Specifications

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Programmable logic controllers (PLCs)
Fieldbus functions
Overview
A fieldbus is the communication
medium used in industrial automa-
tion systems and in process control
to interconnect subsystems that
are spatially dispersed. Distributed
control allows local, hierarchical
control. There is an important
advantage of such a noncentralized
control strategy: it avoids high
processing power and extensive
cabling. Control subsystems can be
located close to the sensors and
actuators in the field. An example of
a fieldbus network is an automobile
assembly line, where the fieldbus
interconnects controllers located at
each assembly stage.
Basic composition of a
fieldbus
A PLC system has a hierarchical
structure in which the upper levels
of the fieldbus network use Ethernet-
based networking. This hierarchy
melds with the other corporate-
management IT systems.
Fieldbuses are bidirectional, digital,
serial networks. CANopen, CCLINK,
ControlNet, DeviceNet, Ethernet,
Interbus, Modbus®, and PROFIBUS
are examples of fieldbus networks.
PROFIBUS DP (decentralized periph-
eral) has become one of the most
commonly used fieldbuses for
factory automation. PROFIBUS DP is
primarily used to network multiple
controllers in decentralized locations.
The physical layer of a fieldbus is
commonly based on RS-485, CAN,
and Ethernet. The fieldbus connects
to a PLC subsystem with a fieldbus
module, as shown below.
The fieldbus module bridges the
PLC’s system backplane to the
fieldbus. The backplane, which is
common to all modules in the PLC
system, can be based on half- or
full-duplex RS-485. RS-485 is ideally
suited to backplane interconnect in
industrial applications because of its
high-EMI tolerance, high speed, and
hot-plug capability.
Fieldbus functions
The fieldbus supports communications throughout the factory.
INDUSTRIAL ETHERNET
COMMUNICATION
FIELDBUS OR SENSOR/ACTUATOR BUS