Dec boards- what are they?

Ethan Dicks ethan.dicks at
Wed Jun 8 12:25:56 CDT 2005

On 6/8/05, Paul Koning <pkoning at> wrote:
> Sure.  I should have been more precise.  The original question was:
> "how can I tell by inspection whether a board is Unibus, Q18, or Q22?"
> Given just those three choices, what I said is accurate -- hex can't
> be Qnn so it must be Unibus by process of elimination.

I think the semantic problem we are having here stems from where one
is starting.  The question we've been answering is a real world
question, "I have this random DEC board in my hand... what does it go
in?" and your question presupposes a subset of that "I have this
random DEC board that is only Unibus, Q18 or Q22.  Which is it?"  In
practical terms, one can't always *know* it's only Unibus, Q18 or Q22,
thus the efforts to answer the larger questions.  I've frequently seen
PDP-8 and other boards mixed together in piles.  Even if one _knows_
that a particular source only had Unibus and Qbus PDP-11s (reducing
the likely range of products), something like a board from an RK611 or
an RKV11 does go _into_ a Unibus or Qbus PDP, but is not itself a
Unibus or Qbus board - the bus-level compatibility is managed at the
backplane level, *not* the board level (for example - the RK611
controller is a set of hex-height cards and a 9-slot backplane that,
dismantled, looks much like a DD11-DK Unibus backplane and assorted
Unibus cards.  The backplane is typically labelled "RK611" as opposed
to "DD11DK", but unless you are a DEC-head, that doesn't mean a whole
lot.  Superficially, without inspecting what pins the wires go to, the
two backplanes look the same.  The same thing goes for the cards...
superficially, the board that's part of the RK611 that has the 40-pin
BERG connector (can't remember the handle number) has a superficial
resemblance to an RL11, a common Unibus controller for the RL01/RL02. 
Same connector, same color PCB, same size, similar component density. 
Unless you locate an identifying mark on the PCB that _says_ RL11 (and
know what one is), the only way to be sure is by looking up the handle
number in a manual or on the 'net).

We really aren't trying to evade your question, but we are trying to
let folks know that barring a numeric list of products, there is no
short list of easy-to-follow rules that _definitively_ distinguish a
particular DEC board as exactly one bus type or another.  There are
rough guidelines that can help you eliminate certain obvious
mismatches, but in the end, what ends up happening is that people like
myself that have worked with DEC equipment over the years either
_know_ what it is because we've seen one before, or we go look it up. 
I myself, with DEC experience going back to 1982, have misidentified
things I've seen at hamfests. One particular example was, IIRC,
mistaking the floating point board set for an 11/34a for some Unibus
CPU.  Now... in terms of your specific question, it wasn't like I
thought it was a Qbus board set (being hex-height and all), but I had
to look at it for a moment to discard some of the more common
possibilities (OMNIBUS, primarily, something I _do_ go looking for at

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