Magtape write rings [Was: Re: Full immersion emulation]

Jon Elson elson at
Sun Mar 5 14:18:27 CST 2017

On 03/05/2017 12:41 PM, Mark J. Blair via cctalk wrote:
>> On Mar 5, 2017, at 09:49, Chuck Guzis via cctalk <cctalk at> wrote:
>> I don't recall many who used those yellow rings for their intended
>> purpose.  It was just too easy to forget to remove one.  You mounted
>> tapes without and reached around the back of the reel with a punch card
>> and tripped the "finger" that detected the ring.
> So, was the write enable state latched at some point in the loading cycle on those drives? That surprises me, because I would have expected the write enable sensor to interrupt write current as combinatorial function on the drive, and/or pass sensor status up to the formatter a combinatorial signal.
On a number of drives, there was a pneumatic latch!  When 
the vacuum motor started, there was a little air pump there 
that produced air bearing air (or in the REALLY bad old 
days, air pressure to blow the tape away from the vacuum 
capstans).  That air pressed a finger against the write 
ring.  if the write ring was present, the finger was 
blocked, and then retracted, and the write was enabled.  if 
the finger was not blocked (no write ring present) then the 
finger extended and locked out as long as air pressure was 
present (until the vacuum motor shut off.)

Some other drives had a ring that was forced back when the 
write ring was present.  The ring had reflectors on it, and 
a photocell read the reflector during the tape load sequence 
to set the write enable FF.

You couldn't just leave a probe against the write ring all 
the time as it would cause the write ring to work out of the 
groove and jam the reel hub.

Mention above about the vacuum capstans. Some really old 
drives had two continuously counterrotating capstans with 
slots in them. Valves applied either air pressure, to make 
the tape float over the capstan, or vacuum, to make the 
capstan grab the tape.


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