Modbus Option Card Properties
Publication PUB091-004-00_0918 21 of 64
Actuator internal spur length as in the table below:
If greater distances are required for the network, then the actuators can be operated at lower baud
rates of 300, 600, 1,200, 2,400 or 4,800 baud. Alternatively, the distances can also be increased at the
higher data rates by using repeaters or fibre-optic links between the devices on the network.
4.3 Cable Types
The network must be connected using a suitable cable. Two conductors plus a shield and drain wire
are required as a minimum, though Rotork recommends the use of a 3 wire cable to ensure the
common mode voltage between all the actuators and the PLC is kept within the RS-485 specified
limits.
Among the cable manufacturers, Belden have the PVC jacketed 3105A single pair 2 core cable, which
meets the minimum requirement. Belden 3106A has an extra core and therefore has sufficient
conductors to include a common wire between all the nodes on the network and meets the Rotork
recommendation. Information on Belden cable may be found on the Belden web site
(www.belden.com) or at http://bwcecom.belden.com/, where a search on RS-485 will reveal technical
papers and cable information.
Fig 13: Typical RS-485 cable
Other cable manufacturers also produce suitable cables for Modbus networks. The quality of the
Modbus network will largely depend on the cabling choices made during planning; and quality of
installation.
The terminals to which the wires connect in each actuator type will be different and the actuator wiring
diagram must be consulted to establish the connections.
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