13 Oct
2015
13 Oct
'15
5:32 p.m.
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
Yes, it works perfectly. It even worked OK after we put it back in the 8/I.
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?
13 Oct
13 Oct
6:24 p.m.
On 10/13/2015 07:32 PM, Michael Thompson wrote:
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.
OK, so the drive must have a head amplifier and a comparator
(sometimes called a slicer in tape technology) to convert
the amplified analog signal to a digital one. Have you
swapped the read amps and comparator cards? (Sounds like
yes, above.) Have you looked at the analog signals? it is
possible a defect in the tape head is causing the read
signal to be different than what should be seen. Looking at
the amplified analog signal might give some hint at the
behavior. (Don't bother trying to look at the raw tape head
signal, it will be really tiny.) I'm not sure where the
read amps and slicer are, whether they are in the control or
the drive. (Seems like the read amp HAS to be in the drive,
though, for noise immunity reasons.)
Jon
17 Oct
17 Oct
7:10 a.m.
On Tue, Oct 13, 2015 at 08:32:49PM -0400, Michael Thompson wrote:
Have you hooked up a digital scope to the read amplifier and seen if the
analog waveform has problems when the glitches occur? Seeing both the analog
and the digital output would help see where the signal is going wrong.
The other is if you have enough memory and channels in your test equipment
you can trigger off a signal indicating that the block number was missed
then look back to see how the block number was wrong. If you have it a
trigger out from the logic analizer can trigger a digital scope to capture
at the same time to see the head amplifier signal.
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.
<https://4310b1a9-a-11c96037-s-sites.googlegroups.com/a/ricomputermuseum.org/home/Home/equipment/dec-pdp-12/dec-pdp-12-restoration/LINCtape_Track-3_Problem.jpg?attachauth=ANoY7crnX8o5yLYK_UhrFx_58_SY0L1ogEtncsO6od4NhnEtVA9JazEEolMQZqGO4o8Z5l_z2g8-Bba6EX1OkRVrjpjutMp_JDMRoWSoMRZk6PnpOdywwKqxjdnHEdFO3Roux9LqFkmKG7oqalE7c8EUL-2ecJb99ZQuUsPvbz7I1oZJPppz8zWCG0174_en4Hm5e2Aqc1vzjESaq6SdUyJ9EDKQIRP_uBsR8lw9sq2DYaIUJk1MxHv2K9YoiWaFgm3V0hfE6T3syLwKBtA6GaoNIv667JwhR0fgf6B-FTXrGeWow5TmVfA%3D&attredirects=0>
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.
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2 comments
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participants (3)
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djg＠pdp8online.com -
elson＠pico-systems.com -
michael.99.thompson＠gmail.com