Figure 18. Recommended Non-Isolated Active Transmitter with HCPL-4200 Isolated Receiver for Simplex Point-to-Point
20 mA Current Loop.
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. The full duplex
data rate is limited by the non-
isolated active receiver current
loop. Comments mentioned under
simplex configuration apply to
the full duplex case. Consult the
HCPL-4100 transmitter opto-
coupler data sheet for specified
device performance.
Half Duplex
The half duplex configuration,
whether point-to-point or
multidrop, gives non-
simultaneous bidirectional data
flow from transmitters to
receivers shown in Figures 22a
and 22b. 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 considerations were
explained in the Simplex config-
uration section.
Figures 22a and 22b 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
Figure 19. Minimum Required Supply
Voltage, V CC, vs. Loop Length for
Current Loop Circuit of Figure 19.
Figure 20. Typical Data Rate vs.
Distance.
optocouplers provide many
designed-in benefits. For
example, total optical isolation at
one end of the current loop is
easily accomplished, which
results in substantial removal of
common mode influences,
elimination of ground potential
Figure 21. Full Duplex Point-to-Point Current Loop System
Configuration.
1-333
HP [ HEWLETT-PACKARD ]
