Modbus MFU Option Card Installation Manual
22 of 64 Publication PUB091-004-00_0918
4.4 Termination Network
In order to operate correctly, all RS-485 highways must be terminated at each end of the main trunk
line using a suitable resistor (120 ohms nominal, half Watt rating). These will usually be near the
scanner PLC and near the last actuator. The resistors are connected between data line A and data
line B. If the termination resistors are not fitted there is a possibility that the network communications
will be unsatisfactory due to signal reflections.
One of the terminations must be active, using biasing resistors. These resistors put a known voltage
across the data highway, so that there is a fixed state when none of the devices on the highway is
transmitting. Without the active termination, there may be some communications errors.
The value of the biasing resistors must be between 450 Ω and 650 Ω.
Fig 14: Active Termination for RS-485 highway. Biasing resistor values are typical.
The MFU has an internal termination resistor which does not require any external wiring to be included
(unlike the previous version of Modbus field unit). The termination resistor, or resistors for a dual
channel, can be electronically switched on through the setting menu within the IQ3.
Termination may also be switched on and off in all actuator types where the MFU is fitted, through the
network, by writing to register 25 and setting it to 1 for ON and to 0 for OFF. Switching termination on
in a dual card will result in both channels being terminated.
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