15 Jan
2014
15 Jan
'14
7:36 a.m.
This is stumping me. What exactly is the 555 being used for? If I can
figure out what it's doing, I can debug it further and solve my problems.
I assume this is the one shown on (pdf) page 26 in the printset on bitsavers.
From what I can see, it's a monostable (R22 and C19
are the timing components) to delay the power-OK signal
(produce by the comparators
in U2). It appears that when the +5V, +12V and -12V lines are all high enough, the
trigger input (pin 2) of that 555 goes high. A little later the output (pin 3) goes low,
causing 'PLB4 DC OK H' to go
high and taking the CPU out of the reset state (see page 43 of the printset).
I would start by looking at the trigger input of that 555. If it's low, then one of
the comparators thinks that one
of the supply lines is too low. Is it?
-tony