Specifications

24 Control and Automation Solutions Guide
Overview
A eldbus is the communications link
between dispersed process-control
equipment. The ocial meaning of
eldbus describes the set of protocols
that have been recognized by the
Fieldbus Foundation. Distributed
control allows local and 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 eld.
An example of a eldbus network
application would be along an
automobile assembly line, where the
eldbus 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 eldbus
network use Ethernet-based networking.
This hierarchy melds with the other
corporate-management IT systems.
Fieldbuses are bidirectional, digital,
serial networks. Protocols that have been
ocially recognized by the Fieldbus
Foundation and formalized in IEC 61158
include PROFIBUS, ControlNet, Interbus,
and others. PROFIBUS DP (decentralized
peripheral) has become one of the most
commonly used deterministic eldbuses
for factory automation. PROFIBUS DP
is primarily used to network multiple
controllers in decentralized locations.
The physical layer of a eldbus
is commonly based on RS-485,
CAN, and Ethernet. The eldbus
connects to a PLC subsystem with
a eldbus module, as shown in the
gure on the following page.
The eldbus module bridges the PLC
system’s backplane protocol to the
eldbus protocol. 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.
Regardless of which eldbus protocol
is used, isolation is required between
the PLC and its distributed subsystems
Fieldbus Functions
INDUSTRIAL ETHERNET
COMMUNICATION
FIELDBUS OR SENSOR/ACTUATOR BUS
The fieldbus supports communications throughout the factory.