4 Dec
2005
4 Dec
'05
10:59 a.m.
>On Tue, 29 Nov 2005, Chuck Guzis wrote:
>> The bigger problem isn't that it's GCR, but rather that it's also
variable
>> rate GCR. I'd remove the quotes from "impossible" on this one,
although,
>> maybe with a razor blade and a magnet and a VERY steady hand...
>
>How fast can you turn the MOTOR ON line on and off? :-)
The real thing that changes between cylinders on the old Mac drives is
the number of bits per revolution. To read/write this you can either :
1) Keep the bit rate from the contorller constant and vary the disk
rotation speed (this is what the Mac did)
2) Keep the spindle speed constant and vary the data rate at the
controller. THis, incidentally, is what Commodore did in the PET drives,
etc. Of course if you do it this way you need a drive with a read filter
that's sufficiently 'wide' not to mind the varying data rate.
The Outbound Laptop Model 125 (ca. 1989 Mac laptop clone) uses a
Citizen brand PC type laptop floppy drive. It reads and writes 800K
Mac floppies as well as 1.4 MB floppies on both platforms. However,
it has a controller card on the end of the floppy drive.
My guess is that the spindle speed is kept constant (300 rpm) and the
data rate is fiddled about with.
The main components of the floppy controller card are an 85C30,
WD37C65, WD92C32 (confused as to why this is there, since the 37C65
has a data separater built in), an LS624 voltage controlled
Probably bevause they needed to do some tricks that the internal data
seperator coudln't handle (e.g. signals were entirely within the IC can
couldn't be monitored or intercepted).
oscillator, a Xicor X9103 digital potentiometer, a
27C256, and a
Well, VCO + digital pot == programmable frequency oscillator. Just what
you need to fiddle around with the data rate :-)
I wonder what the 85C30 is for? THat's a fancy serial chip, used in the
Mac for the 2 serial ports. I wonder if it's used here, with a bit of
extra logic, to handle the data from the floppy disk?
-tony