From: god_free_jones@yahoo.com   
      
   On Sun, 21 Jan 2007 13:54:17 +0100, Knut Roll-Lund  wrote:   
      
   >Michael Black wrote:   
   >> Dave Griffith (dgriffi@cs.csbuak.edu) writes:   
   >>   
   >>>Richard VanHouten  wrote:   
   >>>   
   >>>>I seem to recall that the original Mac floppy drives were 3.5" 180k   
   >>>>drives.  If these are those, they won't be compatible with anything else.   
   >>>   
   >>>No.  The original Mac floppies were 400k   
   >>>   
   >>   
   >> Yes.   
   >>   
   >> Single sided 3.5" would have been 400k (for the Macintosh) or 360K for   
   >> the "IBM PC types".   
   >   
   >   
   >So was the AppleII 3.5" drives too (400K) (and also 1.4M later). The   
   >400k apple drives are special; they have a special card that does a   
   >completely different format (encoding). This is similar to the DiskII   
   >5.25" drives. BTW AppleII is not Macintosh although they shared the 400K   
   >drive format.   
   >   
   >>   
   >> Single density and single sided 3.5" drives would have given about 180K.   
   >   
   >how? Per side, it was normal to use 10 sectors of 256 bytes in single   
   >density. In double density one would either have 18 sectors of 256 bytes   
   >or 9 sectors of 512 bytes.   
   >   
   >Rich said 1 head. Single sided it is. 180K then makes sense only with   
   >double density 40 track. It would be so on TRS-80 model I/III/4 and IBM   
   >PC (with clones).   
   >   
   >Anyway density, single or double, is something decided by the   
   >controller, not the drive. Single density uses FM format and double   
   >density uses MFM format. The MFM format squeezes in almost twice as much   
   >data in the same amaount of "transitions" as does FM. Media could be   
   >bought for SD and DD also but there it just meant that DD media would be   
   >better similar to 48tpi vs. 96tpi. One could even do double density on   
   >the original old Model I drives, one just needed a double density   
   >adapter (could only 35 tracks though).   
   >   
   >5.25" HD is a different matter as there the media differs and the drive   
   >needs to support it. On the PC the HD drives would rotate faster too (in   
   >HD mode).   
      
   I had a desktop PC with a full height 10 meg hard drive and a full   
   height 5.25" floppy drive. The HD had MSDOS 1. something on it.   
   I remember the DOS version did not allow folders.   
   The floppy drive was interesting because it could store 1.1 megs   
   on a DSSD disk. It did that by using 80 tracks, variable SPT, and   
   variable motor speed.   
      
   >--   
   >Knut   
      
   Barry   
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