From: jgd@csd4.csd.uwm.edu (John G Dobnick)
Subject: Re: half-inch tape: transport design
Originator: jgd@csd4.csd.uwm.edu
Reply-To: jgd@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Sat, 25 Jul 1992 04:00:57 GMT

From article <jcmorris.711736167@mwunix>, by jcmorris@mwunix.mitre.org (Joe Morris):
> Since half-inch tape has appeared in threads here, I've got a question
> that's been bugging me for decades: why did most of the mainframe drives
> (especially IBM's 729 and 7330 drives on older equipment and 24xx drives
> on the S/360) use threading patterns which brought the oxide into sliding
> contact with the vacuum column walls?
> 
> The 729 and 2401 drives were very similar in design.  The tape path
> looked something like this:
> 
>      supply reel                      takeup reel
> 
>           0xx        ___________         xx0
>         0 0 0x       |  gate   |        x0 0 0
>           0  x       \_________/        x  0
>              x    xxxxxxxxxxxxxxxxxx    x
>            \ x   x/     (head)     \x   x /
>             |x  x|                  |x  x|
>             |x  x|                  |x  x|
>             |x  x|   <- vacuum ->   |x  x|
>             |x  x|      columns     |x  x|
>             |x  x|                  |x  x|
>             | xx |                  | xx |
>             \----/                  \----/
> 
> Since the tape was wound with the oxide facing the reel hub, the result was
> that the oxide scraped along the walls of the vacuum columns.  A routine 
> chore for operators of tape-based systems was the once-per-shift (minimum!)
> cleaning of the tape columns to remove the accumulated scraped-off oxide.
> 
> Again the question: why were the older drives designed to put the tape
> oxide in sliding contact with the vacuum column walls?

I don't know the "real" reason for this, but I can make a guess.

I've added a piece of the drive you neglected to your drawing above --
the piece labelled "gate".   This was, on some IBM drives I've seen,
a hinged "gate" that opened automatically when a tape was unloaded,
so as to provide easy access to the tape path and the R/W heads.
This "gate" also containd little things like pinch-rollers and tape
guides used to move the tape.

Now, since these old drives were 7-track NRZI, with all their attendant
problems of data clocking, etc., it was *vital* that things like R/W 
head alignment be maintained with _extreme_ precision.  Shift the heads
on one drive, and tapes written there wouldn't be readable on the other
drives.  Imagine the headaches _that_ could cause in a heavily tape
oreinted data processing operation.  And all DP sites that used tape
_were_ "tape oriented", since there wasn't anything _else_ to store data 
on.  (This is before the advent of drums, much less disks.)o

Now, consider what happens to this precious alignment if you are continually
moving and shifting the heads, if they were mounted in the "gate".  They
would very soom be _out_ of alignment, leading to much gnashing of teeth
by the DP operations people.  It would mean, essentially, that you
could only read a tape on the one _specific_ drive that wrote the tape,
and then only if that drive was aligned the _same way_ as when the
tape was written.  A nightmare waiting to happen.

Now, note that with tapes in the columns oxide side "up" (away from the
column walls), the r/w heads would have to be in the "gate" assembly.
Mounting the heads on the fixed (immovable) part of the drive requires
the tape to be loaded the other way around -- oxide "down", and thus
against the column walls.

As for oxide "scraping" on the columns, at least later drives used "air
bearings" to "float" the tape away from actual contact with the walls.
Not that this always worked, mind you. :-)

Now, does anyone really *know* why things were as they were?
-- 
John G Dobnick                          ATTnet: (414) 229-5727
Computing Services Division             INTERNET: jgd@uwm.edu
University of Wisconsin - Milwaukee     UUCP: uunet!uwm!jgd

"Knowing how things work is the basis for appreciation,
and is thus a source of civilized delight."  -- William Safire