Re: IIGS 115,200 Baud Serial Port - How?

[Hugh Hood <hughhood@earthlink.net>]

David (Wilson),

Great observation.

I pulled the schematic on the IIGS and it shows one side of the 3.6864 MHz
oscillator connected to Channel A's (Printer Port) RTxC pin, while the other
side is connected to _both_ Channel A's SYNC pin _and_ Channel B's RTxC pin.

I suppose this is why the IIGS tech note #18 states:

"Because of this single circuit, Write Register 11 (WR11) bit 7 must be set
to 1 for SCC channel A and must be set to 0 for SCC channel B."

Bit 7 is the RTxC-XTAL/NO XTAl select bit, BTW.

> It is interesting to read the "ZILOG SCCZ8030/Z8530 QUESTIONS AND
> ANSWERS" Application note.
> 
> Q. Can RTxC on both channels be driven from the same crystal.
> A. No. A separate crystal should be used for each channel.
> The crystal should be connected between
> /SYNC and RTxC of the respective channels. The alternate
> solution may be to use crystal on one channel
> and reflect the clock out of the TRxC output and feed
> it into another channel
> 
> It seems that Apple worked out how to do it with one crystal by using
> the /SYNC output to drive the 2nd RTxC input.