RS485 Termination

Each differential pair of wires is a transmission line. You must properly terminate the line to prevent reflections.

A common method of terminating a two-wire multidrop RS485 network is to install terminating resistors at each end of the multidrop network. If you daisy-chained multiple instruments together, you need a terminating resistor at only the first and last instruments. The terminating resistor should match the characteristic impedance of the transmission line (typically 100 Ω to 120 Ω).

You can order an optional DB-9 RS485 termination connector (NI part number 182844-01) that contains embedded terminating resistors for easy termination from ni.com/serial.

The following figure shows a multidrop network using terminating resistors.

The following figure shows a full-duplex network using terminating resistors.