Thread by @TubeTimeUS

look what I got! it's the first high resolution video card for the IBM PC!

handwritten serial number.

it's a rev C, so not particularly rare.

zytrex? weird, they seem to have been a parts distributor in the 80s. my guess is that these parts were remarked by them.

it seems to be missing the character ROM chip.

I'll try borrowing the ROM from this old MDA card.

no.

the address lines on the ROM are changing, chip enable is low, but the outputs are always high. 🤔

something tickled my memory about edge-triggered chip enable signals. sure enough, the IBM MDA ROM needs to see edges on chip enable! (the Hercules card ties this line low)

so let's give the chip enable a signal!

I literally dragged the alligator clip around until I got a display. 🤷‍♂️ the layout of the MDA ROM must be different than the Hercules, but it's basically working!

in MDA, the bottom half of the 7x14 font starts at 0x0800, and the CGA thin font (stored in the same ROM) starts at 0x1000. Hercules seems to map it differently, with the bottom half of the font starting at 0x1000 instead.

made a little adapter

probably won't work

well that was easy

here's the adapter pinout.

(all the other pins go straight through)

it's kinda ugly though, it sticks out really far from the card. since i probably will never find a Hercules character ROM, i think i'll modify the card... 🤔

graphics mode works fine btw.

i've written about the Hercules cards before.

https://twitter.com/TubeTimeUS/status/1207816775624089600

removing the old socket

preparing the new socket. the blue wires will go through the board and solder to the through holes from the back.

you can see the blue wires poking through, before soldering

done!