Bias Resistors

NI CompactRIO Device Drivers Help

Edition Date: December 2017

Part Number: 373197L-01

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An RS485 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.

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