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.
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.