Re: More than 4 drives on a daisy-chain?--Maybe!

[dempson@actrix.gen.nz (David Empson)]

Obsbedia2 <obsbedia2@aol.com> wrote:

> This just caught my eye from  the Applied Engineering instruction book for
> the AE 3.5" Disk Drives:
> 
> (from the top of page 3)  "Do not daisy-chain more that two 3.5" drives
> and two 5.25" drives.  The strain on the power supply will be too great."
> 
> If that is the only reason to limit daisy-chaining to 4 drives, then those
> of us with BUGG Power Supplies should be able to chain at least a dozen
> drives together.

It is only one of many reasons.

1. Too much strain on the power supply (covered by using a larger power
supply).

2. Too much strain on the wires used to carry the power to the drive
(cumulative resistance etc.)

3. Signal loading on all of the data lines gets worse as you connect
more drives.

4 (most important): the controller hardware only supports two Apple 3.5
Drives (or equivalent) plus two Apple 5.25 Drives (or equivalent), since
there are only two drive select lines.  You can theoretically connect up
to 127 UniDisk 3.5 drives, since it uses an intelligent protocol
(SmartPort) with an address byte.

All of items 2, 3 and 4 can be worked around by using more than one disk
controller.

There is also the issue of the number of drives supported by the disk
interface card firmware.  This limit doesn't appear to be documented for
anything apart from Apple's SCSI cards (7 for the original card, about
110 for the High-Speed card; both of these limits refer to partitions,
not just physical devices).

Finally there is a limit imposed by the operating system.  ProDOS-8 can
only have 14 units in total, spread over all slots, including /RAM
(leaving you with a limit of 13 real volumes).  GS/OS is limited to 63
volumes of each type (for a native driver) over the whole system, and
may be limited further by available RAM.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand