On 03/10/2017 01:40 PM, Paul Koning via cctalk wrote:
A stretch with no transitions occurs of course at the
block boundary (the gap). It also apparently occurs in
other spots, for a few bit times: tape marks seem to
consist of two frames separated by a few clocks worth of
blank space. Ditto between data and block check frame(s),
if I remember right.
The classic NRZI definition for 800 BPI 9-track is that
there is a 2-character gap after the end of the user data
block, then the CRC and LRCC characters follow. We had some
tapes made on out IBM 360 that had little short gaps all
over them, as they had the machine misconfigured and were
exceeding memory bandwidth. The IBM could read them fine.
I think they only detected the big gap to define the end of
the record. Our DEC controllers all used the 2-character
gap to detect where the block was ending. No way to fix
that if the hardware tape control decided those gaps were
the end of data.
Jon
