From: prp@SSD.intel.com (Paul Pierce)
Subject: Re: half-inch tape: transport design
Reply-To: prp@SSD.intel.com (Paul Pierce)
Organization: Intel
Date: Wed, 22 Jul 1992 01:32:52 GMT

In article <jcmorris.711736167@mwunix>, jcmorris@mwunix.mitre.org (Joe Morris) writes:
|> 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                          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 |
|>             \----/                  \----/
|> 

The loop in the column does not touch the sides all the way down. There is a roller (idler or prolay) on either side of the vacuum column. The rollers are closer together than the sides of the column, so the tape starts away from the sides and only gets close at the very bottom of the loop. So instead of looking like a U with flat sides, the tape in a column looks like a U with the straight parts slanted toward each other at the top.

Blowers push filtered air into the whole top part of the drive, and vacuum pumps pull air from the bottom of the vacuum columns. At the tape loop, more air comes down the inside of the U, so the loop balloons out at the bottom, but some air comes down the outside of the U and keeps the tape loop at the bottom from touching the sides of the vacuum column.

So the tape doesn't touch (much), but that doesn't answer the question of why its oxide side out. I don't know and doubt that the answer is in my 729 CEM's, but I'll look sometime. One thing to notice is that it keeps the oxide away from the rubber capstan and stop wheel.

                   /                  O <- stop wheel
                  /                   ___________________________________
         idler -> |O    capstan -> O |O <- prolay         (head)
                  |                  |
                  |                  | <- oxide side
                | |   vacuum column  | |
                | |                  | |
                | |                  | |
                |/         o          \|
                |\____________________/|
                |          o           |

The little holes along the centerline of the vacuum column go to pressure switches, which is how the drive knows how much tape is in the column.


These ASCII pictures are pretty rough, maybe we should start posting GIF's!

By the way - I may be making some copies of the IBM 650 reference manual. Any interest?

|> Joe Morris

-- 
Paul Pierce			prp@ssd.intel.com
Intel