Ground wire for RS485 or not?

[Watcher]

@ron_sutherland: Very interesting work indeed!

Looking at your schematic, I was wondering if you have considered biasing the RS485 lines with say 100k resistors to +ve and GND respectively for improved noise performance? And termination impedance matching shouldn't  it be 120 Ohm (as opposed to 100) ?

[ron_sutherland]

I think the bias resistors @Watcher is talking about are for fail-safe, which will cause the receivers to see a logic high if no-one is driving the bus. However, Analog Devices and Intersil (which is what I am using) have a more tightly defined receiver threshold level. This allows doing away with the fail-safe bias resistors. There was an article in EDN some years back about this, it basically said the AD devices (at -40mV) don't have enough margin but the Intersil (at -50mV) were right on the edge to ensure fail-safe operation in a typically noisy environment. So to some it up these Intersil chips (all of them on the bus) will see an undriven bus as a logic high (and have 50mV of margin), and thus have no need for the bias resistors.

I'm using CAT5 which has 100 Ohm impedance. Many RS485 cables are 120 Ohm and have a little less copper loss so they can run 1200 meters,  CAT5 probably won't go as far.

[Watcher]

Thanks for the clarification ron_sutherland.
 I ve only been using maxim chips so far for RS485, including the fail safe version.