Nine track file marks, Burroughs

On 01/09/2021 03:20 PM, Dennis Boone via cctalk wrote: 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