PDP-12 Restoration at the RICM
RichA at LivingComputerMuseum.org
Mon Oct 12 20:16:36 CDT 2015
From: tony duell
Sent: Monday, October 12, 2015 11:29 AM
>> 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 ?
>> 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?
> Have you looked for glitches on the other data channels?
>> 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
> IIRC this thing has 3 data channels, so is channel 3 on the outside of the
> head/tape? Could it be some external interference that happens to be picked
> up most strongly by that head winding?
DECtape and LINCtape (same medium, wound opposite directions on the reel) are
10 tracks across. These are pairwise connected serially to produce 5 signals,
called T (timing), M (mark), and 1, 2, and 3. The T tracks are outermost, the
M tracks are next inward from the T tracks, and the tracks making up 1, 2, & 3
T M 1 2 3 1' 2' 3' M' T'
The M tracks are longitudinally encoded (6-bit values chosen such that they
read the same as NRZ backwards and forwards for DECtape, 4-bit values for
LINCtape) to predefine blocks (cf. disk sectors) for data.
So neither of the tracks which make up channel 3 is on the outside of the tape.
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134
mailto:RichA at LivingComputerMuseum.org
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