Subject: More DECtape drive info

From: [email-masked] (Charles Lasner)
Date: 20 May 1994 18:34:57 GMT

Doug's request for DECtape drive info needs to have more stuff explained:

The first drive is the LINC drive.  This was used in the LINC and LINC-8
and uses relays to implement the functions.  However, these relays are an
unusual type: they are capable of switching as fast as 60 Hz, and they
include a coil driver transistor in the case.  They also plug into an
unusual socket complete with mini hold-down straps.  Additionally, the
LINC and LINC-8 support relays as an output device.  There are six of them
and they use precisely the same relay as in the tape drives.  There is a
test program to diagnose sticky relay contacts where you plug the affected
relay into relay register[0] and hook a voltage and the A-D convertor up
to it on two channels.  You get a scope display of the dynamics of the 
contacts as they are switched at high-speed that indicate if they are 
adjusted well, etc.

One particular relay in the tape drive has 110V across the contacts, so 
if it should accidentally make before break, you have smoke!  A common 
ECO was to replace this particular relay with a triac module whose wiring 
is fudged into a relay case with a hole drilled on top, etc.  The module 
itself is then velcroed onto the tape chassis in any empty spot, etc.

The 555 drive used on the PDP-6 and the PDP-5 552 controller is similar.  
However, the LINC drive uses R-series compatible logic levels of -3, 0 Volts 
while the 555 uses so-called "relay logic levels" which are two different 
negative voltages (don't remember the values).

The TU55 includes a jumper module in each drive that can be changed to a
level convertor to make it compatible with the 555.  Other than the 
head-select relays, there are no control relays in either a TU55 or TU56 
(the TU55 is known as the "Solid State" DECtape).

The LINC and 555 drives are belt-driven using a surgical rubber belt with 
cloth inserts.  By now all have worn out.  I have a superior replacement 
belt source where the belts are made of nylon.  The replacements show no 
sign of wear in 12 years, etc.

All newer drives use direct drive motors.  TU55 motor control is 110V 
into SCR's.  TU56 uses power amplifiers to create 40 Hz power into the 
motors.  DC power is used to hold the motors still; less power is used to 
provide tape torque to pull against, etc.  TU55 uses a shaft-mounted 
brake mechanism reminiscent of an automotive air-compressor clutch.  When 
a TU55 is stopped, you can hear the brake apply, etc.

Apparently all of the drive variations in the TU55 world were instigated 
by Astrotype.  This includes the so-called BE01 which is merely a TC01 
with no status panel which not only means no status lights, but also no 
ability to format tapes.  I once upgraded a BE01 to TC01 status by 
canniballizing another TC01 for the relevant cables, panel, modules, 
etc.  It wouldn't format until we hunted down a small missing wire-wrap.  
This missing wrap only affected the ability to format and seemed like a 
sneaky way to disable the function.  (Note: all of the rest of the wrap 
is present to allow the upgrade.)  Once we were done, the only pedantic 
difference between a "real" TC01 and the BE01 is that the nameplate has 
the serial number and ID of BE01 instead of a serial number saying TC01 
from standard production, etc.

Also, there is a unique drive known as a BE03.  In essence, this is a 
dual TU55 attempting to be a TU56.  No unit select is provided; 
presumably the drive is just permanently 0 and 1 or some internal jumper 
can change the drive pair.  The relay-logic/jumper modules are not 
present, so this can only be run on the negative tape buss.  No write 
protect switches are provided.  In fact, the only switches are the 
off-line and motion switches of the TU55 using the same switch 
mechanisms, etc.

Doug has a very early prototype TU55 with the aluminum trimplate on the 
door.  As in the book illustration, it lacks aluminum trim on the sides.  
Newer drives have 4-sided aluminum trim that screws together.  Still 
newer drives use a casting surround that matches the overall appearance 
of machines such as the -8/l and -8/e and are painted antique white to 
match, etc.

TU56 comes in a variant of a "half" drive called TU56H.  Because of the 
built-in power supply, it certainly *weighs* more than half!  Removing 
the dummy cover generally reveals an area easily upgraded.  I have 
demised several TU56H drives to upgrade others to dual status.

A word about TU55/56 guides:

All drives can take the same new ECO guides.  The difference is whether 
to mount with roll pins or pins as well as screws.  Most of the TU55's 
use aluminum guides that cannot be upgraded and which use a flat cover 
plate on each guide.  Very occasionally, the metal will be steel but the 
same overall shape.  The guide plates themselves are made of soft steel 
and are clearly not reversible.

A newer design is always found in a TU56 guide.  In this version, the
front covers are drilled with various holes to support the guide plates
better.  Only a small hole is drilled for the older variant spring.  The 
guide plate area is planed out both in the cover and the guide itself in 
this design.  (The older system uses a deeper cut in the guide and a flat 
cover.)  Some TU55's will be found as ECO'd to the same level as these 
early TU56's.

DEC attempted several ECO's to overcome tape problems, especially when 
the more finicky TD8E came along.  The first attempt was to use a 
reversible ceramic guide plate to replace the soft steel non-reversible 
type.  Field circus liked this, because they could lazily just turn the 
plates over periodically and claim they were "servicing" the machine!  
Additionally, newer rear cheek plates were cut to also be reversible.  
Thus, field circus could dismantle the guides completely and reverse the 
rear plates as well, etc.

However, it was discovered that these reversible front plates actually 
chewed the tape up!  The soft steel plates would develop score lines and 
dirt that caused friction and jerkiness in the tape motion, but wouldn't 
materially hurt the media.  The ceramic plates are tougher, and tend to 
kill the tapes themselves!  A lot of tapes have either a rough edge or a 
"cupped" appearance due to these crummy plates!

Eventually, a better fix was found: The guides themselves were cut deeper 
so that the front plate could march to the rear a few more thousandths in 
case the tape was worn thinner.  Thus, older or damaged tapes could still 
be made to lie flat and be read properly, etc.  (This refers to the tape 
width being less, not the thickness!)

Clearly the ceramic guide plates were pushing too much on the tape, thus 
the cupped appearance, etc.  To counteract this, the front covers were 
redrilled with a larger hole closer to the center of the plate.  A larger 
softer spring was used to allow the tape to lie flatter.

Finally, it was revealed that the reversible ceramic plate was too 
"sharp" for the tape and was a source of wear.  A replacement was 
produced that was exactly the shape of the original one-sided steel 
plate, but made of the same ceramic material.  The only remaining problem 
was a lot of idiot field circus types still assuming that the plates were 
reversible!  (Reversing the non-reversible plate has an affect on the 
tape similar to dragging a razor blade on it!)

A competent machinist can drill the holes in the front covers, and plane 
down the guide area.  Alternate springs can be acquired.  The hardest 
part to get is the ceramic wear plates.  An alternative is to use the 
older steel plates and clean them often.  The reversible plates should be 
discarded.  (Note: perhaps someone can figure out how to modify the 
plates so they are as good as single sinded!  Perhaps some epoxy or other 
resin applied to one side?)

TU56 comes with two series of jumper modules or level convertors.  Most 
PDP-8 applications use a jumper for the relay->negative position, and a 
negative->positive module in the second position on each half-drive.  On 
a PDP-11 TC11 system, both are jumpered since the -11 uses positive 
control logic.  If there are multiple drives, then a straight-through 
cable is used to connect them.  If the drives are on a TD8E, then only 
two drives are supported.  In a contrived case, it's conceivable that 
there would be two TU56H drives with the same cable between them!

But for all other applications, the drives are setup as negative.  If 
there are multiple TU56's involved, the same cable goes between them, but 
the first cable is an asymmetric cable that must have one end inserted 
into the TC01 or TC08 or PDP-12 with the designated other end into the 
first TU56.  The cable is a complicated rainbow cable with no clue as to 
which end is which.  An ECO is to attach labels to it to tell!

Should TU55's be involved, they must come first as they use cabling 
compatible with the controller and are straight-through, etc.  
Additionally, there are head cables to transport the head buss between 
chassis.

TU56 uses an on-board power supply to create sundry voltages needed 
within the drive.  However, an external supply is still needed to provide 
-15 and either +10 or +5 volts.  One of the TU56 modules includes a +10 
-> +5 regulator that will automatically be used if required.  However, if 
the drive is being used for a TD8E, then 5 G888 modules are placed within 
the (first) drive.  (Note: these are the same modules as in the 
TC08/TC15.  The PDP-12 and TC01 use the G882 which is esssentially the 
same card only with negative logic levels, etc.)  In this case, +5 is 
mandatory since otherwise it overloads the regulator!  Thus, most 
TD8E-era TU56's use an external small -15, +5 supply while older drives 
use the older negative unregulated power supplies.  Additionally, the 
TC08 supports a +10 -> +5 regulator on its own chassis, known for getting 
quite hot (and occasionally failing!).  TC01 and TU55 use the older 
supply directly.

There is a problem with the built-in power supply:  A noise-inducing 
grounding problem was revealed.  Apparently field circus invented its own 
fix to the problem.  Unfortunately, it only made the problem worse!  
Thus, it is necessary to ensure their fix is removed, and the official 
one is added.  The unacceptable fix is a wire added internally in a 
roundabout manner.  If present, remove it.

The proper fix can be done either internally or externally.  There are 
multiple power tabs on the supply and a pair of black ground tabs, one 
near the top and one near the bottom of a stack of various power tab 
connectors, etc.  Late production supplies do the ECO internally by 
soldering a wire between these two.  An acceptable alternate is to 
provide those little brass faston expander lugs while the supply is 
inside of the drive and just jumper the two ground terminals together.  
(Note: there should also be a ground wire from one of these to the front 
plate.  This prevents problems due to the pivot point not really being a 
good ground!)  Unfortunately, it's necessary to remove the supply, and 
take it apart to ensure the field circus abortion isn't present!  Having 
both causes a ground loop!

The only exception is a new production supply.  Most look just like the 
earlier supplies, so you still have to take them apart.  (The field 
circus method was quite widespread; drives that were reliable for years 
and then become unreliable are traceable to this mistake!)  However, 
final ECO supplies also include another change obvious from the back 
side: they include a large diameter fuse holder.  This is a fuse for the 
110V receptacle against too much equipment being plugged into the power 
daisy-chain.  The fuse holder is designed to be replaced without supply 
disassembly and is quite obvious if present.  I also have a supply that 
was upgraded to include the fuse holder; it's obviously a refurb job 
because the silkscreen lines don't match, etc.  (And opening that supply 
revealed no internal ECO, either the field circus or the correct one; so 
I added the internal soldered one, etc.)

Drive hubs:

The original drive hub is known as a Hartman hub.  On the LINC-8 drive, 
it's a light mechanism that accepts the reels with a satisfying "snap" as 
you apply the reel.  It's quite easy to remove the reel.  The outermost 
part of the mechanism is a coil spring that gets compressed as you snap 
the reel over it.  The spring is fairly wimpy and has to be "enhanced" by 
removing it from the hub and adding in rubber bands before replacing the 
spring.  In any case, you can get just enough tension that worn reels 
won't slip without needing excessive force to take off new reels, etc.

Unfortunately, on the TU55, someone overdesigned the Hartman hub into a 
disaster!  The reels go on easily, but come off with great difficulty.  
Most TU55 drives show signs of scratches from shoehorns, screwdrivers, 
can-openers, etc. that people have used over the years to remove the 
reels!  Fortunately the back plates are made of aluminum, not plastic!

An ECO is to replace the hubs with TU56-type spider hubs.  Cheap field 
circus-maintained sites would only change one hub on the theory that only 
rarely was the other side needed to be removed.  (Note: which side is a 
take-up reel is determined by whether you use LINCtape or DECtape!)

Some unscrupulous field circus idiots have been found to put old Hartman 
hubs on TU56!  This is the worst possible situation because now you are 
applying screwdrivers to plastic :-(.  Needless to say, this is *not* an 
authorized change!

TU56-type spider hubs come in two varieties.  In one case, the setscrews 
are 180 degrees apart.  The preferable ones are more like 90 degrees, but 
actually a bit more.  (They break less!)

Spider hubs hold the tape well, but not as good as LINC drive Hartman 
hubs.  You have to hand-locate the tape so it doesn't run too much out of 
round, etc.

LINC drives use a one-piece aluminum guide with no moving parts.  In 
spite of the lack of wear plates per se, the gentler drive (it moves at 
2/3 the speed!) neither wears nor wears out tapes.

Most TU56's have a nice plastic front panel behind the drive hubs.  Late 
production drives have a "manufacturing ECO" that deletes the plastic 
panel and merely spray paints the bear cast metal!  (Ugly!)  I have 
deliberately rebuilt a pair of TU56H that were "new" into an "old" 
chassis front!

There is a curious problem associated with the negative level converter 
modules for TU56.  Attempts to write code on both the TC01/08 and the 
PDP-12 to handle tape motion have been thwarted by what appears to be an 
ECO problem with this module.

When a too *new* module is used, it interferes with certain programs:

The OS/8 REWIND and UNLOAD commands were designed on a machine with 4 
pairs of TU56.  (It was also used to produce "certified" DECtapes often 
misformatted!)

The rewind command is supposed to initiate a reverse move command and not 
wait.  If no command is given to the tape system, the END error will stop 
the tape when it's fully rewound, etc.  Should another tape operation 
(such as a read) be started on another drive, the rewinding drive is 
supposed to stop moving.

But on this machine (or similarly afflicted) the tape *continues* to 
rewind while the other selected drive is accessed normally.  (One could 
call this a "feature" but it's not supposed to happen!)  Additionally, 
the unload code often fails on normal tape systems, functioning only on 
similarly defective systems.

In P?S/8 there are keystroke commands to rewind and unload the tapes as 
well.  However, the unload operation is designed to mimic the TOPS10 
equivalent: the tape is rewound into the end zone until the end error 
occurs.  Then, the tape motion is deliberately reversed and then 
cleared.  This has the effect of locking the brakes if on a TU55 and 
merely stopping on the TU56 (after the tape is already wound off; the 
difference is that on a TU56 the tape just flaps, while on a TU55 the 
reel is also stopped).

Unfortunately, on a defective machine the rewind works as in OS/8, and 
the unload merely does a rewind.  Additionally, the P?S/8 binary loader 
program will not properly deselect multiple tapes (again seeming to mimic 
the rewind phenomena).  There is a one-word patch to prevent this, but it 
degrades performance on normal machines!

The only time this situation shows up is if there are multiple TU56's and 
the ECO level of the negative level converter module is too high!  Single 
drive pairs don't exhibit the problem.

cjl