aclo dclo (11/45) flustered-ness

[Jay West]

Tony wrote....
> 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?

> Are you sure that ACLO and DCLO on this supply go anywhere
> else. As I said, I'm working from memory, I guess I'd better check the
> prints.
I could be reading the power system diagrams wrong. But...it seems to me if 
I look at DCLO for example on the lower H742 regulator, P22-9 (DCLO2) is 
sent out to P7-4. This is wired to system backplane pin B28F2 (last slot, 
unibus out). P22-12 (DCLOX) is sent out to P4-3, which is wired to B16U2 
(first slot of the MOS/Bipolar memory I think). I seem to remember reading 
something about the memory being on a unibus B and the cpu on unibus A, 
unless the two are jumpered together. Is it possible one supply provides 
DCLO on one bus and the other supply gives DCLO on the other bus? All this 
same discussion appears to apply to ACLO as well, just different pins.

> And it's there too. Doesn't the interfacing handbook give a Unibus pinout?
Yes, but I didn't see the wirewrap.... see below....

> It might be carried on a PCB trace. It's probably safest to trace it with
> an ohmmeter.
Bingo. The "missing" wirewraps are PCB traces. I assumed (badly) that only 
power was carried on traces on the DD11-K. I was obviously quite wrong. An 
ohmmeter showed my folly quite clearly.

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.

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 
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 :)

Jay