Hi,
RS485 is differential signalling. So theoretically it needs no GND... in the eaning of signal transmission.
However, in most cases the receivers are semiconductors ... and those semiconductors have a limited and specified "common mode voltage range".
This CMVR is given with respect to the receiver IC´s GND.
This GND is a sytem_GND not necessarily EARTH_GND.
On the transmitter side ... you also very often have semiconductors.
They generate the differential RS485 signals. Generated by an IC ... which also refer´s to it´s IC_GND. (Again: not necessarily EARTH)
The usual operational output voltage range of each single RS485 signal is within 0V to 5V referenced to it´s GND.
So to ensure the reciever can work within it´s CMVR (which often is +/-7V wider than output voltage range --> -7V ... +12V) it is the eaysiest way to connect: [transmitter_GND] -- [cable_SHIELD] -- [receiver_GND].
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On the transmitter side ... it is allowed to connect the [transmitter_GND] -- [EARTH_GND]
The same is true for the receiver side.
Now in high power industrial designs with long distance between RS485 partners ... there will be unavoidable voltage shift (bounce) between these two EARTH potentials.
--> to ensure proper operation ... the receivers have this +/-7V wider input voltage range. This allows "hard" potential shift between transmitter and receiver of up to +/-7V.
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When one or both partners are not connected to EARTH_GND .. the partners are considered "floating" to each other. Now when you connect two floting partners ... there will be a shift in potential. It may be in the kV range when ESD is invloved. Each partner will have some stray capacitance to EARTH_GND. So when connecting each other the potentials need to match .. thus there will be a short peak current. Starting with a rather high voltage difference ... and thus maybe high - but short - current peak.
Now imagine the shields are not connected to their own´s IC´s GND .... then this equalizing current needs to flow through the signal lines.
But you don´t want to flow ESD (machine to machine) currents to any signal lines. This would need rather big overvoltage protection.
Here also the GND_connected shield helps.
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To make an industrial system more robust one often uses gavanically isolated RS485 signals .. and the according ICs.
This has two major benefits.
* the voltage shift between two partners (main circuit) can be way higher than +/-7V ... maybe some 100 volts.
* the stray capacitance to GND will be much lower ... so the ESD energy is much lower, resulting in less stress for the ESD protection.e
Using the word GND in the meaning of voltage reference there can be this GND system
A side <--> B side
[EARTH_GND_A] - isolated or not - [SYS_GND_A] - isolated or not - [RS485_GND_A] - usually connected via shield- [RS485_GND_B] - isolated or not - [SYS_GND_B] - isolated or not - [EARTH_GND_B]
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Looks more complicated than it is. It gives you a lot of freedom to use isolation or not.
But in either case it is recommended to connect:
[RS485_GND_A] - connected via shield- [RS485_GND_B]
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If for any reason you don´t want to hard connect the shields with the RS485_GND, then you need to use three resistors:
(from a PROFIBUS recommendation)
VCC - 390 Ohms - Signal B - 220 Ohms - Signal A - 390 Ohms GND
This is termination and signal biasing combined (and adjusting common mode voltage range)
Klaus