Re: 7854 Bad display board


Dan G
 

On Mon, May 18, 2020 at 08:03 PM, Paul wrote:

I was clocking on the 3rd or 4th phase, as those are what's on the edge
connector
Hi Paul,

I recommend taking a look at the "TMS 9900 Memory Bus Timing"
diagram in the TMS 9900 Microprocessor Data Manual, if you
haven't seen it already. It should give you an idea of what CPU signals
you need to acquire with your LA, and how to interpret them.

I don't have the 338 LA, but it appears to have 32 channels, based on
its User's Manual. For initial diagnostics, I think you only need to monitor
the 15 address lines, plus _MEMEN, IAQ, WAIT, HOLDA, DBIN, _WE,
READY and _RESET. You can use whatever channels remain to get a
partial view of the data lines. While not perfect, that should be sufficient
for this task.

BTW, is there a program listing or disassembly somewhere that you were
referring to, or are you just intimately familiar with this beast?
There is no published firmware source code that I am aware of. I have
a side pet project that may change this in the future, but there is no
timeline yet. For now, let's just say that I happen to be familiar with portions
of the 7854 firmware that your scope does not seem to be able to get through.


dan

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