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Couple of ZX81 Interfaces / Boards

Posted: Mon Nov 03, 2014 8:21 pm
by RWAP
Is anyone interested in the following - available for £4 each plus postage - they are untested and just the bare boards:

a) Telex Decoder (voor ZX-81 ELRA)
b) Some sort of i/o board / test board - the main interface is marked OPTREK-R'S - the two connectors at the top are marked IN and UIT. This is then connected to a Maplin Extendiboard GB08J - although a warning only one of the sockets has a key!

Re: Couple of ZX81 Interfaces / Boards

Posted: Mon Nov 03, 2014 11:07 pm
by 1024MAK
RWAP wrote:Is anyone interested in the following - available for £4 each plus postage - they are untested and just the bare boards:

a) Telex Decoder (voor ZX-81 ELRA)
b) Some sort of i/o board / test board - the main interface is marked OPTREK-R'S - the two connectors at the top are marked IN and UIT. This is then connected to a Maplin Extendiboard GB08J - although a warning only one of the sockets has a key!
Hi Rich, the answer is yes. I will send a PM...

Mark

Re: Couple of ZX81 Interfaces / Boards

Posted: Sat Nov 08, 2014 1:43 am
by 1024MAK
Got one of them working. The one with the OPTREK-R'S wording.

I think this is a Dutch produced board. "OPTREK" I think means "pull-up", R'S means resistors. "UIT" means out. At least that's what a translator site tell me :mrgreen:

After looking at the tracks, I worked out that the chips used were one 74LS00 (quad 2-input NAND gates), one 74LS02 (quad 2-input NOR gates), one 74LS244 tri-state octal buffer and one 74LS273 clocked latch with reset. I had the first three in a surplus board that I had to hand. The surplus board has some 74LS374 latches though. Luckily the pin-out is very similar, there only being one difference - pin 1 is a /OE (output enable) instead of a /MR (master reset). So with a minor modification (connecting pin 1 to 0V/GND by fitting a link in place of the capacitor marked "C") the 74LS374 latch should work. I also fitted a 6.8k SIL resistor pack.

Now, what port or memory addressing does it use?

Well, as I traced the tracks, I drew a schematic diagram. Only address lines A12, A13, A14 and A15 are used. /MREQ is used but not /IORQ. So it will respond to memory addresses between F000 hex and FFFF hex (or 61440 to 65535). But there is no connection to /RAMCS to disable the ZX81 internal RAM ghost that will be present at these addresses. So I added a BC549 transistor and 1k resistor to take care of the requirement to pull /RAMCS high when this input/output board is accessed. I also added some decoupling capacitors and a 4 pin header/connector to provide access to the +5V and 0V/GND supplies. I also removed the soldered on ribbon cable to the Maplin board.

This board provides eight digital "TTL" outputs and eight digital "TTL" inputs.

As you can see in the pictures, the outputs work :D. I also tested the inputs and am happy to report that they work as well :D.
photo 2_45.JPG
photo 2_45.JPG (88.97 KiB) Viewed 3294 times
photo 3_50.JPG
photo 3_50.JPG (101.11 KiB) Viewed 3294 times
photo 1_45.JPG
photo 1_45.JPG (94.19 KiB) Viewed 3294 times
Mark

Re: Couple of ZX81 Interfaces / Boards

Posted: Sat Nov 08, 2014 9:41 am
by sirmorris
Image Marvellous work! Image

Re: Couple of ZX81 Interfaces / Boards

Posted: Sat Nov 08, 2014 1:32 pm
by RWAP
Excellent - I am glad you got it working - maybe you could publish the circuit diagram / PCB layout for others if they want to build something similar :D

Re: Couple of ZX81 Interfaces / Boards

Posted: Sat Nov 08, 2014 2:28 pm
by 1024MAK
RWAP wrote:Excellent - I am glad you got it working - maybe you could publish the circuit diagram / PCB layout for others if they want to build something similar :D
Watch this space for the schematic... (Will do a CAD version instead of my scribbled version ;) ). I will post some more pictures of the board including the track side.

Mark

Re: Couple of ZX81 Interfaces / Boards

Posted: Sat Nov 08, 2014 5:51 pm
by PokeMon
Congrats you made it work !
8-)

Re: Couple of ZX81 Interfaces / Boards

Posted: Sun Nov 09, 2014 1:05 am
by 1024MAK
Okay, here is the schematic:
ZX81 input_output Board_e01_75.png
(196.45 KiB) Downloaded 1017 times
I used 74LSxxx series chips, not 74xxx series as shown on the schematic.

Mark

Re: Couple of ZX81 Interfaces / Boards

Posted: Thu Nov 20, 2014 1:21 pm
by 1024MAK
Photos of the top side and the track side attached.

Mark

Re: Couple of ZX81 Interfaces / Boards

Posted: Sun Apr 19, 2015 11:23 pm
by 1024MAK
See also this thread ;)

Mark