PDP-12 Restoration at the RICM

Rich Alderson 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
>> controller.

> 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
are non-adjacent:

	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.


Rich Alderson
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134

mailto:RichA at LivingComputerMuseum.org


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