'Grounding' the layout

[Trip]

I have a problem with spurious signals apparently from a watchman and two different people have asked me about how stuff is grounded and I don't know.

 

I have a PC connected via USB to a DR5000. The DR5000 is connected via loconet cables to:

 

- a digitrax DB210 booster (I'm not using the track power from the DR5000 at all) powered by a DC power supply

- a rr-cirkits SSB-adapter with its own DC power supply

- a DCS50 in 'throttle only' mode with its own DC power supply

 

The SSB adapter is connected via three wires to a chain of rr-cirkits watchman units.

 

So: how are the watchman units "grounded"? (The watchman manual says make sure your Loconet Terminator is grounded. I have no idea what a loconet terminator is.)