Thread by @TubeTimeUS

here's a very weird IBM PC video card. you've probably never seen it before! there's a lot of weirdness here... 🧵

it's the video card from the IBM 3270 PC (the 5271).

this is a PC configured with up to 5 (!) specialized ISA cards that allowed it to emulate an IBM 3270 terminal.

there's a keyboard controller card that needs a special Y splitter dongle. the 3270 keyboard plugs into this dongle, and the other cable on the dongle makes a U turn and plugs into the XT keyboard port!

then there's the "host attach" card which is where you plug in the cable that runs off to the remote mainframe (more likely the terminal controller)

finally, there is the video card, which is the only card of the set that i have! the video card can be expanded with an APA (all points addressable) card for advanced graphics, and a programmed symbols card that allows you to customize the character set.

unfortunately for me, i can't use the video card by itself because it requires the ROM on the *keyboard card* to initialize it!

see, this card has a special 3270 graphics overlay that sits *on top* of the standard emulated CGA mode that it supports.

but the motherboard BIOS doesn't know how to initialize all that special hardware! the option ROM on the keyboard card is supposed to do this.

"easy" you say, "just put in a second video card, like VGA or MDA!" the problem is that the 3270 card uses up *all* the memory addresses that would be used as the frame buffers for these cards! it won't work.

so i power up, i get the beep code for a video error, and then...i can't use it. no display.

fortunately, i have a plan.

here's my innocent little XT-IDE card. it has a ROM socket on there and some nice jumpers that let you configure it to almost any expansion ROM address!

including 0xC0000, which is where the code in the keyboard ROM expects to find itself.

i also found a dump of the keyboard ROM itself and loaded it into Ghidra out of curiosity.

looks like the first thing it does is initialize all the specialized video hardware. this looks very promising.

the pinout isn't documented so this took some experimentation. do you like my adapter? 😂

oh my, it generates video now! the error code is "3250 3270 Display Adapter or Top Card Connector" probably because it can't find the actual keyboard card. hahaha

then it started scrolling a bunch of junk. well, at least it works.

(quick aside: this card is really huge--it is full length ISA! here it is ensconced in my XT.)

so what next? well, i'll try to get my hands on a keyboard card. i could also try patching the ROM to skip over the stuff i don't care about. to be continued...

before i forget, here's the video pinout:
1, 2, 3: ground
4: monitor select: ground=color, float=mono
5, 6, 7: RGB (color mode)
6: intensity, 7: video (mono mode)
8: hsync
9: vsync

video specs that i measured:
color mode: hsync = 24KHz negative, vsync = 63Hz negative
mono mode: hsync = 18.6KHz positive, vsync = 50Hz negative

from seasip.info/VintagePC/5271…, looks like this ROM chip is mapped into two different places in memory. i suppose i could split the ROM and put it in two different XT-IDE cards. 🤔