Termination Network; Fig 14: Termination For Rs-485 Highway - rotork Modbus IQ Pro Technical Manual

Modbus MFU Option Card Installation Manual

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.
RS485
Highway
Single Highway + Analogue Input
RS485
Highway 1
RS485
Highway 2
Dual Highway
RS485
Highway 1
RS485
Highway 1
Single Highway + Repeater

Fig 14: Termination for RS-485 highway

20 of 76
120 ohm
Terminator
Data 1A
Data 1B
Analogue
Input
120 ohm
Terminator
Data 1A
Data 1B
120 ohm
Terminator
Data 2A
Data 2B
120 ohm
Terminator
Data 1A
Data 1B
120 ohm
Terminator
Data 1A
Data 1B
Publication PUB091-003-00_0119