Nine track file marks, Burroughs

[Jon Elson]

On 01/09/2021 03:20 PM, Dennis Boone via cctalk wrote:
> Folks,
>
> I've now seen two Burroughs tapes where some of the expected file marks
> between labels and file data were apparently missing.
>
>
NRZI file marks are only a couple flux changes, drives that 
have squelch logic to suppress unstable read amps could 
easily miss them.  PE file marks have preamble and postamble 
clock data, and it should be quite hard for them to just 
disappear.

Now, one little detail is not all formatters followed the 
specs for interrecord gaps.  The NRZI spec required insanely 
huge gaps, likely designed for very early capstan and pinch 
roller drives.  The gaps were several times the size of 
common data records.  Many later formatters cheated on this 
and used much smaller gaps.  Some formatters might miss a 
file mark due to incompatibility of the gaps.  But, that 
seems less
likely at 1600 BPI, I think the required gap was reduced in 
the standard.

Your tape dump looks very much like a classic ANSI tape 
label format, except for the missing file mark after the 
HDR2 record.  Are ALL those file marks after HDR2 missing, 
or just some of them?
I can see it being quite reasonable for there to be no file 
mark after HDR2, if you were going to write
your own format.  That specific FM is just not actually 
needed if you KNOW the first data record follows HDR2.  So, 
the format could be :

VOL1
HDR1
HDR2
data
data
FM
HDR1
HDR2
data
data
FM

etc.

I can't say I have actually encountered such a tape before, 
but I wouldn't be surprised.  But, of course,
that would not be fully ANSI compatible.

Jon