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QBus Bus Termination.

shevett

Experienced Member
Joined
Jan 28, 2020
Messages
58
Location
Boston, MA
Hey everyone, some more questions on my 11/73 project.

A person commented on one of my videos and told me to make sure I had bus termination...

"Make sure the backplane Q termination resistors are installed (XZ1, XZ2, XZ3, and XZ3 side 1 next to slot 8)."

My history working with Unibus machines always said TERMINATE ZE BUS, but I have slightly less experience with QBus. This machine did NOT come with any terminators (well, slight update, it came with an M9302 unibus termination card, but obviously not socketed into the machine). I don't have anything in the backplane other than functional cards that looks anything like a terminator. Perhaps the BA23 is terminated already?

What I'm finding looking up the BDV11 card is it looks like it has a boot rom and diagnostics. I have a working boot rom and diagnostics. Is it just that the KDJ11B has all this already built in? Do I need to hunt down a terminator?
 
The BDV11 has termination, but only 18-bit as standard, look for 4 mod wires on the top making it 22 bit - The CPU also provides termination, but if you do not have a full backplane, like only 4 boards or so, the CPU termination will probably work just fine.. SCSI termination is a LOT more fussy and critical to get right and "by the book".

If your backplane is rammed - or you are looking at extending to 2 or 3 backplanes, then termination is critical.

I would pay more attention to good forced cooling, especially over the 11/73 CPU - and good anti-static practice when handling the cards...
 
The BDV11 has termination, but only 18-bit as standard, look for 4 mod wires on the top making it 22 bit - The CPU also provides termination, but if you do not have a full backplane, like only 4 boards or so, the CPU termination will probably work just fine.. SCSI termination is a LOT more fussy and critical to get right and "by the book".

If your backplane is rammed - or you are looking at extending to 2 or 3 backplanes, then termination is critical.

I would pay more attention to good forced cooling, especially over the 11/73 CPU - and good anti-static practice when handling the cards.


I strongly agree with 100% of this, with exception of the number of boards. In oemMicronotes #29 it states that a single backplane does not requite termination if there are <20 AC loads, with more capacity depending on the processor termination. You should be able to calculate the actual board capacity. For example a DELQA uses 3.3 AC Loads, DEQNA 2.2 and DZQ11 1.5.

Jerry
 
I strongly agree with 100% of this, with exception of the number of boards. In oemMicronotes #29 it states that a single backplane does not requite termination if there are <20 AC loads, with more capacity depending on the processor termination. You should be able to calculate the actual board capacity. For example a DELQA uses 3.3 AC Loads, DEQNA 2.2 and DZQ11 1.5.

Jerry
Hi Jerry! - thanks for the official DEC line on this, the only other consideration I would throw in, is that there are non-DEC backplanes that are "longer" - I have an GRC 8 slot AB-AB backplane (all serpentine slots), which of course the DEC Micronote won't apply to, and I would expect that to potentially need termination if fitted with a lot of boards.. At present, I have a BDV-11 (22 bit) lurking in the traditional bottom slot just like in the BA11-N, so it is taken care of.

Of course, we may have more than the 20 AC loads anyway in that case - but there could be edge cases where the longer bus without termination could cause issues...
 
I chased this in another email exchange, and also learned the BA23 should have it's own termination:

"Oh yes, Qbus definitely needs termination, but you already have it.
The backplane in the BA23 chassis (H9278A) has four onboard socketed
resistor packs for termination, so a separate terminator board is not
required. You'll see them if you look down to the lower right, on the
PCB underneath the connector blocks."

I'll be double-checking this when i stop doing ${DAYJOB}.

Between the KDJ11B and BA23, I should be GTG.

My current card count is 7. KDJ11B, 512k memory (MSV11? need to check), DHV11, RQDX3, TK50 controller, and two DLV11J's.
 
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