here's a very obscure IBM PC graphics card! it's the video card for the 3270 PC. the smaller card is the keyboard interface, which also contains the video BIOS. let's see if it works.
it's coming up in monochrome mode. I'm not sure how the card is supposed to detect a color monitor.
the card I have is just for text modes. another card allowed for custom fonts (programmed symbol card) and a third card enabled a graphics mode (APA).
much better. I just had to tie pin 4 to ground.
1-3: and
4: monitor select
5: green
6: blue
7: red
8: hsync (23KHz)
9: vsync
the font looks... strange.
it's super hard to photograph this scan frequency. I think it's too close to my camera's frame rate
here's the 3270 native character set. there are some funky symbols there!
heh, the 3270 display buffer acts like an overlay.
trying out the keyboard cable I built. there's an MCU on board to convert the AT keyboard signals to the PC/XT protocol. however I'm getting error 302 on boot. out comes the logic analyzer.
looks like it initializes the AT keyboard correctly.
but then there are some suspicious protocol errors. not sure what is going on there.
then when i press a key, it *almost* works. the upper two traces means that the MCU is prematurely sending a scan code to the XT motherboard port even though only 4 bits of the scan code have been received from the AT keyboard! clearly something is not right.
and the scancode getting sent to the motherboard is 0xF0. that doesn't sound right either.
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