EXOS 2.1 Serial/Network Driver Specification

2. Hardware

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.


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

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 Enterprise, 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.

Next Chapter: Serial Device.

David Bouman. (dsbouma@cs.vu.nl)