During times when the RS485 bus is idle with no data being transmitted, the End of Line Terminating Resistors can cause the voltage between the + and - RS485 communication lines to go to 0 Volts as there is no signal on the bus. At this point the devices on the system will interpret the logic state as being a logic state of 0. Any electromagnetic induced noise on the communication network can be detected as signals that may result in incorrect responses by the devices.
To ensure that this condition is not present Biasing is used. The purpose of Biasing is to ensure that the communication network is in a known state that is not fluctuating.
To Bias the communication network a Pull Up resistor is connected between the Communicating device +5 VDC voltage and the RS485 + signal terminal. A Pull Down resistor is connected between the Communicating Device Common connection and the RS485 - signal Terminal.
Biasing is normally done at the location of the Modbus Master. The Ethernet Gateways on this project are the Modbus Masters. It will be at the Gateways that the End of Line Terminators and the Biasing is to be installed.
In some of the older Ethernet Gateway products Biasing was accomplished by installing external Pull Up and Pull Down resistors. In the next generation of devices, DIP switches were used to enable Biasing of the Communication Signals. The current generation of Ethernet Gateways use programmed functions for the Biasing of the RS485 signals.
I do not see any mention of RS485 Biasing in the manual provided.
Would it be possible to provide details on how the Biasing is to be accomplished.