Ruud Baltissen wrote:
> The remark was made having an HD-drive in combination with a HD-floppy in
> mind.
I see. Apart from changes that are due to other characteristics of the
different r/w head model, which you also have if you use a different DD
drive, an additional change is only to be made if you really use HD disks
instead of DD disks. Without also increasing the data density, there's no
need to use HD disks, because DD disks work fine with 96 or 100tpi track
density (see 880kb format on the Amiga, or 1MB format on 8250).
> In the early days when I had a 8250 but no matching FD's, I used 48 TPI ones
> with reasonable succes. I wonder if they can be used as well in a 1541/HD
> combination.
Why not. I used 3,5" DD formats (i.e. 720 to 800kb) on 5,25" DD disks in my
PC. Worked fine :-)
> > First the stepper is set to track 1 and then the head
> > bump is executed. After the bump sequence, the stepper register is
> > still set to track 1.
>
> Don't you mean track 0 ???
Yes, physical (and logical on PC) track 0, and logical track 1 on a 1541.
> > it needs to make use of an optical
> > track 0 sensor, which is not supported by the 1541.
>
> My idea is to use this sensor in the hardware, not SW. The moment the head
> is on track 0, the sensor disables any stepping pulse when the direction is
> down, like when performing a "Bump". After the Bump the head is above track
> 0 and PB0/PB1 is alligned ie. PB0 = (L).
>
> The only thing needed is a way to let the drive perform a bump after a
> reset: 1) run a small program or 2) hack the ROM and use the 256 free bytes
> at $C000/C0FF.
You don't even need to write the software for this. The ROM of a 1541C
supports this very well, it does a head bump on every reset.
> IMHO the above proposals don't break compatibility.
This all depends on the hardware you have to convert the phase signals to the
step/dir signals ;-) E.g., I thought of a hardware that would trigger on
changes on PB0 (i.e. move head one step) and then know in which direction
this step is meant, and then generate the correct signal on the Shugart bus.
Now what happens with "illegal" phase sequences that have an unknown effect?
I can think of changing only PB1, then the hardware will not see any
movement, because PB0 doesn't change. But on the 1541 hardware, this change
could move a head not at all (if it was just sitting there), but bring it
into an unstable state, that will then lead to a movement with the next valid
step. But into which direction? And if the head is moving at high speed
already, it might make a double track step in the direction it is moving. And
what with high speed manipulations on that port as it is used by drive music?
Nicolas
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