Description for CS/A KEY board, version 1.0
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whatsit	: PET keyboard and RS232 interface
I/O	: 6821 PIA at $810-$817, 6551 ACIA at $818-$81f
Mem	: --

IC1 selects the right I/O are, and A3 is routed to the chip
select signals for dividing the local 16 byte.
No further CPU-side interfacing is necessary.

The keyboard interface is CBM PET compatible (with P3) as well
as it can be used with a C64-type keyboard (with P2).

The 6551 ACIA is a little bit brain-dead concerning handshaking.
That's why I use the PIA's CA2 as /RTS output and the ACIA's
/DSR input as /CTS input. This way these lines can be controlled
independently without disturbing Tx/Rx operations.

In fact one might build an 'ACIA-improvement' which automatically
sets /RTS when a byte arrives, and resets it when a byte is read
from the ACIA data register. This can improve reliability 
considerably, esp. with a high-latency operating system.
More info on the ACIA hack on my RS232 page.