Mystery 8085-related IC identification needed please

Adrian Graham witchy at
Fri Dec 16 07:43:36 CST 2016

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.

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.


Binary Dinosaurs creator/curator - the UK's biggest private home computer

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