PDP-12 at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sun Jun 28 09:02:37 CDT 2015

> The processor and core memory in the PDP-12 are working very well now, so
> we spend some time with the TC12 LINCtape controller. The TC12 is very
> intelligent compared to the more modern TC01/TC02/TC08, and the lobotomized
> TD8E DECtape controllers. The TC12 designers included lots of back-doors to
> make diagnostics more effective. You can even emulate the data coming from
> the TU56 to see if the TC12 processes it correctly.
> The MAINDEC-12-D0GA-A Tape Quickie ran OK and just tests that the TC12
> registers can be written and read back.
> The MAINDEC-12-D3AD-D-D Tape Control Test Part 1 of 2 ran for a long time
> and then displayed "LGP GP=GPC PRESET". It pointed to an M216 module that
> uses the SN7474 ICs that have caused lots of trouble on other modules. It
> tested OK, so we put it back in. We will consider replacing it anyway if we
> see further problems.
> The MAINDEC-12-D3GA-D-D Tape Control Test Part 2 of 2 ran OK as long as
> you held the MARK switch on the console down. The MARK switch on the
> console allows a program to turn on the MARK Track flip-flop in the TC12.
> This is not documented in the manual, but was a hand written note in the
> margin.
> The MAINDEC-12-D3FB-PB Tape Data Test ran for a long time writing patterns
> to tape and verifying that they were written correctly. This means that
> lots of the TC12 LINCtape controller is working, as well as the TU56 tape
> drive. It eventually failed when it tried to verify the block numbers. We
> are not sure that the scratch LINK tape that we used is good, so that may
> be a contributing factor. Maybe we can get the MARK-12 program working and
> we can reformat the scratch tape. We have just a few LINC tapes and need to
> image all of them before we write on them.
> Two more lights on the front panel stopped working. We tested the SN7400
> ICs that send the signal from the registers to the front panel, and they
> are OK. The bulbs are OK, so the transistor that turns on the bulb probably
> failed. We already replaced one for the LINC light, so we know the
> procedure.
> We are getting close to the point where we will need the VR14 display
> working to continue our work. Getting the display working will be quite a
> project. The PVA between the CRT and the shield has degraded and is nearly
> opaque. We will need to remove the outer CRT glass shield and remove the
> degraded PVA. Most restorers just put double-sided tape between the shield
> and the CRT so that it will have the right dimensions in the clamp. We are
> considering replacing the PVA to make is safer. That will be quite a
> project.
> Warren modified the current-loop to RS-232 adapter that he made so it will
> run at higher speeds. We needed to remove "C1" from the W076 console module
> so we could run baud rates faster than 110. After testing, it looks like
> 1200 baud is the best compromise between reliability and performance. Now
> we can load diags 10x faster. Very nice!

We scanned some of the PDP-12 diags, and Al put them on Bitsavers. We will
scan more this week and send them to Al.

Michael Thompson

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