Description for CS/A DUART board, version 1.0
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whatsit	: two RS232 interfaces
I/O	: 16550 UART at $850-$857, 16550 UART at $858-$85f
Mem	: --

The board is pretty straightforward. The '688 (IC8, 8 bit compare)
determines the I/O address. Then IC7 ('138) provides the necessary
separate read and write signals for the two chips. Any access
needs the right I/O address (from IC8) and Phi2 high. Then 
A3 determines the chip to use, and R/-W selects the /RD or the /WR
input.

This board can easily be expanded by moving A4 from the 8 bit compare
(IC8) to A2 of IC7, and using /Q4-7 for two more UARTs.