Confusion about TVS-wiring with RS-485

Question

I'm currently looking into securing my RS-485 connections against ESD. My current understanding is that a TVS diode is put between the data line and GND, so that any discharge will be shorted to ground immediately, without going through other devices (sometimes also against +5V [>USB], not yet sure how that is supposed to work out or where to look it up).

Now I've found some sources that also put a TVS diode between the two data lines (Protecting RS-485 by TI), some that don't and one even omits the ground TVS completely (TI again).

So apparently I don't completely understand how that's supposed to work and what the TVS between the two data lines tries to accomplish and I seek enlightenment so that I may not fry my circuit by accident. The first document explains it in a way I don't completely understand and the second one doesn't explain the reason at all (or I'm too blind to find it).

Answer 1

The TVS between each line to ground limits how far away each line can get away from ground (it clamps the common mode voltage). The TVS between the lines prevents the lines from getting too far away from each other (it clamps the differential voltage).

In a way, the TVS from each line to ground does indirectly prevent each line from getting too far away from each other. But if the common mode limit of the lines is larger than the differential limit, then TVS diodes which are sized to limit the line to ground difference won't protect against exceeding the differential limit.

Answer 2

The goal of any ESD design is to shunt the unwanted current back to the source or to earth. If an ESD event happens on RS485 cables, the best thing to do would be to shunt it to earth, which in most designs is through the PCB ground, then chassis ground.

Because RS485 is differential, if there are TVS diodes to ground on both lines then then any common mode noise will be subtracted out. The diagram below shows a configuration for TVS diodes to ground.

Source: https://www.onsemi.com/pub/Collateral/AND8229-D.PDF

In noisy environments, it may be best to use shielded RS485.

Answer 3

 I don't completely understand how that's supposed to work and what the TVS between the two data lines tries to accomplish

The TVS diodes conduct in reverse bias condition and only when the voltage across them crosses certain threshold (10V, 18V anything, depending on part number). The voltage rating should be definitely higher than the maximum expected voltage on the bus (across the signal lines).

If the relative voltage of the signal lines is less than the voltage rating of the diode, the TVS will be in non conducting state.