HCPL-0721 datasheet(12/18 Pages) HP

12 / 18 page

Optical Isolation for Field Bus

Networks

To recognize the full benefits of

these networks, each recommends

providing galvanic isolation using

Agilent optocouplers. Since

network communication is bi-

directional (involving receiving

Figure 15. Typical field bus communication physical model.

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

network receives maximum

protection from power system

faults and ground loops.

Within an isolated node, such as

the DeviceNet Node shown in

Figure 16, 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 16, power from the network

is used only for the transceiver and

input (network) side of the

optocouplers.

Isolation of nodes connected to any

of the three types of digital field

bus networks is best achieved by

using the HCPL-772X/072X

optocouplers. For each network,

the HCPL-772X/072X satisify 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.

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 CC-Link,

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 15. Each includes

one or more buses, an interface

unit, optical isolation, transceiver,

and sensing and/or actuating

devices.

CONTROLLER

TRANSCEIVER

OPTICAL

ISOLATION

BUS

INTERFACE

TRANSCEIVER

OPTICAL

ISOLATION

BUS

INTERFACE

TRANSCEIVER

OPTICAL

ISOLATION

BUS

INTERFACE

TRANSCEIVER

OPTICAL

ISOLATION

BUS

INTERFACE

TRANSCEIVER

OPTICAL

ISOLATION

BUS

INTERFACE

FIELD BUS

XXXXXX

YYY

SENSOR

DEVICE

CONFIGURATION

MOTOR

STARTER

MOTOR

CONTROLLER

NODE/APP SPECIFIC

uP/CAN

HCPL

772x/072x

HCPL

772x/072x

TRANSCEIVER

LOCAL

NODE

SUPPLY

5 V REG.

NETWORK

POWER

SUPPLY

V+ (SIGNAL)

V– (SIGNAL)

V+ (POWER)

V– (POWER)

GALVANIC

ISOLATION

BOUNDARY

AC LINE

DRAIN/SHIELD

SIGNAL

POWER

Figure 16. Typical DeviceNet Node.

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