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 >
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