Re: Serial speeds in a IIc+?

[David Wilson <david@uow.edu.au>]

Glenn Jones <rg.jones@rogers.com> writes:
>I have been looking over the schematics of the IIc+ and I noticed that 
>the 1.8432 XTAL crystal is setup as external clock source (pin 6 
>connected and pin 7 floating) vs the crystal being connected to both 
>clock pins on the 6551(s).

That is because the IIc+ (and revised IIc and memory expandable IIc)
does not use a crystal (2 pins, driven by the 6551) but rather a
4 pin TTL crystal oscillator module which generates a square wave
whenever power is applied.

>Given that ... would it be feasible to replace the crystal with one of a 
>higher frequency to derive faster baud rates? Like 3.6864 or higher?

Yes, you could replace the 4 pin oscillator with a higher speed unit.
Check the data sheet for the version of the 6551 you find on your IIc(+)
motherboard. I just checked a CSG6551 datasheet and tCCY = 0.5 us = 2MHz
which means it may not work if you try feeding it 3.6MHz.

>I also noticed that the ground side of the crystal has a jumper (R35) - 
>any idea why thats there?

If I recall, the 4th pin on some 4 pin oscillators is a N/C while on
others it may be an output enable.
-- 
David Wilson  School of IT & CS, Uni of Wollongong, Australia