Re: GCR <--CP/M --> MS-DOS and vice versa?

[mjmahon@aol.com (Michael J. Mahon)]

In article <RNOcnRIICapsafqjXTWQlg@giganews.com>, "Bryan Parkoff"
<BParkoff@satx.rr.com> writes:

>    I am working on the parallel port that it connects to floppy drive.  I
>write Parallel Port Device Driver that allows to be manipulated by SOFTWARE
>that will be able to read/write both GCR and MFM encoding without a need of
>hardware controller card.  It can be very tough while it is easy to
>understand GCR structure that is written binary WHILE I have to learn to
>read MFM structure without using MFM hardware controller card.
>    It is only a project, but it is not available for public.  Please feel
>free to provide some ideas.  Also, I am considering to develop USB port that
>will require to have hardware circuit.

Interesting.  I took a shot at doing this very thing (interfacing a Disk ][
to a PC parallel port) several years ago--when 166MHz seemed fast
enough to decode anything on the fly.  ;-)

Then I found out about the speed limitations of the PC's legacy I/O.
No matter how fast your processor is, you won't be able to sample
the parallel port input(s) faster than about 500K times per second.
That is, unfortunately, not fast enough to reliably capture the 1
microsecond read pulses, whose max rate is 250K/sec.

It would require an outboard shift register, appropriately clocked, to
capture the read pulse stream.  If it's just a 2MHz clock, like the
Disk ][ Controller, then it will fill up an 8-bit shift register in 4
microsec.,
(one bit time!), but it is at least doable.  Then you decode the stream
using a software version of the Woz state machine.

A slower capture rate is possible (one nibble every 32 microsec.) if
you use a state machine or other clock recovery algorithm in hardware
to clock the shift register at 250kHz, phase locked to the data stream.
But that is tantamount to interfacing a Disk ][ Controller to the PC
parallel port--which _is_ doable, given power, clocks, etc.  ;-)

-michael

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