Reminds me of
something I did with a TeleVideo 955 once. Turns out
that model has a custom-firmware ROM socket, with an escape sequence
in the stock firmware to execute it. I disassembled enough of the
various pieces to work out much of the interface to the hardware,
and wrote a small video game to run in the custom firmware ROM as a
proof-of-concept exercise. [...]
What an awesome hardware hack for a terminal!
Thank you!
I actually did a lot more. The thing has three socketed ROMs. I
pulled all three and dumped them (at the time I had access to a PROM
burner station at work). It was instantly clear that one was a
character generator and nothing else - the bitmaps were obvious the
moment I looked at the dump in binary. I promptly burned a variation
on that one and as a result now have a tvi955 with an ISO-8859-1 font,
something that may well not exist anywhere else in the world by this
point. :) (The stock font is close but not quite 8859-1, differing in
maybe a dozen characters; a friend of mine who knows fonts looked at it
and says it's probably an early draft of what eventually became
8859-1.)
I also dug out a breadboard and some RAM and a bunch of jumper wires,
and brought out that socket's pins to the breadboard, where I made them
access the RAM instead. I added additional logic so I could write to
the RAM via a serial line, and presto! I had a tvi955 with downloadable
font glyphs! It was a cool hack, but has long since been torn down.
Because the flying wires introduced noise, the per-bit error rate on
reads was very high (for memory), probably 1%-5%. For the character
generator ROM this just produced snow on the screen, but for firmware
it would of course have been catastrophic, so I didn't try to give it
downloadable firmware.
Do you still have it?!?
Yes, though it's been years since I so much as turned it on.
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