PSI BOX Reference
Modes 1-6 & 8.
Addressed Operation.
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The address of the RS232 input output port is as set on the
IEEE address DIP switches. The Centronics port address is one
greater than the RS232 address.
Data written to the RS232 address will be buffered and
sent to the RS232 when the RS232 handshake permits. If the
RS232 output buffer is full then the IEEE handshake will freeze,
with NRFD and NDAC held low, waiting for the RS232 device
to become ready to receive the contents of the PSI BOX buffer.
This ensures that no data is lost!
When data is requested from the RS232 port the PSI BOX
sends any data that is in its RS232 input buffer. The last byte
from the buffer is sent with EOI true. This informs the controller
that the buffer is now empty.
If a byte is requested from the PSI BOX and there is
nothing in its buffer to send then a ’talk with nothing to say’
message is sent, this is the 00 byte with EOI true. This method
of flagging an empty buffer prevents a hang-up in the
handshake.
In a similar manner, when data is requested from the
Centronics input port the PSI BOX sends any data from the
buffer. When the PSI BOX sends the last byte in the buffer it
sets EOI true, indicating to the controller that there is no more
to get. If data is requested when the buffer is empty then a talk
with nothing to say message is generated.
Any data SENT to the PSI BOX for output via Centronics
port is lost since in Mode 1 the Centronics port is input only.
In Mode 1 half the PSI BOX ram acts as the Centronics
input buffer, the other half is evenly split between the RS232
input and output buffers.
Pet To PC Ascii Conversion.
_________________________
From version 4.3 onwards of the PSI BOX, Commodore
Pet to PC Ascii conversion will be performed depending on the
state of mode DIP switch 8. When the switch is UP, ie ON, all
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Chapter 6
