> [...] MicroVAX II [...]

> These machines are the most solid hardware built by anyone, anywhere,
> anytime.  You could drop these things from a third floor window and
> and they'd still run.

Yup.

There's a hardware bug in them; some CPU cards will drop doorbell
interrupts somewhere around a tenth of a percent of the time (IIRC; I
don't recall the exact proportion of the time, but it was very low,
well under 1%, possibly as low as a few per million.)

Why do I mention this?  Well, the problem is that one relevant signal
runs clear across the CPU board.  The capacitance of that etch run is
high enough that it should never have even close to worked.  The only
reason it did is that DEC overdesigned the whole bloody thing by a
factor of about three - the driver capability, combined with the pullup
resistor, makes it marginal.  (There's an ECO for the bug, but you
don't get it unless you specifically ask for it, because damn near
nobody ever uses the doorbell interrupt, and of those that do damn near
nobody cares if one gets missed every once in a while - and some cards
don't suffer from it at all.  The ECO just lowers the pullup resistor,
to reduce the R-C time constant enough for it to work reliably.)

					der Mouse

			       mouse@rodents.montreal.qc.ca
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