IGS-NT Communication Guide
81
6.1.5 Termination Resistors
Because 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 RS-485 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 100–120 Ohms).
6.1.6 Bias Resistors
The transmission line into the RS-485 port enters an indeterminate state when it is not being transmitted to. This
indeterminate state can cause the receivers to receive invalid data bits from the noise picked up on the cable.
To prevent these data bits, you should force the transmission line into a known state. By installing two 620 Ohm
bias resistors at one node on the transmission line, you can create a voltage divider that forces the voltage
between the differential pair to be less than 200 milli-Volts, the threshold for the receiver. You should install
these resistors on only one node. The figure below shows a transmission line using bias resistors. Bias
resistors are placed directly on the PCB of controller. Use jumpers PULL UP/PULL DOWN to connect the bias
resistors.
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