Re: UniDisk vs. Apple 3.5 Drive

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

In article <4m7eru$ptt@the-fly.zip.com.au>,
Alex Maddison <alexm@zip.com.au> wrote:
> 
> I just purchased a 3.5" UniDisk as a second drive for my IIGS and I
> was wondering what the exact differences between the UniDisk and the
> Apple 3.5 Drive is.

They have the same drive mechanism, but a different interface board
within the drive.  The Apple 3.5 Drive has a simple board which
re-routes signals as required.  The UniDisk 3.5 has a small
microcomputer, consisting of a 65C02, RAM, ROM and an IWM disk
controller.  It is possible to convert either drive to the other type
by swapping this board.

The disk hardware in the Apple 3.5 Drive is controlled directly by
software running in the IIgs (the ProDOS/SmartPort firmware or the
GS/OS driver).  Raw data from the disk is sent over the cable, at 2
microseconds per bit (twice the speed of a 5.25" drive).

With a UniDisk 3.5, the ProDOS/SmartPort firmware or GS/OS driver
sends command packets to the microprocessor in the drive, which is
then responsible for doing the actual disk I/O.  The data sent over
the cable consists of command and data packets, and the only encoding
required is to convert 8-bit bytes to 7-bit bytes (the IWM can only
transfer 7-bit data), at 4 microseconds per bit.  The drive does the
"nibbilising" and "denibbilising" (conversion to and from the raw
disk bytes), handling of sectoring, seeking, formatting, etc.

The UniDisk 3.5 is noticeably slower than an Apple 3.5 Drive: data
takes about twice as long to transfer to and from the drive.  The most
significant effect of this is that a different interleave should be
used for optimal performance.

An Apple 3.5 Drive is able to handle a 2:1 interleave, while a UniDisk
3.5 works best with 4:1.  With the correct interleave, the UniDisk 3.5
will take twice as long as an Apple 3.5 Drive to read or write each
track (assuming the Apple 3.5 Drive reads the track in sector order,
which is not necessarily true - see below).

If you use a 4:1 interleave disk in an Apple 3.5 Drive, the transfer
rate for write (and possibly for read) will be halved, because the
drive will require four complete disk revolutions to write each
sector, instead of two.

If use use a 2:1 interleave disk in a UniDisk 3.5, the transfer rate
will be significantly slower: anything from four to eight times,
because the UniDisk will require most a disk revolution to read or
write each sector.


If you are using a ROM 3 IIgs, or GS/OS drivers, or a SuperDrive
controller, then an Apple 3.5 drive is able to read data as fast as
possible: the driver reads the complete track into memory, and then
extracts each sector from the track image (rather than searching for
each sector on the disk as it is requested).

Diversi-Cache uses the same technique to speed up disk accesses on a
ROM 1.

This doesn't apply to a write.

It is not possible to do this with a UniDisk 3.5, because it doesn't
have enough memory to dump an entire track.

> Can I use the two together safely

Yes.  Make sure the UniDisk 3.5 comes AFTER any Apple 3.5 Drives on
the chain, but BEFORE any 5.25" drives.

If you are using GS/OS, you will also need the UniDisk3.5 driver
installed on your system disk.

> can the UniDisk be used on a Macintosh?

No (unless it is converted to an Apple 3.5 Drive by replacing the
interface board).

It can be used on a IIc (with any ROM version except the original
one), IIc+, or a IIe with an appropriate card (Apple 3.5 Floppy Disk
Drive Interface Card, or Apple II 3.5 Disk Controller Card).

> Does the UniDisk have the inbuilt RAM on S5/D2 that the Apple 3.5
> Drive does? 

You are presumably thinking of /RAM5, which has nothing to do with the
Apple 3.5 Drive.  /RAM5 is provided by the IIgs, and accessed through
the slot 5 firmware (or the "RAM5" GS/OS driver).  The Apple 3.5 Drive
and UniDisk 3.5 drive are also accessed through the slot 5 firmware
(or the appropriate GS/OS drivers).

The actual memory for the /RAM5 volume is allocated from the IIgs's
own memory space, and this memory cannot then be used for other
functions.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand