27 Jun
2019
27 Jun
'19
9:28 a.m.
On 06/27/2019 10:21 AM, Warner Losh via cctalk wrote:
Back when I got to school and I was hanging around the computer room on
campus (back when it was THE room on campus with computers), I saw this
half-dollar sized plastic fob on the desk and asked what it was for. The
on-staff operator took a mag tape off the rack, opened it up and set the
end of the tape on the table. She then took the fob and placed it on the
end of the tape and all the iron filings that were suspended in the liquid
inside the fob aligned to the magnetic fields of the tape. They used it to
tell the difference between 800, 1600 and 6250 bps tapes so they could
handle the tapes correctly
800 BPI tapes had no recording over the BOT marker.
1600,
3200 and 6250 tapes had different tracks with a burst of
ones and zeroes across the marker, that uniquely identified
the density.
It had to be a really SIMPLE scheme as the drive itself (not
the formatter) needed to detect this and set various
circuits correctly, like read preamp gain and slicer threshold.
Jon