11
As Figure 15 illustrates, the
combination of Agilent 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
when input V
CC
power is off.
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.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
