On Sun, Aug 4, 2019, 9:21 AM James Knott via talk <talk@gtalug.org> wrote:
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. "For a given data signaling rate <https://en.m.wikipedia.org/wiki/Data_signaling_rate>, i.e., bit rate <https://en.m.wikipedia.org/wiki/Bit_rate>, the NRZ code requires only half the baseband bandwidth <https://en.m.wikipedia.org/wiki/Bandwidth_%28signal_processing%29> required by the Manchester code <https://en.m.wikipedia.org/wiki/Manchester_code> (the passband bandwidth is the same). When used to represent data in an asynchronous communication <https://en.m.wikipedia.org/wiki/Asynchronous_communication> scheme, the absence of a neutral state requires other mechanisms for bit synchronization when a separate clock signal is not available." 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.
Some nice pictures of an IBM unit in this link to a manual, for any
other creative anachronists.
http://ibm-14In .info/223-6988-729-MagTapeCE-InstRef-62-r.pdf <http://ibm-1401.info/223-6988-729-MagTapeCE-InstRef-62-r.pdf>
I used to work on drives that looked similar. However, they were made by a company called Potter, but had the Collins branding on them.
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. --- Post to this mailing list talk@gtalug.org Unsubscribe from this mailing list https://gtalug.org/mailman/listinfo/talk