Re: IWM and un-suitable drives for Macs

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

Charles Stephens <charles_stephens@hotmail.com> wrote:

> I wonder why 1.44 drives don't work then?  I recall that the original Mac II
> didn't support these drives until Apple came out with newer ROMs for the
> machine.  I am certain they use the same IWM ASIC on all of there machines.

They don't.  The upgrade for the Mac II and SE required replacing two
chips: the ROM and the disk controller.  The IWM chip was replaced with
a SWIM chip, which adds support for MFM and high density.

The SWIM chip was standard from the IIx (1988) onward, but Apple didn't
use it to replace the IWM in the ROM 3 IIgs (1989) or the IIc+ (1988).

I have never seen any detailed information on the SWIM chip, but in
principle supporting high density would require doubling the bit rate,
i.e. one byte is received or transmitted every 8 microseconds.  This is
too fast for a IIgs running at 2.8 MHz, but an 8 MHz Mac (SE) can keep
up. 

Apple used a faster 65C02 on the SuperDrive controller (probably 4 MHz,
but I haven't checked).

> The Inside Mac and Apple IIgs reference manuals seem to have the same pin
> out for both.

The only Inside Mac volumes that cover hardware are volume 2 (original
Mac 128 and 512), volume 4 (Mac Plus).  These predate the SWIM chip.

Volume 5 covers the Mac SE and II, but it doesn't include any hardware
information, and was published at the time these machines were released
(1987), before the SWIM chip was available.

I haven't looked at volume 6 in detail, but Apple had stopped providing
detailed hardware information by then, so I'd be surprised if it
included any further information on the IWM or SWIM.

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