PDP-12 at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sat Jul 11 20:02:52 CDT 2015

 J. Victor Nahigian donated some M221 and M222 boards for the processor and
TC12 LINCtape controller. They are in pretty bad condition, but are
repairable. Warren wrote a test program for the M222 boards, and some of
the just donated boards actually work OK. It will be nice to have some

Dan (the donor) brought over the now lubricated top of cabinet fan. He had
to make some special tools to disassembled it but was able to lubricate the
dry bearings. We will reinstall it next week. He also dropped off the
system logbook and more diagnostic manuals. More scanning for Bitsavers! It
is really interesting to see the service history of this system since it
was new.

The LICM scanned some of the missing MAINDEC documentation so we were able
to run:
MAINDEC-12-D1AC-D Extended Memory Control (EXTMC12)

MAINDEC-12-D8CC-D_KW12A_Clock_Test Fails test #11 where the AC should be
0000 and is 7777 on the CLRB instruction.

MAINDEC-12-D8CA-D-(D) KW12 Real Time Clock Diagnostic (KW12TST) shows the
same failure mode as D8CC.

MAINDEC-12-D6BA VR12 Display Test Runs, but we have no display connected.
We connected the 'scope to the outputs of the VC12 display controller when
MAINDEC-12-D6BA was running. Without a way to interpret the intensify
signal, and no persistence in the phosphor, the resulting image was not too
good. At least we know that the display controller is responding to
commands and outputs signals that look reasonable.

We reran MAINDEC-12-D3AD-D-D Tape Control Test Part 1 of 2.
It still fails at 3400 with the error message "LGP GP=GPC PRESET" printed.

We reran MAINDEC-12-D3GA-D-D Tape Control Test part 2 of 2.
That runs fine.

I bought the matching terminator for my current probe and was able to give
it a try today. The normal setting for PDP-12, PDP-8/I, and PDP-8/L core is
320mA. When we debugged the core we set the voltage regulator to the middle
of the high and low voltage settings that caused periodic errors. The low
going part is the read current and the high going is the rewrite current.
The result turned out to be 316mV for the read. Not too bad!

Michael Thompson

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