B.M.S. tape format

[Tony Duell]

This is on-topic if you think the HP9830 is a computer :-)

I'v been thinking about the internal cassette tape drive of that machine, 
with particular refeerence to being able to read/write the tapes in some 
other machine. By chance I was looking at the technical manual for the 
Racal Thermionic (the company name, it doesn't use valves!) Digideck P72. 
This unit has obvious similarites to internal tape drive on the HP9830, 
in fact some mechanical parts will interchange between the 2 units.

Anyway, it appears the data format is very similar too. 

There are 2 tracks on the tape. A pules on one track is a '0'. A pulse on 
the other track is a '1'. And a pules on both tracks together is a marker.

The format of a byte on the taoe is 

M b b b b f b b b b M

Where 'M' is a marker (and both markers are part of a given byte, by 
default, therefrore, there are 2 markers between the data bits of 
adjacent bytes on the tape), 'b' is a bit of the data byte and 'f' is a 
flag bit. The flag bit, being in the middle, appears in the same position 
in the serial-parallel conversion shift register no matter wheter the 
tape is moving forwards or backwards. I think HP used it to identify the 
bytes in the fiule header or something.

It's not chear whether HP record 2 markers between each byte, or just 
one. But the Digideck can do either.

Anyway, the Racal manual refers to this as 'B.M.S. format'. Has anyone 
heard of this? A google search doesn't find anyhting that looks relevant.


-tony