Using a 3.5" 720k disk...

[Chuck Guzis]

On 19 Apr 2009 at 20:13, Philip Pemberton wrote:

>    - If you add something to sync to the index marks and generate a
> hard-sector "emulation" index pattern from that, you can't put a HS
> disc in that drive

Not so--it'd work the same way my PIC solution does--while it's 
sampling the drive speed (and not passing index pulses) it's looking 
for sector holes.  If it finds them, it hangs back and allows index 
pulses to flow through.  If it sees an index pulse every 166/200 msec 
or so, it knows to start generating sector pulses on the next rev.
 
> What about just detecting the speed of the input index pulse? If
> they're turning up faster than, say, once every quarter-second, then
> bypass the index-pulse multiplier. Basically have a multiplexer that
> allows the output to be sourced from /Index_In, or /Index_Generated.

Exactly what my PIC code does.  But the base issue is really the ISV 
characteristics of the drive's speed control.  Direct-drive 3.5" 
drives are probably much better than old 5.25" belt-drive drives.   I 
suspect that consistent speed control issues  may have originally 
been the reason for NEC specifying 8x512 format for a "360K" 
diskette.

--Chuck