Reading MT/ST Tapes

Chuck Guzis cclist at
Fri Jul 30 21:34:14 CDT 2021

On 7/30/21 6:22 PM, Paul Berger via cctalk wrote:

> The MT/ST did pretty good for being a electro-mechanical device,all the
> logic was relays in it.  I seem to recall many years ago one of the old
> OP guys telling me that it write in stripes across the tape. It would
> have to be some very simple format because it would be hard to have the
> thing sync on headers with only a little relay logic.

Like a motion picture film projector.  Brings the tape to a complete
stop for each character and then scans across it with a single head;
going in, the character is read, going out, the character just read is
checked.  I assume (but am not sure) that if the check fails, a retry is
attempted.  The head moves at 45 ips and records bits at a 45 degree
angle relative to the tape axis.   This is so the tape can be scanned
without moving the head for a mark in the control track (reading
parallel to the axis of tape movement) or reading characters with the
tape stopped (reading orthogonal to the tape movement.  Obviously,
precise tape positioning is important (even at 20 cpi), hence the
sprocket feed.

In off-list conversations with others, I keep trying to impress on the
younger folks that this is basically an electro-mechanical device with
heavy emphasis on mechanics.  After all, the people who serviced these
things were typewriter repair people.  I doubt that the innards of the
MT/ST were much more complex than those of the Selectric itself. (One of
these days, I'll get up the nerve to replace the motor drive belt in my
Correcting Selectric III).

But when you've grown up with microprocessors, I guess it can be hard to
envision a world with only rudimentary electronics.


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