With PROFIBUS, bus interfaces must be electrically isolated from the process supply. Optical transmitters are used for this purpose with RS 485. This is intended to avoid completely any ohmic coupling by noise signals from the process.
With PROFIBUS all signals are carried symmetrically on two wave guides. Care should be taken that these two wave guides always lie close together, so that coupled interference can occur as common mode interference - the same on both wave guides - rather than as a difference.
To amplify this protective effect, in a PROFIBUS cable both wave guides are stranded, forcing them to lie close together and making them alternate closer proximity to the source of capacitive interference, while the loops have small areas that alternate the coupling direction.
For RS485 transmission, shielded cable is specified as mandatory with PROFIBUS. This shield comprises a wire mesh to allow the diversion even of considerable energy and a foil that provides an effective screen even against high frequency interference.
A shield is only effective if it has a large area connected to protective ground everywhere - at all possible connections.
To allow the correct implementation of this requirement, plugs provided for the PROFIBUS have been designed with the corresponding preparations.
The cable specifications for RS 485 – PROFIBUS DP/FMS - are different from the cables for MBP – PROFIBUS - PA.