PDP-12 Restoration at the RICM

[Mike Ross]

I have no constructive input to share - except the strong memory of
the bloke who maintained my pdp-12s for 25 years before I collected
them saying that the TC12 was an "absolute bugger" and the hardest
part of the machine to troubleshoot! Good luck and don't feel bad
about having a hard time with it!

Mike

On Mon, Oct 12, 2015 at 3:42 AM, Michael Thompson
<michael.99.thompson at gmail.com> wrote:
> We swapped the TU56 and TU55 drives between the PDP-12 and the PDP-8/I. We
> found that the TU56 uses a double-sided paddle card for the control signals
> and the TU55 uses a single-sided paddle-card. The TC12 tape controller in
> the PDP-12 will work with either tape drive if you have the right cable. We
> borrowed a control and data cable from the PDP-8/I. The TU56 in the PDP-12
> was wired for +5V, but can use +10V or +5V. The TU55 needs just 50ma of
> +10V, so we ran a wire from a +10V backplane pin to the TU55. After these
> modifications the TU55 operated correctly in the PDP-12.
>
> The TU55 behaved a little better than the TU56, and sometimes would
> actually boot OS/8. We continued chasing the issue and found glitches on
> data channel 3. We have swapped every module and cable that relates to data
> channel 3, but have not been able to fix the glitches. The glitches are
> there with both the TU55 and the TU56, so the problem is in the TC12
> controller.
>
> We put the TU56 back in the PDP-12, and the TU55 back in the PDP-8/I. The
> TU56 behaves worse than the TU55, but that may be due to the timing
> adjustments in the TU56. I have to make the same adjustments in my personal
> TU56, so by next I will know how to adjust the TU56 in the PDP-12.
>
> We are running out of things to try for debugging the TC12 controller in
> the PDP-12. At this point we are really stumped as to why the TC12 doesn't
> work correctly. We are going to try a few remaining ideas like running the
> TC12 from a laboratory power supply.
>
> We would welcome any ideas that you have.
>
> On Sun, Aug 16, 2015 at 10:00 AM, Michael Thompson <
> michael.99.thompson at gmail.com> 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.
>>
>> 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
>> controller.
>>
>> 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
>>
>
>
>
> --
> Michael Thompson



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