Re: Apple 2c Variants

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

Leslie Ayling <layling@intercoast.com.au> wrote:

> Hi all,
> 
> Just a quick question about the different varieties of Apple 2c's.
> 
> Not counting the 2cplus, what are the *physical* differences in
> the various 2c machines between those that support the UNIDISK,
> and those that don't?

I'm aware of three major versions of the IIc logic board, and four
firmware versions.

The firmware version number is obtained via the PRINT PEEK(64447)
command in BASIC, but it isn't easy to distinguish between the first two
versions of the logic board without opening up the machine.

The original firmware (PEEK(64447)=255) uses a 16 KB ROM and does not
support the UniDisk 3.5.  The original logic board was designed to
support either a 16 KB or 32 KB ROM (using bank switching), with 16 KB
being the default setting.  To install a later firmware revision (all 32
KB), you need to do two modifications, which are at marked locations:
cut a tie, and solder a blob.

The only major difference on the second version of the logic board is
that they changed the way in which the serial port clock is generated.

On the original logic board, the serial port clock is derived from the
master system clock, and the resulting frequency (1.7897725 MHz) isn't
quite right, resulting in baud rates which are about 3% lower than they
should be (e.g. 9600 bps on the original IIc is actually 9321 bps).
This can cause communication problems with some devices that cannot
handle such a large discrepancy.

On the second version of the logic board, the serial port clock is
derived from a 1.8432 MHz crystal oscillator, which gives the correct
baud rates.

They might have also changed the default setting for the ROM size, but I
doubt it, as this version of the logic board can be used with the
original firmware.

The second version of the firmware (PEEK(64447)=0) added support for the
UniDisk 3.5.  As noted above, it is a 32 KB ROM, and requires a
modification to the first (and possibly second) version of the logic
board.

The third version of the firmware (PEEK(64447)=3) added support for the
memory expansion slot, which appeared in the third version of the logic
board (presumably released at the same time).  I'm not aware of any
other significant changes on this logic board, but I expect that the
default ROM size was changed to 32 KB.

The fourth version of the firmware (PEEK(64447)=4) had bug fixes.

> Is it simply a ROM upgrade to support the UNIDISK 3.5, or are there
> some other hardware differences involved?

The ROM upgrade (with cut-and-solder modifications) is all that is
required to support a UniDisk 3.5 on the original IIc logic board.

> Has anyone managed to get the Apple3.5 drive (from a GS for example)
> working on the 2c with an appropriate driver?

This is not possible.  The Apple 3.5 Drive requires the SmartPort
interface to operate in a faster mode (2 microseconds per bit cell
instead of 4 microseconds), which is too fast for a 1 MHz 6502 or 65C02
to handle without special hardware support.  In theory, it might be
achievable with an acelerator, but there are several other pieces of the
puzzle missing in the case of the IIc (additional control signals are
required, there is no firmware support, and there isn't any system-owned
RAM which can be used for translation buffers).

The IIgs (2.8 MHz 65816) is fast enough to support the Apple 3.5 Drive,
as is the SuperDrive card (4 MHz 65C02, I think).  In both cases, there
is enough RAM available.

The Apple IIc+ is the only other Apple II which directly supports the
Apple 3.5 Drive.  It has special hardware support which eliminates the
need for the extra RAM and deals with the speed issue, and the firmware
includes the necessary drivers.  (It also has the equivalent of a 4 MHz
Zip Chip built in, but this wasn't necessary for supporting the Apple
3.5 Drive given the special hardware.)

-- 
David Empson
dempson@actrix.gen.nz