Big LED Screen

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The Big LED Screen

(I thought there was a page for this big LED sign thing, but I'm not seeing it. Recreating.)

I (User:jbm jbm) spent the evening poking at the LED screen. Time flew, and I need to get to bed, so I've reassembled it and closed it up, so the notes below are a little hazy. Here's where I am so far:


  • Originally controlled by a 386; mobo is shot.
  • The 386 connects via ISA to a "buffer board" which looks to be a memory buffer and power conditioner.
  • The buffer board stores data into a couple memory chips, which are then accessible to the daughterboards which drive the actual LEDs.
  • There are four daughterboards, in two chains of length two. Each of these daughterboards is connected to a single "section" of LEDs (ie: there are four big "sections" of LEDs).


Boring. Broken. Did I mention boring?

Buffer board

Tonight (2008-12-30) I worked from the backend up a bit, but eventually gave up. I then moved to the ISA frontside and worked down, which was far more productive.

The device appears to sit at ISA IO ports 0x180 through 0x183. The addresses are decoded by U51 (74688 comparator), which then hits the OE2 on U52-19 (74541 driver iirc). This is then used to feed U53 and U54 (both 74574 D-flip-flops). These appear to be there to combat fan-out. I'm not entirely certain where these go, but it seemed like they were going into the RAMs.

The low bits of the ISA address selection sit on the rightmost two pins on the top row of the header, SA1 and SA0, in that order (Just hook the connector up and use the multimeter if that's nonsensical). I haven't traced them through yet; I was in the middle of it when my time ran out. They look to run over to the empty CPU(?) slot on the right side of the board.

The output bus to the daughterboards looks to be pretty narrow. I have a hunch that it's serial, which is a bit of a bummer.


I haven't gotten to prying one of these loose yet. It seems... difficult.

  • I once had luck gently prying those daughterboards off of the main screen board. If I recall correctly, I found two UCN5832A (File:Ucn5832.pdf) 32-bit shift registers. This would tend to support your theory of serial running on the output bus onto the daughterboards. -- jof
  • That's totally correct, and I've got the connector pinout, even. The long 10-pin pigtails are as follows:
 1 - UCN-40   CLK (serial clock)
 2 - GND
 3 - UCN-4    STROBE (latch driver)
 4 - GND
 5 - UCN-2    SIN (serial in)
 6 - GND
 7 - UCN-3    GND
 8 - GND
 9 - GND

These daisychain through the chip (ed: it's a nice little driver, I'm rather enamored of it).

Here's my raw notes on signalling, straight out of the notepad:

Looking at the sign doing STROBE: sequences .13ms apart, within each sequence, 5 peaks @+5V, 4us each high, otherwise the signal is low.

Looking at CLK: We do a bunch of lcokign, the strobe, etc. 8 CLKs in 5us, entire process takes 125us, appx 200CLKs. This gives an input rate of 1.6MHz(!)

Experimenting with an Arduino, I get 3 cycles in 20us: 150kHz. Arduino driving sign directly: fail. We'll need a little memory controller here probably.