Re: Apple IIc and 3.5 drives

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

In article <2D62775E@mkt.eicon.com> jimr@EICON.COM (Jim Royal) writes:

[snip]
 
> The UniDisk 3.5 is an intelligent drive, which means its controller card is 
> built into the drive.  The Apple 3.5 Drive does not contain a controller 
> card.

OK so far.

> The UniDisk 3.5 is slower than the Apple 3.5 Drive because the
> controller circutry is an old, slow design.

Nope.  The UniDisk 3.5 is slow because it reads individual disk blocks
into RAM within the drive, then transfers them to the computer.  The
data transfer uses the SmartPort protocol, which sends one bit every 4
microseconds.  The Apple 3.5 Drive transfers data to the computer at 1
bit every 2 microseconds.  

The extra level of buffering and the slower transfer rate are the main
reasons for the UniDisk 3.5 being slower.

> Hence, the disk interleave on the UniDisk is, I believe, 4:1.  The
> disk interleave for the Apple 3.5 Drive is 2:1.

These are just the defaults.  In fact, the default format for the Apple
3.5 Drive under ProDOS-8 is also 4:1.  GS/OS defaults to 2:1 interleave
for these drives, because it usually reads data in larger clumps
(ProDOS-8 always reads disks one block at a time).

If you use a 4:1 interleave disk, the effective data transfer rate
will be the same for both drives.  The Apple 3.5 Drive can transfer
data twice as fast from a 2:1 interleave drive under GS/OS, or if the
drive is connected to a SuperDrive card, or on a ROM 03 IIgs under
ProDOS-8.  An unaccelerated ROM 01 IIgs may be unable to read data fast
enough from a 2:1 interleave under ProDOS-8, which results in a
significantly slower effective transfer rate.

The same applies to a UniDisk 3.5 - if you use 2:1 interleave, an
entire disk revolution is required to read each block (with 4:1
interleave, at least two blocks can be read per revolution), which
results in slower data transfer.

In summary, to maximize disk access speeds:

With a UniDisk 3.5, or an Apple 3.5 Drive on a ROM 01 IIgs under
ProDOS-8, use 4:1 interleave.  If 2:1 is used, the disk access speed
will be halved (or worse).

With an Apple 3.5 Drive on a SuperDrive card or ROM 03, or under
GS/OS, use 2:1 interleave.  If 4:1 is used, the disk access speed will
be halved.


I don't know if the IIc+ is fast enough to read 2:1 interleave without
extra delays.


> On the other hand, if all you've been using with your IIc are the 5.25 
> drives, using the UniDisk 3.5 will be a real treat.

No argument there!  Apart from having much more disk space, the
UniDisk 3.5 is somewhat faster than a 5.25" drive.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand