Digital Field Bus Communication
Networks
monitor it. With the advent of
digital field bus communication
networks such as DeviceNet,
PROFIBUS, and Smart
Distributed Systems (SDS), gone
are the days of constrained
information. Controllers can now
receive multiple readings from
field devices (sensors, actuators,
etc.) in addition to diagnostic
information.
The physical model for each of
these digital field bus communica-
tion networks is very similar as
shown in Figure 17. Each includes
one or more buses, an interface
unit, optical isolation, transceiver,
and sensing and/or actuating
devices.
To date, despite its many draw-
backs, the 4 - 20 mA analog
current loop has been the most
widely accepted standard for
implementing process control
systems. In today’s manufacturing
environment, however, automated
systems are expected to help
manage the process, not merely
CONTROLLER
BUS
INTERFACE
OPTICAL
ISOLATION
TRANSCEIVER
FIELD BUS
TRANSCEIVER
TRANSCEIVER
TRANSCEIVER
TRANSCEIVER
OPTICAL
OPTICAL
OPTICAL
OPTICAL
ISOLATION
ISOLATION
ISOLATION
ISOLATION
BUS
INTERFACE
BUS
INTERFACE
BUS
INTERFACE
BUS
INTERFACE
XXXXXX
YYY
SENSOR
DEVICE
CONFIGURATION
MOTOR
CONTROLLER
MOTOR
STARTER
Figure 17. Typical field bus communication physical model.
Optical Isolation for Field Bus
Networks
network receives maximum
protection from power system
faults and ground loops.
transceiver and input (network)
side of the optocouplers.
To recognize the full benefits of
these networks, each recommends
providing galvanic isolation using
Agilent optocouplers. Since
network communication is bi-
directional (involving receiving
data from and transmitting data
onto the network), two Agilent
optocouplers are needed. By
providing galvanic isolation, data
integrity is retained via noise
reduction and the elimination of
false signals. In addition, the
Isolation of nodes connected to
any of the three types of digital
field bus networks is best
achieved by using the HCPL-x710
optocouplers. For each network,
the HCPL-x710 satisify the critical
propagation delay and pulse
width distortion requirements
over the temperature range of 0°C
to +85°C, and power supply
Within an isolated node, such as
the DeviceNet Node shown in
Figure 18, some of the node’s
components are referenced to a
ground other than V- of the
network. These components could
include such things as devices
with serial ports, parallel ports,
RS232 and RS485 type ports. As
shown in Figure 18, power from
the network is used only for the
voltage range of 4.5 V to 5.5 V.
11
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
