Asteroids MPU troubleshooting

Whew! Well, now that that's over, and we're back on-line, let's power up
the engines and get back in orbit.

ASTEROIDS!

I still need help, and I am willing to offer up some tidbits of recently
gained knowledge to the database for troubleshooting these boards.

1) grounding the /VW line (pin 7 of the '157 at K2), or other RAM address
select lines, will cause the MPU to generate the beeps that indicate RAM
failure for one of the RAMs (if it hasn't already). This is a good way to
tell how far your board has come up. If it CAN do the beeps to tell you it
thinks it has a bad RAM (which you just caused by momentarily grounding
that pin), it is pretty far along. The ROMs are probably good, the RAMs
are probably good, and the sound is good (well, OK, that last one is a bit
obvious). At this point if you still have the watchdog reset problem, the
cause is beyond this. (and I'm still trying to figure that out).
Presumably it is in the vector generator section of the board.

2) Grounding the HALT line (pin 2 at L10) will cause the MPU to indicate
watchdog reset problems in a good board. This indicates that the CPU is
looking for that HALT line to continue operation, and if it doesn't get it,
it doesn't execute that next section of code that will output the watchdog
timer reset. So... a bad vector state machine circuit will freak the MPU
into watchdog reset.

That's where I am now. I now know I have a bad vector drawing circuit, but
I don't know where the problem is, all the signals I have seen look good,
even the watchdog reset, though it is obvious now that the TIMING of the
signal is too far apart to stop the clock before the reset is generated.
Again, I am looking for input from people on known problem chips, possible
points on the board to probe for data, anything to keep quantifying the
operation of this board.

Thanks guys.

Ace

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Received on Thu Jun 18 14:50:23 1998