unreadable HP cartridges

Jim Battle frustum at pacbell.net
Wed Jun 22 20:08:59 CDT 2005

Al Kossow wrote:

> Has the on-tape format of 98xx HP85 or 264x carts been documented 
> somewhere?

Funny enough, I happened to be looking for something else and found this in the 
HP 9825B Desktop Computer Service Manual, page 4-12:

	Digital information is stored on the tape using a delta
	distance encoding scheme.  The "1" distance is approximately
	1.75 times longer than the "0" distance.  The magnetic polarity
	is irrelevant, only the distance is important.  Since the tape
	is driven at a constant speed, the time elapsed between flux
	transitions is measured.


	Reading from the Tape
	Reading from the tape is more complex than writing it.
	Variations in timing and tape speed from system to system
	must be accounted for.  To allow for these variations the
	circuit detects the ratio of the "1" time to the "0" time,
	not the actual elapsed time.  So the first thing that has
	to be done when reading a particular tpae is to esablish
	the duration of a "0" time on the tape.  Tape protocol
	dictates that a gap will be followed immediately by twelve
	flux transitions with the "0" spacing.  These are used to
	esablish a reference for the data which follows on the tape.

	(then there are some details about the actual circuit)

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