> Just out of interest, be aware of one of the common misconceptions 
> about
> RS485. Although many refer to it as a "2 wire" this is misleading. 
> The data
> signals are relative to GND, and all nodes must share a common GND. 
> It is
> not (in it's standard configuration) an isolated bus.

You may cvery well be correct BUT I can';t see why gnd is needed for 
signalling purposes. As long as wires signal differentially at the 
standard levels ground potential would not matter for signalling 
purposes. Certainly if devices had limited common mode voltage 
capability and if ICs were used which did'nt like eg 230 VAC to 
ground, then you could get problems if you applied 230VAC common mode 
to both leads ;-)

> I have actually seen an RS485 network with all devices isolated from 
> GND
> and using just a single twisted pair. By some strange quirk it 
> appeared to
> "mostly work", "most" of the time, although it suffered many 485 
> chip failures.

Makes sense. If you had a floating bearer it could assume any voltage 
relative to ground. If it did and a device connected cared about 
voltage to ground then failures could occur.



        RM

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