30 Mar
2007
30 Mar
'07
6:44 p.m.
Tony wrote....
Right.
An open-collector output is what the name implies. The output pin is
connected to the collector of an (NPN) transistor, the emitter of which
is groudned. So if that transistor is turned on, the pin is low, if it's
turned off the pin floats
You (the desigern) get to provide the collector load for that transistor.
Here it's just a resistor to +5V. So if the transistor is turned off, the
pin appears high (pulled up via the reisstor).
The thing is, you can connect many outputs together without problems
(unlike with normal 'totem pole' outputs where having 2 outputs connected
togtehr will cause high currents to folw if they're in different states).
With opne-collector outputs, if any transistor is turned on, the signal
will be low. If they're all off, it'll be high. Hence the term
'Wired-AND' (or 'Wired-OR' if you're using negative logic) for such
connections of multiple open-collector outputs. I've seen schematics (and
I think DEC were among them) where there's an AND gate symbol drawn
around the point where all these signals get joined together just to
remind you what's going on.
What you have to realise first is that, like most
Unibus signals (because
that's what they are, they are carried on the Unibus cables), ACLO and
DCLO are open-collector driven. They're pulled high (to +5V) by resistors
on the bus terminator cards, and can be pulled low at any point, by any
device.
I was thinking that open collector signal levels were low and float, not
low
and hi. From my testing, you're obviously right though, see below. I guess
the pullups are what take the float to hi? I got +5v & GND for my logic probe from CA2 &
CC2 and powered up just the
BA11K. BF1 & BF2 both showed low as I'd expect (since the cpu was powered
off). This was a sanity check on my part to make sure AC/DCLO were pulled
low. I powered up everything and BF1 & BF2 went hi. I hit load on the RL02
and the drive started to spin up. At just the time I'd expect the heads to
load, the fault lite came on. BF1 & 2 were still hi. If the RX02 is powered
off, the drive will not fault, it will spin up fine. Also, if the RX02 is
off and everything else on, the drive loads fine and boots RT11. The moment
I plug in the power for the RX02 the RL02 immediately faults (but the cpu
keeps on running right where it was). One thing I need to check, I am not
positive (because it's all rather loud) that when the RL02 faults that it
spins down the drive. I don't recall hearing the noticeable decrease in
volume. I would have thought it would spin down upon any fault. Maybe it is
and i just didn't hear it.
IIRC, not all 'faults' cause a spin-down.
What this has told you is that the problem is not due to ACLO or DCLO.
Now that's useful information. When you're trying to sort out a fault
like this, knwoing where it _isn't_ is avtually very useful.
I can start going through the prints on the RL02 as Tony suggested, looking
at all the signals that can trip the fault light. Is that the right
approach? I mean, the problem obviously isn't in the RL02 or controller - as
Well, it's what I'd do next, obviously (as I suggested it). The reasoning
behind doing it that way, is that you have a definite problem (the FAULT
light comes on). So your next move should be to find out _why_ that light
is coming on.
they work flawlessly if the RX02 is turned off. This
would make me think the
problem is more likely something about the RL02 or M8256.
At least I'm learning a lot :)
Good!. That's one reason I don't like taking things like this to private
e-mail, I feel that everyone (including me, I might add) learn a lot from
such discussions, even if we don't ahve that particular machine. Fault
tracing is much the same on any hardware, actually.
-tony