From: kr-lund@nogarbage.online.no   
      
   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).   
      
   --   
   Knut   
   (delete 'nogarbage.' for email)   
      
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