Tape reel data recovery from MERA-400 polish computer

[Jon Elson]

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