25 Jun
2015
25 Jun
'15
10:02 a.m.
From: tony duell
There are 3 40 pin Berg headers, one row of each
appears to be ground
Ah, hadn't noticed that! But then again, I hadn't looked at them closely
yet! :-) Yes, they do connect to ground - all the UNIBUS ground pins are
ganged together, and connected to the A-row Berg pins on all 3 connectors.
So every other wire on the 40-conductor flat cables should be ground - that's
even better than the classic BC11A, where almost every other line is, from
what I can see, simply left floating (which is better than nothing, but not
as good as grounding them, is my understanding).
> I was wondering if maybe the M9015 was an M9014
with termination
> resistors, or something (the way the QBUS versions come with and with
> termination)
I would be very surprised. Unibus is normally
terminated at the ends
and not in the middle.
Right, but the very similar QBUS does have terminations (of a sort - the
rules for when you need terminations on QBUS extensions are so complex that I
don't really grok them yet) 'in the middle', so...
I just couldn't find out _anything_ about M9015's, so I was just guessing in
the dark. Real data gratefully received.
From: John Wilson
I was kind of assuming that there's some
impedance-matching (etc.)
problem with using ribbon cables for more than one hop.
Hmm. Well, I dunno; that may be beyond my (minimal :-) level of analog
expertise. I would have assumed that it's the _change_ from one impedance
level to another that's the issue (you can get a reflection off the
junction), so whether one's using long or short cables between a pair of
M9014's, it shouldn't be _that_ big a deal (modulo propagation delays, which
_are_ an issue with length). Perhaps someone else can opine?
But I hope we can do fairly long runs with the 40-conductor (aka BC05L-xx),
that could save us when we run out of BC11A's, if that strange Flexprint flat
white cable the BC11A uses is no longer available.
Noel
PS: From what I can see so far (done half the pins), the M9014 and M9042
do have an identical pinout on the Berg connectors.
25 Jun
25 Jun
10:38 a.m.
On Jun 25, 2015, at 1:02 PM, Noel Chiappa <jnc at
mercury.lcs.mit.edu> wrote:
That doesn?t seem likely. While DEC engineers on average didn?t understand transmission
lines as well as, say, Cray engineers, the definition of ?termination? is something that
you learn in EE 101 and are unlikely to forget.
paul
From: tony duell
...
> I was wondering if maybe the M9015 was an
M9014 with termination
> resistors, or something (the way the QBUS versions come with and with
> termination)
I would be very surprised. Unibus is normally
terminated at the ends
and not in the middle.
Right, but the very similar QBUS does have terminations (of a sort - the
rules for when you need terminations on QBUS extensions are so complex that I
don't really grok them yet) 'in the middle', so? 
10:41 a.m.
[M9014]
[One row on each Berg is ground]
Ah, hadn't noticed that! But then again, I
hadn't looked at them closely
yet! :-) Yes, they do connect to ground - all the UNIBUS ground pins are
ganged together, and connected to the A-row Berg pins on all 3 connectors.
I am not at all surprised they are grounded, it's the only logical place for them
all to end up.
So every other wire on the 40-conductor flat cables
should be ground - that's
even better than the classic BC11A, where almost every other line is, from
what I can see, simply left floating (which is better than nothing, but not
as good as grounding them, is my understanding).
I am surprised. DEC didn't waste copper like that. It's been a long time since I
worked
on a BC11A, but I thought alternate wires were grounded. Maybe a track right along the
edge of the PCB where the cable comes off (so you can't see it). I will check.
Right, but the very similar QBUS does have
terminations (of a sort - the
rules for when you need terminations on QBUS extensions are so complex that I
don't really grok them yet) 'in the middle', so...
I never really understood the Qbus rules either. The ones I saw make very little
electrical sense (unlike Unibus, which is designed as a transmission line). I don't
think I've ever taken Qbus outside one backplane, as a result just about anything
will work. On Unibus too, you can generally get away with no terminator at the
far end if you are in one backplane, or even one mouting box. Can be useful
for testing...
Hmm. Well, I dunno; that may be beyond my (minimal :-)
level of analog
expertise. I would have assumed that it's the _change_ from one impedance
level to another that's the issue (you can get a reflection off the
junction), so whether one's using long or short cables between a pair of
M9014's, it shouldn't be _that_ big a deal (modulo propagation delays, which
_are_ an issue with length). Perhaps someone else can opine?
Well, Unibus is terminated into 180 Ohms and 390 Ohms, isn't it? The thevenin
equivalent is thus around 123 Ohms. Most ribbon cables have a characteristic
impedance when used with alternate wires grounded of around 100 Ohms (I
seem to remember that is certainly right for the twist-n-flat ones). That's a
small mismatch, but I don't think it is going to cause big problems.
-tony
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