Description for CS/A IEC board, version 1.1
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whatsit	: IEEE488 interface and CBM serial IEC interface (for C64)
I/O	: 6821 PIA at $820-$82f, 6522 VIA at $840-$84f
Mem	: --

IC1 together with half of IC4 selects the right I/O area for both
chips. No further CPU-side interfacing is necessary.

The ieee488 interface is not CBM PET compatible. The reason for this
is that CBM decided to spread the IEEE488 lines over three I/O chips.
I didn't want to do this, so I decided to move them around.

IC2 (PIA) just provides a lot of I/O lines for the IEEE488 interface.
As I don't use a bi-directional driver (like newer PETs, older do it
like I do) both ports are already occupied with the data lines.
Port B of IC3 (VIA) provides the rest of the I/O lines for the IEEE488
interface. 

The implementation supports slave mode operation, i.e. when an ATN
comes from the bus (another master sets it), NRFD must go low to
signal that there are devices on the bus. This is done by 
xor-ing the bus ATN line with a PATNA output line. If they
differ NRFD is pulled low, blocking the bus until the CPU
has recognized the ATN situation, changed ATNA and is waiting
for commands.

If, however, the computer is on but no software for slave mode is 
running, this would block the whole IEEE488 bus forever. That's why
there is jumper J1 to disable this feature.

The serial IEC bus is made with the VIA port A. The same note for
slave operation holds true here, too. Only it is the DATA line
to be pulled down.

As a PET compatibility feature :-) VIA CA2 is routed to another
connector, which can be connected to the video board (CS/A VDC)
to provide PET compatible character ROM switching.
Also VIA CB2 is put to drive a piezo beeper, as it is in the
PET from the 4*** series on.

The serial IEC bus is Commodore-compatible (i.e. it uses the 
open-collector architecture as in the Commodore computer itself),
while the IEEE488 interface is _not_ standard in electrical 
terms. The bus should have different resistor arrays and 
drivers. However, it worked for me, but you are warned.