PDP-12 Restoration at the RICM (tony duell)

Michael Thompson michael.99.thompson at gmail.com
Tue Oct 13 19:32:49 CDT 2015

> Date: Mon, 12 Oct 2015 18:29:11 +0000
> From: tony duell <ard at p850ug1.demon.co.uk>
> Subject: RE: PDP-12 Restoration at the RICM
> > We swapped the TU56 and TU55 drives between the PDP-12 and the PDP-8/I.
> We
> Does the TU55 work correctly on the 8/I ?

Yes, it works perfectly. It even worked OK after we put it back in the 8/I.

> > 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
> What exactly do you mean by 'glitches'? Are these on a TTL level signal,
> an analogue
> output of the read amplifer, or what?

You can see the logic analyzer trace of many of the TC12 signals here.

The labels at the far left of the image include the backplane slot and pin
number for the probe location. The signals were TTL level by the time the
logic analyzer saw them.

Have you looked for glitches on the other data channels?

Tracks 1 & 2 look fine.  We swapped the probes for track 2 & 3 just to make
sure that it wasn't a logic analyzer problem. A 'scope connected to the
differential signals shows the same track 3 glitches. The glitches were
present with the TU55 and the TU56 tape drives. We used different control
and data cables for the TU55 and TU56. We swapped the G882 modules between
tracks, and from the TC01 in the PDP-8/I and did not see a change. We
swapped the W603 module in slot F16, the R107 in F12, and the W512 in F13.
The LTR WRITE ON L signal is inactive and stable. We tried several tapes,
including ones formatted and written by DEC and the glitches are present on
track 3.

Michael Thompson

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