Modbus MFU Option Card Installation Manual
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Publication PUB091-003-00_0119
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). These will usually be near the scanner PLC and
near the furthest 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. In some cases network data line biasing may also be necessary, however to minimise
this requirement the actuator transceiver chips include secure turn round components.
For IQ / IQT, SI / EH and Q actuators that use the MFU, there is a termination resistor inside each
actuator. One side of the resistor is brought to a terminal whilst the other is already connected to data
line A. Linking the Terminator terminal to data line B completes the connection to place the resistor
across the data lines.
Fig 14: Termination for RS-485 highway
RS485
Highway
Analogue
Input
120 ohm
Data 1B
Data 1A
Terminator
Single H Analogue Input
Dual Highway
Single H Repeater
RS485
Highway 1
RS485
Highway 2
RS485
Highway 1
RS485
Highway 1
120 ohm
Data 1B
Data 1A
Terminator
120 ohm
Data 2B
Data 2A
Terminator
120 ohm
Data 1B
Data 1A
Terminator
120 ohm
Data 1B
Data 1A
Terminator