Re: Apple IIc serial port problem?

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

In article <CtMBwG.1G9@griffin.cuc.ab.ca>,
Sean Dockery <dockery@griffin.cuc.ab.ca> wrote:
> autry@uxa.cso.uiuc.edu wrote the following article:
> 
> | What's this about certain Apple IIc serial ports having a hardware problem?
> | 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 original Apple //c was installed with the incorrect crystal for its
> modem port.

No, it didn't have a crystal at all.  Both 6551s are clocked from the
system 14.31818 MHz clock, via a divide-by-8 circuit (1.7897725 MHz).

The revised motherboard added a crystal to one of the 6551s, with the
output also going to the other 6551.

> The result was that higher modem speeds were not synchronized with
> the speed of the ACIA (6551) driving the modem port, and interrupts 
> would be missed.

No, receive characters are corrupted because the modem is sending data
too fast (as far as the IIc is concerned), and the 6551 gets out of
sync.  On the transmit side, bits are sent too slowly, and the modem
may "stretch" a bit.

There is no problem with interrupts, you just get (and send) corrupted
characters.

> I've seen the reference number around, but I don't have it; perhaps someone
> out there would be kind enough to reproduce it for you.