Interesting RK8E fault

[Josh Dersch]

Hi all --

Finally got all the parts together (and my act together) to actually get an
RK05 lashed up to my PDP-8/e -- only took a decade or so :).  I fixed a few
problems with the RK05 and it appears to be behaving very nicely.

The RK8E controller is mostly working properly but fails interestingly when
running the formatter, and during the exerciser -- on cylinder 128 and 192
and very infrequently on cylinder 64 it will get a cylinder mismatch when
doing the seek.  When running the formatter during the verification pass,
on cyls 64 and 128 if I retry the read it'll continue without issues, but
it's never successful on a retry on cylinder 192.  I tried hooking it to
the RK05 in my 11/40 and it exhibits the same behavior, so I'm guessing the
drive isn't at fault.  And the error is consistent across packs (of which I
have only two).

Apart from that fault the drive and controller seem to work fine -- I wrote
out an OS/8 pack with Adventure on it (or at least the first 191 cylinders
of it) and it works without issue.

Reading the RK8E service docs and schematics, the cylinder address compare
is done by reusing the CRC buffer, so I suspect the issue is in or around
there -- the big problem is that debugging it is rather painful since that
logic is in the middle board of a three board set, with jumper blocks on
top -- so bringing it out on an extender isn't an option.  I'm curious if
anyone's seen this issue or is so very familiar with the logic that the
fault is obvious.

I suspected the 7496 shift register at E14 which takes in the cylinder
address to be compared w/the header on disk, and I went ahead and replaced
it in the hopes that I'd get lucky, but no go.

Anyone have any advice?

Thanks,
Josh