[Info-vax] VAX 6000 on Young Sheldon

[gah4]

On Saturday, February 11, 2023 at 10:23:10 PM UTC-8, David Jones wrote:

(snip)
> The appendix describes lrc and crc for NRZ1 format (and crc only for 800 CPI) but says explicitly that PE format has neither.

OK, but NRZI needs it a lot more!

NRZI depends on their being at least one flux transition ('1' bit) in each character.
(Oops for whoever decided to use even parity on 7 track tapes.)
All tracks clock together.  A tiny head azimuth error, and it fails.

PE clocks each track separately.  No problems with head azimuth. 
Another way to look at it, is that PE puts a parity bit on each bit.

I thought that GCR uses CRC, but haven't actually thought it for 40 years.