Re: upgrades for //c

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

In article <93Jun04.081422.30045@acs.ucalgary.ca> sbdocker@acs.ucalgary.ca (Sean Brendan Dockery) writes:
> 
> The 1200 bps limitation that is continually being mentioned is with
> regards to the built in port 2 firmware; it will only let you toggle
> between 300 and 1200 bps.  (This is that dumb terminal you can get to
> by typing "IN#2" following by "^AT" from the "]" prompt.)

Nope, wrong.

The original IIc (unmodified motherboard) has a problem handling 1200
baud or higher because the 6551 ACIAs are not being clocked at the
right frequency.

They should be clocked at 1.8432 MHz to get "spot on" baud rates, but
they're actually being clocked at 1.7898 MHz (14.31818 divided by 8).

The resulting 3% error is not significant at low baud rates, but the
faster you go, the more likely it is that you will have problems
communicating with a device which expects the baud rate to be closer
than that.  For example, if set to 9600 bps, the IIc is actually
running at 9322 bps.

This is only a problem for some people - most modems seem to be able
to handle the slightly different rate without problems.  My user
group's IIc is unmodified, and it has been used with a 2400 bps modem.
I've also used it occasionally at 9600 bps (with a direct connection to
another computer).

I think receiving is more likely to be a problem - the IIc may
occasionally mis-receive bytes.

The "fixed" motherboard actually has a 1.8432 MHz crystal providing
the clock signal for the 6551's, eliminating the error.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand