grounding - RS485 network ground pin - when to connect?

As many know, one can implement a simple node to node rs485 communication by using only two wires, A and B. Well, the standard specify to connect the ground of the two node together.

from Wikipedia:

In addition to the A and B connections, the EIA standard also specifies a third interconnection point called C, which is the common signal reference ground.

I stumbled upon tens of articles that speak about this third connection but still couldn't understand the concept.

1. Why can't the receiver just act as a simple voltmeter? measuring the voltage between A and B?


2. If both nodes are battery operated(different battery for each node) does the ground connection make any difference?


3. Why is it better for (outdoor) nodes to be earthed when cables are long?


4. How is this ground connection good for protection from transient?

Answer

Note: I don't claim to have the most scientific answer but i will try to explain things the way i understand them.

1. 
   

   I don't know if The Voltmeter argument is 100% valid , since it is a low speed device , a more appropriate example would be the Battery operated oscilloscope , or a differential oscilloscope probe.

   
   


2. 
   

   Remember the definition of Voltage which is a potential difference between 2 points. The Signals A and B are differentiated by the electrical components inside the Amplifier (mainly transistors) which all have absolute maximum ratings between their base and collector/emitter .. This is referred to as differential amplifier maximum Common Mode Input Voltage with respect to its own ground. Hence the A and B voltages are meaningless without specifying what they are referenced to. For example if the difference between A and B is 2.5v but this voltage is 20v shifted above the receivers amplifier supply , would would the amplifier see (2.5 or 22.5) ?

   
   


3. 
   

   4: The long distance outdoor cables are more prone to noise or ESD or whatever source of charge or current that may enter the bus ( that has specific impedance and DC resistance) so if amount of charge/current is high enough multiplied by the higher resistance (of long cable ) would cause a larger voltage spike at the receiver that may cause damage. Earthing in this situation can be used to provide path for the spikes hitting the shield , and may be used as a stable ground reference.

   
   

Whether the RS485 receiver circuit can be changed to be floating and fully capable of acting like the voltmeter/Oscilloscope might be entirely possible by adding extra components, isolation circuits, etc.. but with added cost, complexity and size which is all beyond the capability of a small IC such as the MAX485.