Description for CS/A PETIO, version 1.1
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whatsit	: Commodore PET compatible I/O 
	  (keyboard, IEEE488, tapes, userport)
I/O	: 6821 PIA at $810-$817,
	  6821 PIA at $820-$82f, 
	  6522 VIA at $840-$84f
Mem	: --

This board combines parts of the functionality of the earlier
"KEY" and "IEC" boards in a way that the result is Commodore PET 
compatible. the design is pretty straigthforward.
IC1 and IC2 select the I/O chips.

IEEE488

The ieee488 interface is now CBM PET compatible.  CBM decided to spread 
the IEEE488 lines over three I/O chips that were previously spread on
two different CS/A65 boards, but are now on one board only.

The 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 IC9 (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 JP7 to disable this feature.

VDC connector

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 IC10 CB1
is routed to the connector, so it can get the VDC horizontal sync
signal as input. This is the way interrupts are generated in the PET.

Beeper

Also VIA CB2 is put to drive a piezo beeper, as it is in the
PET from the 4*** series on.

Keyboard

The keyboard interface is CBM PET compatible (with P7 and P8) as well
as it can be used with a C64-type keyboard (with P1).

Tapes

The PET has two tape connectors. The tape requires 9V operation,
which is not available on the bus. Thus I decided to only put the
signals for the tape to two separate connectors, one for each tape.

Userport

The PET has standard I/O lines from the VIA on the userport, but
also a number of special signals. Not all signals are provided,
especially the VIDEO and HDRIVE signals are not provided.


Special feature

As the PET I/O uses basically all available I/O lines, I had to 
decide what to sacrifice to add more features. So I decided to 
reuse the tape lines:

1) The "IEEE488 device mode" (or "slave mode") requires an additional
   line that is borrowed from the tape 1 motor control line CB2 on
   the keyboard PIA.

2) To use a C64 keyboard in a full matrix design (as opposed to the PET
   where a 74LS145 expands the line number to a single selected line)
   the lines that are otherwise used must be relocated. This is done
   with JP6 that connects the IEEE488 EOI line to the tape 2 motor
   control on VIA PB4
.