but I'll give that a go
too.

Cheers!

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