BZ vector memory won't shut up

I am totally confused by now. Have a BZ board that reports ram errors, every read 0x000 always came back FF. All data buffers checked out good.

Pulled all the roms thinking that more than one thing might be reporting data back to the bus. Didn't help. Checked the vector memory data buffer LS245 @ F2. !Buffen was low, ok so vector memory is also reporting data, pulled F2 and main memory tests out ok.

So I figure !buffen is broken traced it back to the vector address selector LS157 @ D4. Pin 9 low here. Check its inputs (pins 10, 11), hmm Pull-up resistor on pin 10 is ok. High data there. Pin 11 is the !O2 clock from the micro. This clock is running but !buffen is not changing. Have to replace the 157 and see what happens, unfortunately someone has previously replaced it and done a TOTAL hack job. Going to be a touch of work on that one.

The question I have is how does the micro coordinate the two memory busses? What exactly is the O2 clock from the micro doing in this situation. I can't even start to figure out why you would want the vector generator data to be either set or read on this basis.

I am an empty book, please fill my pages.

David

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Received on Sat May 12 18:56:26 2001