Slothie -yes you are correct the one I linked to above and another I found online have patch wires, close up here of both for convenience:
One seems to roughly follow the VGA layout the other looks like it is almost the same patch pattern but, totally reversed! the one that seems to be following the VDU for direct connect also has some resistors patching something. Would be nice to work out what is going on there.
https://www.eevblog.com/forum/projec...4-restoration/
I should have credited this link as the source of the VDU board I posted with the edge connector on it. I have just noticed looking at it again I think it carries out the trick I am planning. As you say XOUT should be on the bottom - I suggest on the pin below no A27 as that is unused so would be easy to bend back the pin on the connector and then just bridge the pads...
I can just do a similar thing with B15 the Reverse Pages (input) pin - bend the pin up and patch it over to B8 which will mean it appears on the IO Chip A3 line... it even has a neat via I can pick it from or the hole itself...
We just need to work out what the default effect is on all those ports at reset... Maybe one or two need inversion to ensure display is on or off etc...
The only public software I know of for the VDU is the falling man animation in Dec 1979 PE - that does not seem to write to the I/O chip so doing this is unlikely to break major software...
https://www.americanradiohistory.com...9-12-S-OCR.pdf
SH- you may like the submitted subroutine handler in there as well if you haven't seen it - clever bit of code for larger programs... The offset calculator is also better than the one in SCIOS as it patches the whole program.
The other circuit I want to try and may interest others is in the 1980 August Issue of Practical Electronics which is a circuit by Anthony D. Love of Swansea that uses a 74LS32 to allow Bit 6 of the character code to invert individual characters to Black on white! Page 48
https://www.americanradiohistory.com...0-08-S-OCR.pdf
As regards the Memory - there are some notes posted somewhere (by SH I think) on the gates they used on the Issue V to remove the multiple PROM images so it is possible with only what is on the board.
The would mean that the additional chip you add would be for the alternate memory option - this should also mean only that change has a routing difference to the real MK15 Issue V. UTsource have AM2111 at 5.95 each at the moment - they do not show stock levels but seemed happy when I put four in my basket - delivery is slow or steep though...
SH - thanks for doing all that testing... - at least you have a sketch (it was an arduino you used?) now you can share for people to test a VDU board
I have received my two MM80C95N as well which I will try in the VDU when I build it and get it working. One of them certainly works on the keypad IC11 i/f which we are discussing in the other thread. I didn't think as a CMOS part it would have enough fan out but, maybe the other chips are not being driven as it is a bus... so they need the EN or CS asserted for that. I will study a bit more the VDU schematic how they are used there, as this is at the limits of my electronics knowledge...