Overvoltage protection for RS485 ports:
The maximum permissible differential voltage which is permitted to reach RS485 transceivers from the outside is limited to approx. ± 12...14V according to data sheet specifications of the chip manufacturer. Voltages which exceed these values will inevitably result in damage to or even destruction of the chips.
Shielded cables which run completely within a building should not generally see such high differential voltages, since coupling onto the transmission line is manifested as common mode interference: The potential of all cable leads is increased equally. In such applications the standard Interface Model 86201 can be used without any problems, since its galvanic isolation ensures that equal potential increases do not cause any damage.
When using the interface converter on transmission lines that run outside of buildings or when the cabling conditions are unclear, use of the 86203 Interface is advisable. This interface type features integrated overvoltage protection which limits the maximum occurring differential voltages to +/- 9V using fast protection diodes.