Re: Old V. Late //c (Not //c+)

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

In article <4rljjr$p74@agate.xara.net>, Richard <rtk@scotborders.co.uk> wrote:
> 
> From my research I was unaware that the later //c (the one with UniDisk
> support, AppleTalk and Memory expansion) featured Mini-DIN 8 connectors.
> Does anybody have an example of a //c (not plus) with these for my museum?

No.  The IIc+ is the only IIc model which has a Mini-Din-8.  All
non-plus versions use the same DIN-5 connector.

> I do know what the motherboard fix is for - it ISN'T for port speed - it
> is a fault with the built-in Terminal which caused the machine to lock-up
> at speeds over 1200baud.

That's the first I've heard of such a reason.

The motherboard modification replaces the 74LS161 which is used to
generate the serial port clock (by dividing the 14.31818 MHz system
clock by 8 to get 1.7897725 MHz) with a 1.8432 MHz crystal oscillator.

The 6551 is supposed to be clocked at 1.8432 MHz to generate the
correct baud rates, so the original IIc motherboard's shortcut results
in baud rates which are about 3% too slow.  This is generally not a
problem when sending data to a printer, or with dumb 300 bps modems,
but intelligent modems (1200 bps or faster) tend to be much more fussy
about the baud rates, and 3% is enough of an error to throw some
modems out of sync with the computer.

The net result is that your transmitted and received data gets corrupted.

This might cause the built-in terminal to lock up, but that would only be a
side effect of the real problem.

Note that this doesn't affect all modems - I didn't have any trouble
with a ZyXEL U-1496E (at 9600 bps) connected to a IIc with the
original motherboard, and I think I was using the built-in terminal
software as well (it had a little trouble keeping up with the data, so
I tried Z-Link instead, which worked fine).

Apple also added several features to the serial firmware in the UniDisk 3.5
ROM version ($FBBF = $00) - masking of line feeds, and support for
XON/XOFF are mentioned in IIc technical note #7; this is independent
of the motherboard upgrade.

> The free replacement isn't a big problem

Yes it is.  Apple doesn't provide it any more.

> see ROM 00 to ROM 01 upgrade for IIgs - even the UK still supports
> this, although they don't have the parts! I wouldn't bother, ROM 00
> machines are v. rare.

The problem is that the occasional ROM 00 IIgs turns up because it was
originally bought by a school, and never used as anything other than a
fast colour IIe.  If you actually want to start using it as a IIgs,
you have to upgrade it to ROM 01 to be able to run any software
written since 1988.

In theory, the ROM 00 to ROM 01 upgrade might still be available (it
isn't ten years since the upgrade was released).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand