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
> 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