Hello. I am a huge fan / lover / geek for LED signs/screens and have some
experience with them. One day I hope to have one of the giant modular
systems like they use behind stages, currently my full color rig is only 3
meters wide by 1 meter high (but can eat 12 amps full white full bright!)
(More reply below)
On boot it's supposed to say "UNIT OPERATIONAL" on the display before
plugging in the keyboard ribbon cable. There is garbage displayed instead.
When the keyboard is plugged in, the display responds to start/stop scroll
(green and red keys), and messages can be input. The memory (yellow key) and
control (blue key) functions don't seem to work.
There actually is a microprocessor (8048 or 8051), but inside the keyboard
only.
Another observation: The display only scrolls a couple of characters, jumps
back to the beginning and repeats forever.
But the actual display board has a 24-pin Harris PROM (fuse link), seven
2102's 1K memory for the 7 rows of LEDs and many 7400 TTL including 4-bit
counters, comparators and adders. No CPU at all, so it must be a state
machine, and a fairly complex one at that unless some of the "smarts" are
actually in the keyboard microprocessor. The cable from the keyboard connects
directly to the PROM without any buffering.
I have found that only the lowest 4 bits of the memory address lines have
activity on them, which explains why two characters scroll before it repeats
(16 columns). I can't find anything strange looking (e.g. non-TTL levels) on
the middle and upper nibbles except that they don't move.
There does not seem to be any information on this unit online. I'd like to
find a repair manual (probably too long a long shot), but even a schematic
would save a lot of hair pulling!
Most of the units I have toyed around inside of -- the screen operates
with rows and columns. The text is "scanned" on the LEDs using
shift registers to drive the columns, and then something to trigger the
rows (there are a lot less rows than columns.) I guess it scans it out of
the RAM ICs, but that means the pendant would most likely do the "font
generation" into the RAM?
Do you think the PROM is actually a table that is the state machine? Where
they needed a repeating pattern of bits so it's blown into the PROM then
the address lines are stepped through producing the needed data bits to
drive it?
"Unit operational" would have to be stored in the prom....
Could dump the prom and look for the data. There is a utility used for
video game hacking, a Java app called Tile Molestor that will graphically
render data from a bin file and let you change color depths and offsets to
see patterns.
- Ethan
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: Ethan O'Toole
