No VMA from 6808

Still working on the dead-dead Monday Night Football pinball that I’ve asked for and received help on this forum before.

I’ve faked the missing power-on reset generator IC by bringing the RESET! low. Unfortunately for some reason I’ve also had to bring the NMI! low afterwards to get the CPU to start.

Since using Leon’s Test EPROM doesn’t produce the results programmed I’ve booted the CPU without any EPROM in the first address. This make the CPU hunt for an answering address and allows checking the address and data buses.

In any case, with or without an EPROM in the first socket I get no VMA signal. I’ve resocketed the CPU with the VMA Pin 5 unconnected to anything just to make sure the two AND gates aren’t dragging it down.

Still no VMA signal. What would cause the 6808 not to produce it?

Yes, brand new 6808, twice in case of a bad run; RAM doesn’t matter with no program; have the 4Mcps clock; IRQ is pulsing;E is going out; R/W is going out.

Data Sheets and circuit schematics can be found here.

Your link is broken.

Hi. Have you been able to scope out a functioning 6808 system? I ask in case the actual behaviour is a bit funky compared to the spec.

I’ve got a couple of arcade boards with 6808 I think and I can slap on my bus/logic analyser and watch the state of VMA on CPU init if you like?


Sorry 'bout that.
Fixed Link.

I’m going to take another look at a functioning Data East 6808 on Monday. I looked at the CPU signals with the Test EPROM so I’m going to record the signal with no program.

Pull and check the resistance of each pin of RA5 to common (usually one of the end pins; often this is marked with a dot or other indicator). One of the resistors in the array may have shorted and is pulling VMA low.

I checked for resistance to ground but I’ll try this.