On Wed, Jun 03, 2009 at 12:04:20PM -0700, Luke Crook wrote:
> I have a Rabbit turbo tape cartridge for the C64 from Eastern House. A wire
> runs from the cartridge that is meant to be jammed into the cassette port,
> connecting to (I think) the Cassette Read pin. * *Are there any particular
> advantages to this kind of tape fast loader?

Unlike the PET and the VIC-20, whose tape port is connected to a
MOS 6522 VIA, the CASS READ signal of the C64 is connected to the -FLAG
input of a MOS 6526 CIA.  That input can only detect falling edges.
The corresponding input on the VIA can detect falling or rising edges,
and if memory serves, you can also read the current state of the line.

Could it be that the authors did not want to work around this hardware
limitation of the C64?  For decoding, it should suffice to know the time
intervals between successive falling edges.  The Tangerine Oric-1
(a 6502/6522-based computer with a primitive teletext-like video chip)
writes "half-cycles" on its tapes.  I implemented read and write support
for the Oric tape format in the "c2n" utility, even though the C2N232
firmware only reports the times between falling edges.

	Marko

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