Description: SCSI Hard Disk Drives: Interleave Information (11/94)
Header: SCSI Hard Disk Drives: Interleave Information (11/94)

 Article Created: 2 March 1987
 Article Reviewed/Updated: 09 November 1994

 TOPIC -----------------------------------------------------------

 This article pertains to the interleave of Apple external SCSI hard drives of
 all sizes.

 DISCUSSION ------------------------------------------------------


 To initilaize a SCSI hard disk drive you want to use on a Macintosh:

 1. Always use Apple HD SC Setup.

 2. Always initialize from the type of Macintosh the drive will be connected
   to. For example, if you wish to connect a Apple HD40 SC to a Macintosh
   Plus, initialize that hard disk with a Macintosh Plus.

 If you want more than one type of Macintosh to access the SCSI hard disk,
 always initialize it from the slowest system for the best average
 performance. You must do this because, with Apple HD SC Setup, different
 types of Macintosh computers require a SCSI hard disk with a different
 interleave.

     Computer                 Interleave   relative speed

     Macintosh II and later      1:1            fast
     Macintosh SE                2:1            medium
     Macintosh Plus              3:1            slow

     Apple II                    3:1            slow

 Interleave is the ratio of consecutive sectors a CPU can read from or write
 to. This means that a Macintosh II is fast enough to read from or write to
 every consecutive sector, a Macintosh SE can only read every other sector,
 and the Macintosh Plus is only fast enough to read one in every three
 sectors. An Apple II also has an interleave of 3:1. The Apple II can support
 a 3:1 interleave because the SCSI card has a 8K RAM buffer enabling it to
 buffer data until the CPU is ready for it.

 Delays occur when you connect a "slower" Macintosh to a hard disk initialized
 on a "faster" Macintosh. For example, a hard disk intialized from a Macintosh
 II has more data on every consecutive sector than a Macintosh Plus can
 consecutively read, necessitating a full revolution of the platter before the
 Macintosh Plus can access the sectors it knows it missed. There's a delay of
 the revolution of only one or two sectors when a "fast" Macintosh accesses a
 hard disk intialized from a "slower" Macintosh.

 For the change in interleave ratio between the Macintosh Plus and the
 Macintosh SE, there are three reasons: improved performance, hardware
 handshaking, and changes in SCSI firmware. First, the Macintosh SE performs
 more quickly, even though it uses the Macintosh Plus' 68000 processor running
 at the same speed. The Macintosh SE has a change in hardware design that
 allows the video circuitry 1 long word access to the RAM for every 3 made by
 the processor, as opposed to the 1 video access for every 1 made by the
 processor on previous Macintoshes. Next, hardware handshaking was
 implemented, allowing cleaner and faster communication between the Macintosh
 SE and the drive. Finally, the SCSI firmware was cleaned up and optimized for
 the SE. Previous bugs and patches caused the SCSI driver to not perform as
 quickly as it should.


 Article Change History:
 09 Nov 1994 - Reviewed for technical accuracy, revised formatting.

 Support Information Services
 Copyright 1990-94 Apple Computer, Inc.
 Keywords: