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

[Brent Hilpert]

On 2017-Mar-05, at 11:27 AM, Chuck Guzis via cctalk wrote:
> On 03/05/2017 10:41 AM, Mark J. Blair via cctalk wrote:
>> 
>> 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.
> 
> Yup.  You loaded the tape, then bumped the "finger".   As I mentioned,
> this worked on 60x and 65x drives.  Since the autoloaders closed the
> door as part of the cycle, this probably didn't work for 66x.
> 
> This actually worked pretty well--you never ran the risk of leaving a
> ring in inadvertently.  The consequence of forgetting to enable write
> was usually far less dire than mistakenly writing to a tape that was
> supposed to be read-only.


In my limited experience of drives, but I expect it's typical, the write enable
is electro-mechanically latched: if the write ring is present at load, a solenoid
is activated which pulls the sense finger further in to hold the finger off the write ring,
so it won't be a friction and wear point while running.
The solenoid is released at unload.