Help with a busted MSV11-L?
jnc at mercury.lcs.mit.edu
Sun Dec 7 12:49:30 CST 2014
> From: Jon Elson
>> the anomalous voltage I see (2V) is after the output (i.e. to the
>> QBUS) data latches, but before the bus transceiver
> Output from the memory chips? Well, then, you may have to replace them
> all, as one chip may be jamming the bus.
No, "after the output .. data latches", which are between the memory chips
and the bus transceiver.
> Do any of the affected driver chips run hotter than the others? That's
> the "finger test", you replace any anomalously hot chips first
That's a useful idea, thanks, I'll try that.
> The 2V level sounds like maybe NO chip is driving that node. A shorted
> output (or input) would likely pull one way or the other, 2V is about
> the resting point of a TTL input.
Ah, more useful info. The way the two output data laches are wired, the
output control on one is wired with the negation of the output control on the
other, so one or the other ought to be trying to drive the line at all times.
So perhaps the one that _should_ be driving the line normally is the bad one,
then (since it's clearly not...) Thanks for the tip!
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