Mystery 8085-related IC identification needed please

allison ajp166 at
Fri Dec 16 09:44:57 CST 2016

On 12/16/2016 08:43 AM, Adrian Graham wrote:
> Hi folks,
> I have an 8085-based viewdata telephone system on the bench that's proving
> to be a labour of love in trying to get it running with zero documentation -
> there are only 6 known examples that I've come across and all but 2 of them
> are in museums, none known to work. If any of you fancy searching it's an
> STC Executel 3910 and at least two of the hits you'll get will be my machine
> before I bought it.
> I've been in contact with all museums who have one, no luck on docs though
> the Museum of Computing which is local to me MAY have some (Jason the owner
> has 2 3910s himself), I just need to get down there and search for it :)
> Kind-of-fortunately the viewdata side of things seems standard - all 74LS
> TTL with an MC3242AP running 16x 4116-2 DRAMs, 27128 EEPROMs etc. The
> viewdata side of things is powered by a Plessey MR9735-002 teletext
> processor supported by a pair of 2112 RAM chips and an SAA5070 "LUCY".
> ROM selection is done by a PAL but my EEPROM reader has verified that's OK
> and I have a dump of it as well as all the ROMs.
> Unfortunately it's suffered battery leakage and it was seemingly stored on
> its back in a very damp environment so some of the capacitors at the rear of
> the board have rotted and bits of the analogue board for the 5" TV had
> rusted to nothing, though I've replaced those.
> What I originally thought was an analogue board issue that I posted about
> here has turned into a total lack of timing issue so I've been tracing out
> all the circuits and building a schematic of the board. All was well until I
> came to the two chips you can see in the centre of this pic that have no
> markings (9B and 10B on the board):
> They aren't 'standard' 14-pin DIPs in that they don't follow the
> GND-on-pin-7-Vcc-on-pin-14 layout. Pins 6 and 7 on both are wired together
> (not to GND) and form the RESET signal for the 8085 via the 7414 at 10A,
> source for this signal is unknown currently. Pin 5 on both appears to be Vcc
> and pin 10 is GND or at least are pulled high and low respectively.
> The XTAL you can see with its supporting resistors is connected to the 7404
> at 12B, pin 12 of which goes to pin 1 of 10B (one of the mystery chips) and
> pin 1 (X1) of the 8085.

IF I had to throw a wild guess either a baud rate generator chip, an
N-stage counter
like CD4040 or 4060 or one of many low number cmos (CD4xxx parts) wired
as oscillator.
most of those had non TTL style pin outs.


> Any clues? I'm going to search for reference 8085 boards (and I guess 8086?)
> layouts to see if there are any similarities in timing circuits but for now
> I'm stumped.
> Cheers!

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