PDP-12 Restoration at the RICM

Jay Jaeger cube1 at charter.net
Sun Aug 16 17:18:12 CDT 2015

On 8/16/2015 9:00 AM, Michael Thompson wrote:
> 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.

Just a gut hunch, based on the symptoms.  Could you have a bit pick or
drop in the block number in the TC12?  Given the behavior, one might bet
on a bit pick.  Perhaps intermittent.

Second gut hunch is that it would be hard to see how the drive could
cause this UNLESS the TC12 uses one side of the redundant tape channels
in one direction, and the other side in the other direction.

Will be interesting to hear what you really find.


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