PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sun Aug 16 09:00:29 CDT 2015

We did a lot more debugging on the TC12 LINCtape controller.

We saw a 500ns glitch in the LMU MOTION signal that corresponded to a short
slowdown in tape speed. We will investigate this next week.

We entered the LINC instruction to check a single block (0707) in the left
switches and a block number (0777-0000) in the right switches. When we
pressed the DO key it should go to that block on the LINCtape. With large
block numbers (07xx) and with the tape positioned half way through the tape
it worked OK. With lower block numbers it sometimes could not find the
block and searched back-and-forth on the tape. The logic analyzer showed
that the block numbers were correct in a sequence of several blocks, and
then it will read a bad block number. The TC12 would tell the TU55 to turn
around, it would read a good block number, realize that it was going the
wrong direction, and turn the tape around. It would then read a good block
number, and then a bad block number, and turn around.

At this point we don't think that we are working with bad tapes, but the
problem might be in either the TU56 tape drive, or the TC12 LINCtape
controller. We see bad behavior in both devices so we will do as Charles
Lasner suggested and swap a TU55 and TU56 between the PDP-12 and the
PDP-8/I. This will let us test the TU56 with a known good TC01 LINCtape
controller, and test a known good TU55 with a questionable TC12 LINCtape

We ran the A-to-D converter test and were rewarded with a display on the
VR14 that showed correct operation of the knobs and A-to-D converters.

Michael Thompson

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