, both
inputs and outputs are inverted between these external lines and the
appropriate bits of the ports, so the actual values of the bits will be
clear for a high line and set for a low line.
The serial/network hardware consists of two outputs and two inputs.
The outputs are the bottom two bits of an output port (port
0B7h) as below. The other bits of this output port are not
used.
b0 - DATA OUT
b1 - STATUS OUT
Each of these outputs is connected to an open collector inverter with a
pullup resistor. Thus for example setting bit 1 of this port will pull
the STATUS OUT line low. Clearing this bit will allow the
STATUS OUT line to float high unless any other connected
machine is pulling it low.
The two inputs are available as bits of a general purpose input port.
The other bits are used for cassette inputs and various other things. The
port number is 0B6h and the relevant bits are:
b4 - DATA IN
b5 - STATUS IN
For use as a serial interface these inputs and outputs are used separately to provide half duplex communication (data can be sent either way but only one way at a time). For use as a network the STATUS IN and STATUS OUT lines are joined together, as are DATA OUT and DATA IN.
Note : Throughout this document the signal levels referred to are the actual levels on the external lines. On the