Using protection (TVS) diodes for RS-422/485

[sweethomela8]

I'd like to put in some diodes to protect from overvoltages on the differential inputs to the RS-422/485 receiver. How can this be done?

 

[IanP]

The basic protection includes two shunting devices, one on each wire (see picture below) ..
There are small variations of the above configuration, which may include fuses and additional TVS between both data lines, but as I mentioned in majority of cases the basic configuration is sufficient ..

**broken link removed**

IanP

 

[sweethomela8]

Do the TVS diodes have to be bidirectional? Can they simply be unidirectional and reverse biased with a breakdown voltage of 12-14 volts?

 

[dick_freebird]

RS-422 spec includes a receiver common-mode range of +/-7V (spec)
and 10V (survive) which a single-ended diode would bind up.
At the transmitter, such clamping would be OK.

You'd like to have some idea just how transmitter, receiver
and earth grounds are related, before you take any shortcuts.

If (say) your transmitter and receiver are running off power
supplies that are galvanically isolated, there might be no decent
current return from "earth ground" to either of the "protected"
devices. If you don't have a good low impedance current
loop then your clamps may not be as effective as you imagine.

The threat current loop has to be closed tightly, so you need
to know where it comes from in order to return it there as
cleanly as possible, preferably through a route that does not
involve the electronics at either end.