Figure 14 RS-485
An RS-485 network should be wired as indicated in Figure 15 and terminated at each end of the network with a
120-ohm resistor. An on-board terminating resistor is provided in the modem and can be engaged by operating the
single DIP switch on the end plate next to the RS-485 terminals. The DIP switch should be in the
“1” (on) position
to connect the resistor. If the RS-485 device that the modem is being connected to does not have a termination
switch, a 120-ohm resistor must be fitted manually across the RS-485 terminals. Only devices at each end of the
multi-drop RS-485 cable need to have a termination resistor enabled or fitted.
Figure 15 Multidrop Serial
Fail-safe Biasing
Fail-safe biasing is a simple voltage divider that is connected to the RS-485 bus and pulls the terminal voltages
high or low when the communication state is idle, rather than leaving them in a floating state which could cause
data corruption. If you are connecting a serial device that does not support fail-safe biasing and a 115S expansion
I/O module is also not fitted, biasing resistors must be wired to each RS-485 terminal to ensure correct operation.
Resistor values depend on the supply voltage. See Figure 16 for resistor value calculation and wiring.
NOTE The WI-MOD-xxx-E-5W does not support fail-safe biasing on the RS-485 unless a 115S Serial
Expansion module is also connected and has its termination switch enabled.
Figure 16 Resistor Value Calculation
