Description for CS/A CPU65B/Gecko board, version 2.1
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whatsit	: CPU + RAM + ROM
I/O	: 6522 VIA at $c10 - $c1f, 16550A UART at $c00 - $c0f
Mem	: RAM at $040000-$047fff, ROM or I/O at $048000-$04ffff

The board is pretty straightforward. it is derived from the CPU65B/Plain
board, and contains some additional I/O. Also it can be used with an 
external CPU.

General operation

With a local CPU it maps the 8 or 32k static
RAM (IC5) to the lower 32k addresses. The upper 32k addresses are
divided between the I/O area ($e000-$efff or $e800-$efff, see J3) 
and the ROM (rest). 

The first half of IC2 selects the I/O area. The second half then 
enables either the /IOSEL line or the ROM (depending on the output
of the first half) as long as it is in the upper 32k addresses.

I/O area

The UART is used to implement a standard RS232 interface.
It also controls a LED.

The VIA handles a CBM serial IEC bus interface that can be enabled
for slave mode (work as disk drive).
In addition to that it has a 4 bit input + 4 bit output buffer
which can be used to build a keyboard with up to 64 keys.

The VIA PB7 is used to switch a 40kHz frequency generator
(IC17, R6, C8 and part of IC18) on and off. 40 kHz is the
frequency used in infrared control devices, for remote controlled
CD-players for example :-)
This switched signal is fed to some infrared LEDs that can drive
my audio equipment nuts... :-O

External CPU mode

To use the board, i.e. the memory and the I/O chips from an external
CPU, you have first to remove the 6502 from its socket on this board.
Then J5 must be set to allow memory access to be controlled from the 
/MEMSEL bus signal. With J6 it is possible to either make 
the local I/O space (UART and VIA) visible to the external /IOSEL 
signal (connect the CS/A bus /IOSEL with the /G input of IC9 (pin1))
or to map the local I/O space to the local memory space that is
accessed with the /MEMSEL signal (connect /G with /Q2 output of the
second half of IC2 (pin10).
For local CPU mode, connect all three lines from J6.