Decoding Pinball Bus Signals Part 2

The connection from the CPU board to the driver board uses 34 pins, in a standard 17×2 format. In the machine, it’s connected together by a short (4” or so) ribbon cable withi IDC (insulation displacement) connectors. If you were around when IDE drives were common, you’ve dealt with these connectors.

Look through my old stuff, I couldn’t find any cables that had 34 pins, but I did find an old 40-pin IDE drive cable with 3 connectors. I taped off one end the connectors, looked carefully at the schematics for the board, and wired up the logic analyzer to the third connector.

Then is was a simple matter to unlock the pin, remove the backglass, pull off the existing connector, and hook up the new cables. Turn on the pin, it boots up fine, and… none of the lights work. turn it off, pull the cable, replug it, turn it on, no lights. Put the factory cable back on, things work fine. My cable, no lights.

Not really sure why it didn’t work, but I was able to get a signal off of the analyzer. Sort of…

Hmm. What do we have?

Well, let’s start with the good news at the bottom. Channels 8 and 9 are the row and column strobes, and they are putting out nice signals, ones that should be easy to grab with the Arduino.

Channels 0-7 are supposed to be the data. What we see are really thin pulses (they do have some width to them, but not much) on some of the channels, and nothing on the rest of them. What’s going on? Well, I’d like to stress that I don’t know. What I do know is that the boards are built using TTL logic, and TTL inputs have a relatively low impedance, which limits the number of inputs you can drive with a single output (this is known as “fan out”). The data lines that I’m trying to read go to multiple boards, so my guess – well, actually, it’s more of a hope than a guess – is that the load of the analyzer plus not having the lines well terminated is causing the issue.

So, it’s off to Vetco to pick up some real 34-pin connectors and ribbon cable. I’ll make up a nice short cable, and we can see how that works with the analyzer.

If that doesn’t work, we’ll try the Arduino directly, which has nice high-impedance inputs, so it should put minimal load on the bus.

So, what do you think ?