Re: Apple IIc serial port problem?

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

In article <312ehg$r2g@vixen.cso.uiuc.edu>,
autry david <autry@uxa.cso.uiuc.edu> wrote:
> 
> 
> What's this about certain Apple IIc serial ports having a hardware problem?

The original IIc motherboard has a design shortcut to save a couple of
components, which results in the serial chips being clocked at a rate
which is slightly too slow (1.7897725 MHz instead of 1.8432 MHz).
This causes the IIc to send characters slightly slower than the
selected baud rate (the bits get stretched, which may cause problems
with some devices, particularly modems at 1200 bps or faster), and it
may miss (or misinterpret) receive characters, especially at higher rates.

The fix involves a motherboard swap.  The revised IIc motherboard used
a 1.8432 MHz crystal to produce the correct speed for the serial ports.

> How do I know if my IIc is one of them?  (Other than I can't get anything to 
> transfer 100% correctly over the serial port to my mac.)

The only way to tell for sure is to open up the machine and locate the
motherboard version number.

You can be pretty sure by checking the ROM version.  Get into BASIC
and type:

PRINT PEEK(64447)

This will give you a number which identifies the ROM version.  Version
255 is the original IIc ROM, which almost definitely means you have
the original motherboard.

0 is the upgraded ROM which supports the UniDisk 3.5 drive, has a
mini-assembler, and some new serial firmware commands.  You may have
the original motherboard (with an upgraded ROM) or you may have a
replacement motherboard as well as a new ROM (in which case there is
no hardware problem with the serial ports).

3 and 4 are newer ROM versions which support the memory expansion
card.  If you have one of these versions, you almost definitely have
the third version of the motherboard, which also has a memory
expansion slot.  It is possible to upgrade the ROM on either of the
earlier motherboards to this version, but this is highly unlikely (I
know of one case of this combination).

5 is the Apple IIc plus, which is basically a different computer.


If you can demonstrate a problem with the motherboard, you are
supposed to be able to get a free upgrade from an Apple dealer (Apple
Computer will reimburse the dealer for the upgrade).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand