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. > > http://www.binarydinosaurs.co.uk/stcexecutel01.jpg > > 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". > > http://txlib.mb21.co.uk/main.php?g2_view=core.DownloadItem&g2_itemId=2034 > > 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): > > http://www.binarydinosaurs.co.uk/stcexecuteltimingcircuits.jpg > > 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. Allison > 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! >
