25 Nov
2015
25 Nov
'15
4:18 p.m.
From: Robert Jarratt
When the power is OK the output of the inverters is
low, so the
transistors are off, presumably allowing the signals to float high.
When the power is not OK, the inverters are high, turning on the
transistors and shorting the signal to ground.
That is correct.
One thing to watch for: in a machine which does not have bus pullups on the
backplane (some do, but many, especially early DEC ones, do not), if you run
it without a CPU card plugged in (e.g. to test the power supply), BPOK/BDCOK
will be at 0V even if the power is OK because there's no pullup to pull it
high (unless driven low).
This may also affect some of the lights, e.g. the 'Power OK' light in some
DEC boxes - it won't come on, even though power is in fact fine, and
working.
Noel