Subject: TU55 DECtape drives

From: [email-masked] (Douglas W. Jones,201H MLH,3193350740,3193382879)
Date: 19 May 1994 14:34:43 GMT

For all those who remember or save pre 1970 DEC hardware, I'm interested
in trying to pin down the variations on the TU55 DECtape drive.  DECtape
was DEC's original answer to the market that was eventually satisfied
by the floppy disk.  A DECtape reel is 4 inches in diameter by an inch
thick, with 3/4 inch wide tape.  It is block addressable, bidirectionally
readable, and as conventionally formatted for the PDP-8, it had 1474
blocks of 129 words, 12 bits each; that translates to 285219 kilobytes,
which is not too different from early floppy disks.  Furthermore, DECtapes
were well known for their fault tolerance.  It was common to be able to
recover data from a reel reel of DECtape that had been unrolled, crumpled,
and even punched with holes.

Back to the story.  DEC made two major varieties of DECtape drives, the
TU55 (essentially pre 1970) and TU56 (essentially post 1970).  Below is
a bit of ASCII art showing the basic configuration of a pair of these
drives, mounted in an EIA standard 19 inch mounting rack:

          | |_____________| |
          || s s s   s s s ||             s = switches
          ||   A       A   ||   <-- TU56  A = record/playback head
          ||(O) (O) (O) (O)||            (O)= spindle
          ||_______________||
          ||         s s s ||             Note: Drive have no capstain,
          ||   A           ||   <-- TU55  no pinch rollers, and only
          ||(O) (O)        ||             a few switches (load unload,
          ||_______________||             forward reverse, and unit
          | |             | |             number selection).
          | |             | |

Now, to my quest!  I have 4 TU55 drives in my collection (none yet
restored to operating condition).  Here are pictures of the two basic
varieties I have:

           ____________________________________
           ------------------------------------
           |                  --------------- |   Serial number 166
           |  --------------  | ||| |-| ||| | |   looks like this,
           |  |   - /\ -   |  | ||| |-| ||| | |   roughly.
           |  |  /_||||_\  |  |             | |
           |  |        __  |  |             | |   Aluminum trim along
           |  | _\/_  /  \ |  |-------------| |   the top and bottom
           |  |  /\   \__/ |  |  DECtape55  | |   only, and an aluminum
           |  |____________|  |-------------| |   nameplate on the door
           |                  |_____________| |   over the wirewrap.
           |__________________________________|
           ------------------------------------
           ____________________________________
           | -------------------------------- |
           ||                 ---------------||   Serial number 439
           || --------------  | ||| |-| ||| |||   is about like this.
           || |   - /\ -   |  | ||| |-| ||| |||
           || |  /_||||_\  |  |             |||
           || |        __  |  |             |||   Aluminum trim along
           || | _\/_  /  \ |  |             |||   the top, bottom and
           || |  /\   \__/ |  |             |||   sides, and nothing
           || |____________|  |             |||   on the door but black
           ||                 |_____________|||   paint.
           ||________________________________||
           ------------------------------------

In both of the above pictures, the ||| symbol represents two adjacent
rocker switches.                   |||

In both of the above pictures, the |-| symbol represents a whopping big
thumbwheel unit select switch.     |-|

My final two drives, serial numbers 2078 and 2104, were made under the
Astrotape OEM label.  They've got a simplified user interface, with
no unit select thumbwheel switch and only three rocker switches, one
on each side and one where the unit select is.  Other than that, the
silkscreened Astrotape logo on the door, the lack of a cooling fan
behind the front panel, and differences in the 110 volt wiring, they're
identical to number 439.

I understand that there was later variant with plastic trim around the
front panel.

So!  If you have some TU55 DECtape drives, could you fill out this little
survey for each one you can find?

   Serial number:
   Has it got an aluminum nameplate?
     If not, has it got a painted on logo?
     If it has a logo (aluminum or painted) what is it?
   Has it got aluminum rails around the edges?
     If so, are they just top and bottom?
     If not, has it got plastic trim around the edges?
   Has it got a cooling fan over the motors?
   Is it currently in operational condition?

					Doug Jones
					[email-masked]

---

From: [email-masked]
Date: 19 May 94 22:16:43 MDT

The piece of TU55 that I have looks similar to this. However, I have aluminum
trim on all four sides. I also don't have the star hub; both of my hubs are
round.

>    Is it currently in operational condition?

Sadly, no. You see, the guy I got this from collects old stuff as raw
materials for his projects. He's interested in motors, switches, etc. The
only reason I have the face plate is that he didn't know what to do with
it, so he made me a kind of artsy thingy from it. I only have the face
plate.

Kind of a depressing thing to do to a machine, I know. He's the same guy
that I got my PDP-5 front panel from; if I had had any idea what the machine
was at the time, I would have held out for the entire thing (it's not like
it had PDP-5 written on it or anything; just a collection of assorted
logic modules with a brushed stainless front panel. The front panel doesn't
even say PDP-5). I also managed to rescue the CPU half of a classic 8 from
him; the only motors, etc. were the fans, which he wasn't terribly interested
in. I couldn't get the rest of the logic because he was determined to open
up the core to see what it looked like inside.

Sigh; now I'm all depressed...
-- 
----------------+------------------------------------------------------
Roger Ivie      | Don't think of it as a 'new' computer, think of it as
[email-masked] |     'obsolete-ready'

---

From: [email-masked] (Chris Grace)
Date: Mon, 23 May 1994 06:15:33 GMT

Douglas W. Jones,201H MLH,3193350740,3193382879 ([email-masked]) unleashed on the world:
: For all those who remember or save pre 1970 DEC hardware, I'm interested
: in trying to pin down the variations on the TU55 DECtape drive.  DECtape
: was DEC's original answer to the market that was eventually satisfied
: by the floppy disk.  A DECtape reel is 4 inches in diameter by an inch
: thick, with 3/4 inch wide tape.  It is block addressable, bidirectionally
: readable, and as conventionally formatted for the PDP-8, it had 1474
: blocks of 129 words, 12 bits each; that translates to 285219 kilobytes,
                                                        ^^^^^^^???????
: which is not too different from early floppy disks.  
 If early floppies were this big why bother with hard drives?
(Obviously a typing error, but couldn't resist it. A 285MB floppy?
-- 
Chris Grace      TransData Corporation Ltd        Auckland       New Zealand
[email-masked]  [email-masked]  CIS:76616,2042  Bix: cgrace
Phone: +64 9 275 3191                                     Fax:+64 9 256 0088