Hi

i need to specify a mechanical relay to switch a modbus connected piece of equipment from on modbus network to another. Hope to use a DPDT relay but wonder what contacts to specify.

 

gold flashed contacts would work best. get the ones rated for very low wetting current.

--
Bob
http://ilbob.blogspot.com/

 

You should also carefully link (but not switch) the grounds so you don't have a ground surge during the relay change. In other words, using relay to flip TX/RX will be okay, but make sure your grounds have some common linkage via surge-protection and perhaps some low-resistance/high-wattage resisters (or PTC resistance).

Without this, you could even have devices resetting at the relay switch due to a ground spike causing a 2.8v or 3.3v CPU/Micro to see voltage sag/surge etc.

I know the company robustdc.com makes a solid-state A/B switch for RS-232 or RS-485 which includes optical isolation & surge protection. It is commonly used for I/O-driven fail-over.