Subject: PDP 8/L repair log - hopefully the LAST installment!

From: Charles <[email-masked]>
Date: Thu Mar 27 20:16:01 2003

The 8/L is finally fixed! The final interesting failures were:

The Group 2 microinstructions (CLA, IAC, etc) would not work.
Changed the relevant 7400 - still only weak wiggles. Tracked it
down to a dead short on the AC CLEAR line. Turns out that one of
the ground bus strips that run the length of the backplane had
buckled just enough to touch the exposed wire at the bottom of the
pin it was supposed to make a circle around! That was really fun
to locate.

Lastly, the TTY receiver card M706 would only work when on an
extender card. When in the backplane it would receive only one
character and "hang" until power cycled. I spent a lot of time in
a fruitless search for a bad backplane contact. Finally I found on
the stop bit flipflops that their PRESET pins were floating (a
frequent bad habit of DEC in their early design was to leave
uncommitted TTL inputs to float, which works fine as long as it's
an electrically "quiet" environment. Which a card handling 20mA
TTY signals is NOT). A quick 1K pullup to +5 and it hasn't dropped
a bit since.

I've just finished constructing my "DF32x4" disk simulator (PC
boards in a 1U [1.75" high] rack chassis). I'm starting to debug
now - so far I had a bad IDC connector and (of course) no spare. I
will make Eagle .SCH and .BRD files available to anyone who wants
them IF it ever works ;)

-Charles

[demime 1.01a removed an attachment of type image/jpeg which had a name of mvc-457e.jpg]

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From: Marvin Johnston <[email-masked]>
Date: Thu Mar 27 20:24:00 2003

> pin it was supposed to make a circle around! That was really fun

I had a similar problem in the early 70's with a PDP-16; periodically
one of the chips would "blow its top" and I'd come in to find the
machine not working and the chip blown apart. Checking found nothing,
and replacing the chip solved the problem for another couple of months.
I finally had occassion to pull the computer and check out the back
plane for another reason, and found the same thing you did ... except it
was *major* intermittent :). The buss line was IIRC -15VDC or so and
periodically would touch one contact thus blowing the chip apart. Only
took a year to solve it.

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From: Ethan Dicks <[email-masked]>
Date: Thu Mar 27 20:52:01 2003

--- Charles <[email-masked]> wrote:
> The 8/L is finally fixed!

Congrats!

> extender card....

A) What rev M706 card do you have?  (early rev M-series modules look
like they were done on a whole-board grid with signal lines taped
where needed; later boards have just as much copper as required,
with the chips as close to the backplane as routing allows)

B) Would you write up where you attached your pullups?

None of my M706 cards have behaved badly like that, but if I run across
a setting where I see that behavior, I'd like to benefit from your
efforts.

> will make Eagle .SCH and .BRD files available to anyone who wants

That sounds great!  I'm interested.  I have a few -8/Ls and an -8/I that
could use a disk device.  I even have one BM08, giving me 12K on one
-8/L.  I'd *love* to be able to show off OS/8 on it.

Are you using a microprocessor on your DF32x4? FPGAs?  PALs?  TTL?
How are you handling the cabling issue?  Do you have a way to load
the emulated disk devices "out of band"?  (i.e., can you get data into
it in any way other than the databreak interface to the -8?)

> [demime 1.01a removed an attachment of type image/jpeg which had a name

What's the picture of?

-ethan

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From: Lawrence LeMay <[email-masked]>
Date: Fri Mar 28 15:17:00 2003

> -8/L.  I'd *love* to be able to show off OS/8 on it.

Count me in as well, finding mass storage for my /L is nearly impossible.

-Lawrence LeMay