THVD4411: THVD4411

Pierpaolo Guida said:

Which is the best solution to protect it from harsh environment? like surge, EFT or more potential overvoltage

Using external devices to help clamp the voltage to be below the abs max of the device's pins. I've linked a document below which walks through some external parts for RS485. 

https://www.ti.com/lit/ug/tiduas1b/tiduas1b.pdf?ts=1730833930929

Pierpaolo Guida said:

overcurrent?

For current, you could use a series resistor to help limit it. In the document i referenced, it uses a 10 ohm pulse proof resistor which would also help in situations where you have shorts.

For RS485/422 drivers, you want to limit the size of these resistors. Normally I would say 10 ohms would be enough since RS485 technically is spec'd to handle a short between A and B pins.

RS232 is designed for to handle shorts to GND though some datasheets may require/state the short may only happen on one of the outputs. For these pins, you can probably size up the resistors to be larger than 10 ohms but I probably wouldn't go larger than 100 ohms since it will form a resistor divider with the Rin pin's 5k termination resistor.

The other option is to use a fuse. I would probably size the fuse to be larger than the max short circuit current the output pin is spec'd for.

Pierpaolo Guida said:

which is the best schematic solution?

Ideally, choosing a device with internal protection. Though some devices may not be rated for stuff like surge since the power of a surge event is so large compared to esd/EFTs. Again I would recommend looking at the TI design i referenced earlier.

Pierpaolo Guida said:

another thing: is better to protect signals from common signals (GND) or from SHIELD/EARTH/CHASSIS connection?

I'm not entirely sure, this is outside of my normal expertise. I normally see ESD devices referenced to common GND and if there is a Chassis GND I sometimes see resistors from the common GND tied to the Chassis GND to lower loop current.

-Bobby