Re: Why can't Gs 800k drives read 720k IBM disks?

[JB3819@JB3819.SBC.COM]

In article <797758948.AA00397@tlp.apana.org.au>, <peterm@tlp.apana.org.au> 
writes:
> 
> This doesn't explain why cards like CTI's FDC-10 for the Apple II can produce 
> 800K ProDOS MFM 3.5" diskettes using standard IBM-style drives. I believe 
> that the real reasons the IBM-style drives were 720K were:
> a) IBM's inherent conservatism. 720K was easily achieved and worked well.
> b) 720K was twice the capacity of the 5.25" diskettes that were of the same 
> era.
> Similarly, Apple seems to have a love-affair with multiples of 400K since DOS 
> 3.3...
> 
> My 2c worth...
> 
> *<8o)X
> Regds,
>  Peter M.
> 
Short answer:
   How it gets written is more important that what gets written.

Longer answer:
   The electrical encoding scheme used to write bits onto the media is 
enough different that heads set up for GCC can NOT read MFM and vice 
versa.  Whether it is 720K or 800K is a different question and your 
(a) and (b) may apply.  So, if you use a controller card that'll 
handle an IBM drive (MFM) you can, within physical limits of the drive, 
write whatever you want on it, including 800K ProDOS formatted data.  
However, because of the encoding differences between GCC and MFM, an 
Apple drive (GCC) can't even read the BITS on the diskette.

P.S. I may have misremembered the acronym for the Apple encoding scheme.


JABouchard  "Poindexter"
j.bouchard3@genie.com