Data General 6023 75ips vacuum column tape start up

Tom Jennings, 13 February 2005

The tape drive wasn't too hard to bring up, no deep debugging dead-ends, no particularly serious problems, though I did snap the tape a bunch, but I tested with the scroungiest tape I could find -- one marked "MISSING BOT".

This is the post-event email I sent to the classic computer list last night. It does contain pretty much everything I did. I've added photos (not allowed on the list) of things referenced in the text.

The home-made manometer. It's pretty simple, a transparent tube in a bucket of water. Tube is marked off in inches (I needed only 10" and 20"). The tube is really long because it's for another project and I didn't want to cut it up.


This is the new vacuum column seal made of RTC silicone rubber applied with my finger. See the text.






From tomj@worldpowersystems.com Sun Feb 13 21:55:44 2005
Date: Sun, 13 Feb 2005 21:54:49 -0800 (PST)
From: Tom Jennings 
To: "General Discussion: On-Topic Posts Only" 
Subject: NOVA4/X -- running!

Must be the alignment of the planets, favorable for old
computers...

I got the 6023 75ips tape drive working. Pretty straightforward
debugging.  With Bruce Ray's hint, the first thing I checked was
the vacuum switches (there's five, one for "master vaccum supply"
and then two each (low side, high side) in each column as limit
switches) and three of them were bad. Found that out a few weeks
ago. Swapped switches around with the other drive, didn't "run".

So I got scientific (sic). I made a manometer out of a long
plastic tube, a bucket and some automotive vacuum hose, some
masking tape and a tape measure. Attached each vacuum switch to it
with a tee, and hose in my mouth as vacuum supply and adjusted
each switch as close as possible (10% spec). One was way off, the
"master supply" switch has a different setting and I had mixed
them up, from the donor.

It attempted to load, but acted strangely, sucked tape into the
buffers (vacuum columns) then THWACK the tape, then go slack.
Twice it snapped the tape! (I'm testing with a known-bad tape).

Yesterday and today I spent about two hours tracing through logic
to see why the spindle motors wouldn't move (not even for manual
unload).  Traced right up to the driver amp -- no power supply!
Duh. ... turns out the DC circuit breaker popped. Sheesh.

While the breakers were popped, during testing I initiated LOAD,
and fondled the reels such that it the servo was happy,
positioning the tape loops in the buffer. Limit switches etc all
worked OK. During this though, I found that the lucite cover over
the vacuum columns wasn't sealing; it'd warped, lifting up at the
corners, leaking a lot of vacuum. For now, I taped it shut.

Resetting the breaker a few times, I determined that a logic
problem causes the two spindle motors, in the middle of the load
sequence, to rotate at high speed in opposite directions --
drawing HUGE currents, tug of war with the tape, which snaps or
wrenches into the spool. The takeup side decided that LOAD
succeeded, and was searching for BOT. The supply side decided that
LOAD failed, and was rewinding!

More servo problems. Turns out it was another vacuum leak!  The
lucite cover seals one column from the other, and it was leaking.
I made a seal by wiping silicone rubber in a film about 0.010"
thick with my fingers, tapered to match the lift of the warped
corners. Tape now loads!

Now I tweaked the final servo pots, it all behaves properly.

Now that I have actual DG terminals, I plugged one on instead of
minicom on a laptop. Display works, no keyboard. Terminal wants
CTS or RTS asserted. Fixed!

Reset, self-test, try to boot the tape:

!100022L

Nothing happens.

Long story short: the cables to the disk and tape were plugged in
wrong! Ouch. Luckily, identical paddle card wiring and pinouts.
I'd removed and marked them, but managed to install them wrong
anyways! Off-by-one problem.

Power up, (disk still off), self-test, boot:

!OK 000000 !I !100022L

DTOS (diagnostic tape operating system) boots! I typed ACCEPT, it
ran some tests, then many, many tape retried, !FATAL ERROR (tape
read error).  Clean head and capstan. Reboot, ACCEPT... runs! but
I halted it, powered on the 6070 disk, (cringing awaiting head
crash.... head load OK!), reboot DTOS, run ACCEPT... it discovers
the tape (duh) and disk!

ACCEPT (accept list of equipment found, run all applicable diags
one pass) ran... halted when I accidentially hit a key during
GALLOPING ROWS mem test? Seems odd...

Reboot, repeat, hands in pockets... runs many tests... I leave
running and go eat dinner.

An hour later, I find the tape slack. There's a crease in the tape
and a transparent spot (!), but no sign of a jam, I think the
transparent spot triggered BOT (likely an error). On the screen is
the DTOS exec saying "NMMD x V 3.0" which is where it loads the
tape file; it was searching... so without reset I reload the tape,
press ONLINE... and it starts searching again! It gets to
approximatel the same thickness of tape... JAM! WHACK! power goes
out!

1) The tape caught somehow, likely that crease, cross-wound on the
capstan. These reel motors are POWERFUL, likely break your finger.
That's what it tapes to swing a 5lb reel of tape at 75 ips.

2) The jam causes motor current draw to skyrocket. I've got the
terminal, CPU, tape drive, and disk, all plugged into a quadbox
extention, plugged into the (heavy duty) outlet strip on my
workbench...  the little breaker on that tripped.

Well that was stupid. The disk heads were loaded at the time, but
wasn't operating (it needs formatting anyways) and I
double-checked that the head-unload mechanism (monster capacitor +
N.C. relay) worked, so it's fine. I checked.

But I likely ruined the already-creased DTOS tape (will find out
Tuesday night) and the other DTOS tape Bruce lent me won't boot;
read error.


But it was a lot of progress anyways!

I took some pictures, like the goofy manometer, I'll put 'em up
this week.

From tomj@worldpowersystems.com Mon Feb 14 09:09:23 2005
Date: Mon, 14 Feb 2005 09:08:21 -0800 (PST)
From: Tom Jennings 
To: "General Discussion: On-Topic and Off-Topic Posts" 
Subject: Re: NOVA4/X -- running!

On Mon, 14 Feb 2005, Tore S Bekkedal wrote:

>> It attempted to load, but acted strangely, sucked tape into the
>> buffers (vacuum columns) then THWACK the tape, then go slack.
>
> That's the same thing happening with the NORD-10's drive. Although it
> fortunately cuts out in time, before harming the tape :)

Well I learned a lot when the servo motors were turned off: That
let me verify that the limit stops worked precisely right.

(For those not familiar with vacuum column tape buffers, there's a
deep rectangular box, open at one end. At the bottom of the box is
vacuum generated by a pump (a noisy pump). There's two boxes, one
upside down, for each reel. It sucks a loop of tape down into the
box. Picture a loop of tape sucked into the box, but held so that
it goes half way; it wants to suck all the way to the bottom of
course. Then...)

For each column, there's two limit (vacuum) switches, one near the
open end. The switch near the bottom should have vacuum on it
(tape blocks off the bottom of the box), the one at the top should
not (tape is sucked past the switch).


That's the limit stuff. The servo loop is actually
straightforward, sort-of. In addition to the limit switches, the
column has a row of small holes into the column box, which on the
backside go into a manifold; as the tape moves up and down the box
more or less holes are passed by the tape. The holes towards the
bottom of the box apply vacuum, the ones above the tape loop are
at atmospheric pressure.  Therefore, given a whole bunch of givens
(pump capacity >> flow of the little holes, etc mostly static
physics) the amount of vacuum (OK pressure you pedants) in the
manifold is proportional to how far down the column the tape is!

Then there's this hokey-looking (but reliable) piston/lever/spring
thing that bends a strain guage, sniffing the vacuum manifold.
This produces a resistance proportional to the amount of tape in
the box.

This is the feedback for the 10-ampere at 12-volts opamp that
drives the tape motor. Linear! Heat! Tape-snapping torque! Dumb as
a bag of hammers. It really needs some fast and sensitive
current-limit (rate ramp-up) to avoid tape-snapping.

Its the sort of thing you make a microprocessor do. Ahem.


Anyways, the point of this: I did not do this step, but you could:
manually arrange for the right amount of slack tape to fill the
buffers half way, and tape down one of the reels. UNPLUG THE
MOTORS from their drivers.  Go through the "LOAD" sequence, with
one hand on the free reel, and position the tape into the columns
correctly. This is fairly easy.

Now you've isolated the free-reel's servo system for open-loop
testing: you can put a DVM on the feedback resistor strain guage
thingie and rock the free reel back and forth and see it produce
an error voltage.

Or better yet, put a "big" resistor in series with the motor to
limit it's torque to something absurdly low (so it won't tear all
the skin off you hand and snap the tape). The D.G. drive wouldn't
mind this, other designs might get upset. DG's tape servo is
simply a gargantuan discrete opamp, all linear and ceramic .1-ohm,
50-watt resistors. No PWM, no nothing! Many watts! Tres simple.

I wouldn't have known to do this at the time, but with the system
LOADed, and the reels taped down into place, motors off,
physically checking for vacuum leaks (pressing covers, wiggling
hoses, etc) would have produced a BIG feedback signal change.
This would have been Wrong and a big hint on what to fix.



Oh yeah, I forgot to say, the hoses that pressed onto the plastic
vacuum switches were stretched out and likely leaking; they were
loose, on and off. I cut off .25" and fixed that. It was an
accumulation of problems. The other end of the hose, that pressed
onto a brass nipple, was NOT stretched out, so it was a
plastic::plastic problem, probably one of them outgassing etc.