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RS485 TVS diode function

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nandhu015

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Hi

sm712.jpg

What happens if only 2 wires connected of RS485 connections leaving the Ground?

Will it be effective using TVS diodes?

Nandhu
 

Hi,

This circuit limits voltage to about -7V and +12V.
This is the typical valid input range of an RS485 receiver (wrt GND).
And each receiver/transmitter is referenced to it's GND.

This protection circuit needs to be connected very close to the RS485 transmitter/receiver. Distance some mm, with fixed connections. Usually it is placed directely on the PCB.
It is useless somewhere along the cable. It is useless in the cable_side_connector.

Any non_isolated RS485 connection needs a GND connection, too.

Klaus
 
No it will not work.
 

WITHOUT the ground it gives around +/- 20V protection across the RS485 lines only, that's not enough to prevent damage on most devices.

WITH the ground it gives about +13V to -8V protection from both lines to ground, for most devices that would be adequate.

So removing the ground makes the circuit useless. There should still be a ground connection between the transmitter and receiver(s) for RS485 to work as it can only tolerate a limited common mode offset. RS485 is differential but unipolar, if the ground is reasonably good, a single Zener limiting the positive excursion should be adequate.

Brian.
 

Please be descriptive.

If you connect RS485 lines but not GND pin, TVS will not work properly.

When an ESD event occur, ESD apply both RS485 pins high voltage with respect to GND and to conclude your component will burn.

The reason to connect GND pin of TVS is at the inputs of RS485, TVS are keeping the voltage level of RS485 lines at a safe level with respect to GND. No GND connection no safety.

Final result will be the same as you did not add TVS to your design.

Best Regards
 

TVS diodes at both ends, each connected to the transceiver ground can well protect the transceiver against ESD events.

Another question is if the connection might fail due to excessive common mode voltage.

It should be noted that non-isolated RS-485 interfaces are generally at risk to be damaged by large common mode voltage differences, with or without the suggested ground connection. Thus their usage should be limited to local busses, e.g. inside a control cabinet.
 

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