M9202 Unibus connector inverted

From: no <oliv555_at_arrl.net>
Date: Tue Jun 12 00:49:19 2001

Tony Duell wrote:
>
> >
> > I came across some M9202 "Unibus connector inverted", and I'm wondering
> > if anyone knows what they are used for?
>
> IIRC, the M9202 is a Unibus jumper (used to connect the Unibus Out of one
> backplane to Unibus In of the next) with 2' of cable folded up between
> the PCBs. It thus adds a slight delay to the signals.
>
> The are supposed to be some DEC rules about when to use M9202s and when
> to use M920s (the plain Unibus jumper). My experience suggests that for
> most systems you can use either jumper and it will work.
>

True. And it gets more sensitive as you add peripherals and increase
the total unibus (aggregate) length. Our 11/45 system, 2cpus sharing
memory, has a 5' cable btween the KB11 backplane and the first SPC.

I found out the hard way one night when that cable went bad that the
system would flat not run with either a M920 or M9202. It liked the
spare 6' cable we had on hand, though.

When I mentioned this to our field rep (Link, not DEC) I was told that
our system was "tuned" to a certain length due to its complexity and
that all unibus cables be replaced only with those of equal length.

> One thing you can do with the M9202 is drill out the rivets hold it
> togther. This will leave you with full-length PCBs with handles on them
> linked by 2' of cable. This is ideal for bypassing a backplane (pull out
> the jumpers connecting it to the ones on either side and put this
> 'cable' between Unibus out of the previous backplane and Unibus in of the
> next). Which can be useful when you're tracing grant problems.
>

Good troubleshooting aid. That 5' BC11 works great for this, and long
enough to extend to the expander rack.

> -tony


.... nick o
Received on Tue Jun 12 2001 - 00:49:19 BST