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

Jon Elson elson at pico-systems.com
Thu Jun 27 11:28:22 CDT 2019


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


More information about the cctech mailing list