Recovering the ROM of an IBM 5100 using OCR (among other things)

[Jon Elson]

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