Digital Field Bus
Communication
Networks
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
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
The physical model for each of
these digital field bus communica-
tion networks is very similar as
shown in Figure 16. Each
includes one or more buses, an
interface unit, optical isolation,
transceiver, and sensing and/or
actuating devices.
information.
CONTROLLER
BUS
INTERFACE
OPTICAL
ISOLATION
TRANSCEIVER
FIELD BUS
TRANSCEIVER
TRANSCEIVER
TRANSCEIVER
TRANSCEIVER
OPTICAL
ISOLATION
OPTICAL
ISOLATION
OPTICAL
ISOLATION
OPTICAL
ISOLATION
BUS
INTERFACE
BUS
INTERFACE
BUS
INTERFACE
BUS
INTERFACE
XXXXXX
YYY
SENSOR
DEVICE
CONFIGURATION
MOTOR
CONTROLLER
MOTOR
STARTER
Figure 16. Typical Field Bus Communication Physical Model.
the network is used only for the
transceiver and input (network)
side of the optocouplers.
false signals. In addition, the
network receives maximum
protection from power system
faults and ground loops.
Optical Isolation for
Field Bus Networks
To recognize the full benefits of
these networks, each recom-
mends providing galvanic
isolation using Hewlett-Packard
optocouplers. Since network
communication is bi-directional
(involving receiving data from
and transmitting data onto the
network), two Hewlett-Packard
optocouplers are needed. By
providing galvanic isolation, data
integrity is retained via noise
reduction and the elimination of
Isolation of nodes connected to
any of the three types of digital
field bus networks is best
Within an isolated node, such as
the DeviceNet Node shown in
Figure 17, 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 17, power from
achieved by using the HCPL-0710
optocoupler. For each network,
the HCPL-0710 satisifies the
critical propagation delay and
pulse width distortion require-
ments over the temperature range
of 0°C to +85°C, and power
supply voltage range of 4.5 V
to 5.5 V.
HP [ HEWLETT-PACKARD ]
