On 2019-08-04 08:09 AM, Russell Reiter wrote:
> Also interesting is that NRZI seems to have two definitions. Non
> Return Zero Inverted or NRZ-IBM.
>
NRZI was created by IBM, specifically for use with tape drives. They
were one of the earliest, if not earliest to use mag tape. I read the
technical reason for "inverted" many years ago, but I have forgotten the
details. Often that sort of thing is done to obtain best performance
from something. One such example was the use of odd parity. From a
strictly error detection point of view odd or even will work, but with
odd, there will always be one "1" bit for clocking, as I mentioned.
From looking at the manual, inverted might be a reference to their NOR & XOR logic gates.
Check out the wiring patches on the unit in the manual you can see what a cluster fork that could turn out to be if you had to troubleshoot it, especially where line voltage is used for sync.
From your Wikipedia link it is indicated that NRZI was designed to work with or without a clock sync. A term I never heard before, off keying, refers to using the actual line polarity to determine if the logical state is 0 or 1; that is where line clock tic is not used.
Here's the second paragraph from your link. There are secondary data sync methods when there is no specific timing signal multiplexed into the stream.
It goes on to say that NRZ draws half the bandwidth of RZ encoding. I guess that's why there are numerous validation methods built into NRZ. Thats an attractive feature for in house IT, at a time when they have to cobble their own systems together with parts from different manufacturers.
How often would you do routine servicing, as opposed to repairs? They look like huge dust magnets to me and I can't see dust and magnetic tape playing well together.
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