Bias Resistors

An RS-485 transmission line enters an indeterminate state if no nodes are transmitting on it. This indeterminate state can cause the receivers to receive invalid data bits from noise picked up on the cable. To prevent a line from receiving these data bits, the transmission line can be forced into a known state by installing two bias resistors at one node on the transmission line. Doing so creates a voltage divider that forces the voltage between the differential pair to be greater than 200 mV, the threshold voltage for the receiver. For more information about RS-485 bias resistors, refer to Using Your Serial Hardware.