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As Figure 15 illustrates, the

combination of Hewlett-Packard

current loop optocouplers, HCPL-

4100 transmitter and HCPL-4200

receiver, can be used at the

isolated end of current loops.

Cross talk and common mode

coupling are greatly reduced

when optical isolation is imple-

mented at the same end of both

loops, as shown. Full duplex data

rate is limited by the non-isolated

active transmitter current loop.

Comments mentioned under

simplex configuration apply to

the full duplex case. Consult the

HCPL-4200 receiver optocoupler

data sheet for specified device

performance.

Half Duplex

The half duplex configuration,

whether point to point or multi-

drop, gives non-simultaneous

bidirectional data flow from

transmitters to transmitters

shown in Figures 16a and 16b.

This configuration allows the use

of two wires to carry data back

and forth between local and

remote units. However, protocol

must be used to determine which

specific transmitter can operate

at any given time. Maximum data

rate for a half duplex system is

limited by the loop current

charging time. These considera-

tions were explained in the

Simplex configuration section.

Figures 16a and 16b illustrate

half duplex application for the

combination of HCPL-4100/-4200

optocouplers. The unique and

complementary designs of the

HCPL-4100 transmitter and

HCPL-4200 receiver optocoup-

lers provide many designed-in

benefits. For example, total

optical isolation at one end of the

current loop is easily accom-

plished, which results in

substantial removal of common

mode influences, elimination of

ground potential differences and

reduction of power supply

requirements. With this combina-

tion of HCPL-4100/-4200 opto-

couplers, specific current loop

noise immunity is provided, i.e.,

minimum SPACE state current

noise immunity is 1 mA, MARK

state noise immunity is 8 mA.

Voltage compliance of the current

source must be of an adequate

level for operating all units in the

loop while not exceeding 27 V dc,

the maximum breakdown voltage

for the HCPL-4100. Note that the

HCPL-4100 transmitter will allow

output loop current to conduct

Consult the HCPL-4200 receiver

optocoupler data sheet for

specified device performance.

For more information about the

HCPL-4100/-4200 optocouplers,

consult Application Note 1018.

Figure 15. Full Duplex Point to Point Current Loop System Configuration.