Double Buffer RK11-C
jnc at mercury.lcs.mit.edu
Mon Dec 5 13:32:56 CST 2016
> look at the lower right line of lights on the panel: ... and three bits
> of Major State; now look at the RK11-C prints, connector B32:
> ... Postamble, Checksum, Data, Header, Preamble.
> One thing I have been wondering about is that "RK11-C" - that implies
> that there was a -B, etc. I wonder if this panel goes with one of them?
Well, now that I look at a few more things I'm pretty certain the panel in
that image goes with some currently-unknown RK11 predecessor to the RK11-C.
Note those 5 'state' lines/lights, and then look at the 'Major States' RK11-C
print (RK11-C-04, pg. 14 of the PDF version, RK11-C Enginering Drawings
Feb1971). In the upper left corner there are a row of 6 flops, each labeled
with one of those states (plus one for 'Idle'), arranged in a chain. So one
light for the output of each flop...
Now look at that display panel: 3 bits for 'Major State' - implying it is
binary coded - likely implemented with a counter?
Notice also the signal "COUNT MSR" ('Major State Register', I expect) - just
what you'd expect to see if the major state had previously been held in a
counter, not a string of flops. Why go to all the trouble to synthesize that
signal (on the next page, RK11-C-05, "MSR Control") when you cou;d have used
the individual composing signals to clock each flop?
So my _guess_ is that in the previous version, they'd used a counter, but had
had some problems (perhaps it was a binary counter, not Gray code, and the
decoding into states was producing glitches), and had therefore switched to
the string of flops.
(This whole process makes me feel like a paleontologist, reconstructing some
unknown dinosaur from a fragment of one bone, using a lot of complex reasoning
from small clues contained therein! :-)
It would be most interesting to know if there are any signs anywhere of
predecessors to the RK11-C.
My suspicion is that they were produced in very small numbers - perhaps as
prototypes, only internal to DEC. (If they'd had problems with glitches in the
major state counter, they would not have wanted to release it as a product.)
Or if it was released as a product, perhaps they were all recalled and
replaced with RK11-C's because of the issues.
As evidence for this, I point to the Spare Module Handbook, which lists only
the RK11-C and -D - but _does_ list the KT11-B, a rara avis indeed. (More
dinosaur bones... :-) This argues that the predecessor did not exist in the
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