Free: Stack of Victor or Sirius floppy disks

[Ethan Dicks]

On Fri, Mar 4, 2011 at 2:22 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
>> CBM disks did that (one of the 65xx processors could affect the
>> low-order two bits of a divide-by-N chip), and so did 400K Mac floppy
>
> Actually, Commodore drives (at least the 8050 and 1541) kept the spidnle
> speed constant and speeded up the data clock on the outside tracks.

I did not intend to imply that CBM drives changed the spindle speed.
Unfortunately,
I failed to explicitly state the beneficiary of the divide-by-N chip
as being the data
clock, so I can see how it can look that I suggested that it was the motor speed
that was variable.

> Yes. The 400K drive (which I believe is much the same as the drive Terry
> was trying to fix) has an input for a PWM motor speec control signal. On
> the mac, a counter is loaded from 8 bits of an unused video RAM location
> at the end of each scan line (the other 8 bits go to a similar PWM
> cirucit for the sound IIRC).

Interesting.  I did know the mechanism to that level of detail - I just knew the
speed varied enough to be audibly detected.

> The Apple 800K drive is alos variable-speed, but IIRC the speed control
> is handled entirely with the drive. The speed is set depending on what
> cylinder the heads are on.

I wasn't sure what was up with the 800K drives, so I didn't say anything.  I
did recall that there was some difference from how the 400K drives were
designed, but I didn't know what.

Thanks for clearing up any confusion I may have caused.

-ethan