--- Rick Bensene <rickb_at_bensene.com> wrote:
> When I remove the RX211 and insert a NPG/Interrupt Grant jumper, the
> machine runs beautifully.
Always a good sign.
> Is this a sign of a sick RX211, or some kind of bus loading problem?
If you have multiple backplanes, bus loading is something to sit down
and calculate - each peripheral is documented (in the handbooks of the
period) with its AC and DC loads.
Having personally run 11/34s with a full BA-11 (three backplanes
including the CPU) loaded to the gills with peripherals (serial
interfaces and disk, mostly), I haven't yet run into a problem
that appeared to be bus loading. I would suspect the RX211 if
the rest of the box seemed reasonable.
One thing to identify is what else shares its BR level. ISTR most
DEC cards were either BR4 or BR5. Remember that an interrupting
device breaks the grant chain for all peripherals to the rear of
the bus. There's a small chance that the offending device is at
the same BR level as the RX211 and in *front* of it. If possible,
try putting the RX211 at the front, then test. No reason to leave
it there permenently - it should be a robust enough device to go
anywhere in the Unibus (which is not true of some cards on the Qbus,
especially when an RQDX1 is involved, but that's another story).
Without knowing details about the states of the Unibus arbitration
signals (that you almost certainly don't know), it's tough to guess
what might be wrong. There is a "PDP-11 Bus Handbook" out there -
slim and pale blue. It contains much wisdom if you can find a copy.
Also, don't be afraid to use older docs, even back to the 11/20 - the
Unibus did evolve a little over the years (MUD slots vs SPC, etc.),
but the underlying handshaking didn't mutate much. That's why you
can take a PDP-11/20 card and drop it in an 11/84.
-ethan
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Received on Tue May 06 2003 - 13:04:00 BST