with only three wires and two of those labeled to be carrying power, the signaling will be some type of asynchrounous serial. 


further guessing IC1 looks like an optocoupler and Q1 seems to be connected too, so I'm guessing NET idles high, using a pull-up via the opto-coupler and signaling is done using Q1 as a pull-down something likethis:

<!-- Begin schematic: In order to preserve an editable schematic, please
     don't edit this section directly.
     Click the "edit" link below the image in the preview instead. -->

![schematic](https://i.sstatic.net/qn8hm.png)

<!-- End schematic -->