Re: Mac Plus floppy interface

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

Patrick Schaefer <patrick.schaefer@topmail.de> wrote:

> However, there will be a couple of problems: 
> 1) The Mac runs its IWM at nealy 8 MHz, the Apple II supplies 7,15 MHz.
> Someone mentioned that it is possible to configure the IWM for both
> clock rates, if not you will have to add a hardware modification
> (multiplexer) that supplies both clock rates.

The IWM has a mode bit that basically tells it which host computer it is
running in.  On an Apple II, the mode bit is set to tell it that the
master clock is about 7 MHz (actually 7.15909 MHz, or slightly less for
a PAL-based IIe or IIc, which uses a slightly lower master clock
frequency).  On a Mac, the mode bit is set to tell it that the master
clock is about 8 MHz.

The IWM actually needs to run at an internal cycle time of about 1 MHz,
so it uses this mode bit to control an internal timing chain which
counts off seven or eight external clock signals to generate one
internal clock.

In other words, you don't need to worry about this - the system software
or firmware will set this bit correctly, and it should be left alone by
any application code.

> 2) Your Mac doesn't start from the hard disk. It seems that the Mac ROM
> only checks for smartport devices during startup and doesn't expect any
> Ph0..3 step motor drives. Therefore a ROM modification will be
> necessary.

The Mac doesn't support SmartPort devices (such as the UniDisk 3.5)
either.  It only supports "dumb" 3.5" drives (the Mac 400K and 800K
drives, the Apple 3.5 Drive, or in later models the SuperDrive), and the
Apple HD20, which uses a different access method.

> The Mac floppy interface is decribed in the "Macintosh Family Hardware
> Reference". From these schematics you will see that there is no hardware
> difference between the IIc/IIGS and Mac implementation (except for the
> 400k PWM signal that you can ignore and the different clock rate
> mentioned above).

Ah yes, the PWM signal.  This goes out on pin 10 of the Mac's floppy
connector.  Can this signal be turned off and the pin changed to an
input?  If not, there will be a problem using a 5.25" drive, because
this pin is the write protect input from a 5.25" drive.

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