Tips on reviving a TU56 + TD8E?

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Thomas Moss

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Oct 7, 2020, 2:10:04 AM10/7/20
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Hi All,

I recently bought a TU56 for my PDP-8/e, and am looking for some
advice on getting it to work.

Over the course of a few weeks I fixed odds and ends with the drive
(motor run capacitors, damaged wires, transistors, etc.), and for a few
days the right drive worked very reliably. I could read existing tapes,
write to them, and format fresh tapes.
(A write-up to this point is here: www.vcfed.org/forum/showthread.php?76935)
(A good source for new motor run capacitors: https://www.ebay.com/itm/142631210678)

However, this changed when I wrote a file to an existing tape, only to
find that the directory structure had somehow been corrupted in the
process. Out of curiosity I mounted a scratch tape that I knew I could
format before and tried to format it again, the result was a
BLOCK NUMBER ERROR PHASE 4.

When running the TD8E MAINDEC it's apparent why - when set to display
the block number in the accumulator for the tape I tried to format,
every third bit appears to stick or glitch out. The tape I think I
corrupted however appears to count up and down the blocks correctly.
(Video here: https://streamable.com/bh8f7p)

I tried swapping the G888s around to see if it made any difference, but
nothing changed. I then turned my attention to the cabling and the TD8E.

Somewhere along the line things got worse, and now running TDFRMT just
results in a "SETUP?" error when trying to format a tape.

I've tried running the TD8E MAINDEC (DHTDAB), and managed to get it to
pass the following tests:
-Operator Intervention
-Data Register
-Command Register
-Initialize
-SDLC, SDLD, SDRC, and SDRC and AC Clear
-Single Line Skip Instruction and Logic Test
-Quad Line Skip Instruction and Logic Test

However, once it reaches the Timing Error Skip Instruction and Logic
Test, it fails with the following:
>TIMING ERROR SKIP INSTRUCTION AND LOGIC
>TIMING ERROR STATUS BIT NOT SET IN COMMAND REGISTER
>
>TIMING ERROR DOES NOT CLEAR WRITE FLIP/FLOP

I also found some tests that check the Data and Command Registers by
accepting input from the switches and displaying the result in the MQ.
These work fine, although trough doing so I discovered that I had stuck
bits on the MQ on one of my Major Registers boards. I swapped with a
spare for now and will look at that later.

Lastly I found a copy of the source for TDFRMT so I could see what
exactly was causing the "SETUP?" error. I noticed there's several things
that can cause that error (the JMPs to ERCHK), so I replaced the first
JMP ERCHK with a halt and sure enough it did halt there.

/SEE IF THE DRIVE IS OK
003224 6774 RSTSM, SDLC /LOAD CR TO CLEAR TIMEING ERROR
003225 6775 SDLD /LOAD DATA BUFFER TO CLEAR S Q FLAGS
003226 1162 TAD DT0400 /SET WRITE
003227 1027 TAD DTA /GET UNIT
003230 3257 DCA SAV /STORE IT AWAY
003231 1257 TAD SAV
003232 6771 SDSS
003233 5232 JMP .-1
003234 6774 SDLC
003235 1257 TAD SAV
003236 6774 SDLC /LOAD THE TRANSPORT
003237 6776 SDRC /READ THE COMMAND REGISTER AND CHECK IT
003240 7006 RTL
003241 7004 RAL
003242 7500 SMA /CHECK WRITE TO BE SET
003243 5260 JMP ERCHK /WRITE IS NOT SET <<<---Halts here if changed to HLT.
003244 7004 RAL /CHECK WLO
003245 7510 SPA
003246 5260 JMP ERCHK /WLO
003247 7004 RAL /CHECK SELECT AND TIMING ERROR
003250 7710 SPA CLA
003251 5260 JMP ERCHK /SELECT OR TIMING ERROR
003252 4777' JMS NUDTA /CHECK OTHER DRIVE IF ANY
003253 5213 JMP RSTSM-11 /CHECK OTHER DRIVE
003254 5655 JMP I .+1
003255 1400 STMK
003256 0000 CNTERL, 0
003257 0000 SAV, 0

Does anyone have any hints on what I should check from here?


Regards,
-Tom

mo...@sdf.org
SDF Public Access UNIX System - https://sdf.org
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