Playchoice coundown timer

[stratjakt]

I don't think I'll ever find the countdown display for my PC10, but I think it should be possible to build one.

So far my search for a schematic of the original has been fruitless, but I'm sure someone has a single monitor PC10 (standup, countertop, conversion) out there who can take a really close up photo of it.. 

Even if not, I'm sure it can be figured out from the block diagram.  Hopefully some of you geniouses can help me figure this out, and I can build a working replacement, and give the schematics back to the community and all that..

Anyhow, the block diagram shows the following lines going to the display board:

DD3, DD2, DD1, DD0
AD4, AD1, AD0
~IOWR (~ = not)
5V, GND

So, what I'm guessing is, AD4, AD1, and AD0 address one of the display elements (not sure why its 4, 1, 0 - misprint?), at which time IOWR is taken low to activate it (IO Write?), an number (in binary) from 0 to 9 is sent via DDx, and then IOWR goes high again, and the display is latched..  This would happen perhaps 4 times per second (one for each display), or maybe the mobo is sophisticated enough to only modify the displays that need it.

It's a 4 digit display that displays 0000 thru 9999.  The AD line scheme sort of confuses me.  I'd expect either 4 lines (to directly select a display), or two lines (to select it via binary).  Perhaps AD0 and AD1 select the display, and AD4 is some sort of a control line - an "address ready?"

It would be easier to figure out if I had a schematic of the single monitor motherboard, but I can only find a full schematic of the dual-monitor motherboard online, which doesn't have this half of the circuit.

I envision a relatively simple circuit, though, with 4 7-segment drivers (7447 if i remember), a 16-bit latch, or maybe 4 4-bits..  I guess I'll start doodling out a schematic.

Bottom line, a really good legible photo of the beast in question would make this project a snap..  I dont want to leave it in freeplay mode, it just sits at the game select screen burning the crap out of the monitor..