HCPL-4100 Datasheet, PDF (9/12 Pages) Agilent(Hewlett-Packard) – Optically Coupled 20 mA Optically Coupled 20 mA

Applications
Data transfer between equipment which employs current
loop circuits can be accomplished via one of three configu-
rations: simplex, half duplex or full duplex communication.
With these configurations, point-to-point and multidrop
arrangements are possible.The appropriate configuration
to use depends upon data rate, number of stations, number
and length of lines, direction of data flow, protocol, current
source location and voltage compliance value, etc.
Simplex
The simplex configuration, whether point to point or
multidrop, gives unidirectional data flow from transmit-
ter to transmitter(s). This is the simplest configuration for
use in long line length (two wire), moderate data rate, and
low current source compliance level applications. A block
diagram of simplex point to point arrangement is given in
Figure 12 for the HCPL-4100 transmitter optocoupler.
Majorfactorswhichlimitmaximumdatarate performance
for a simplex loop are the location and compliance volt-
age of the loop current source as well as the total line
capacitance. Application of the HCPL-4100 transmitter
in a simplex loop necessitates thtat a non-isolated active
receiver (containing current source) be used at the opposite
end of the current loop. With long line length, large line
capacitance will need to be charged to the compliance
voltage level of the current source before the receiver
loop current decreases to zero.This effect limits upper data
rate performance. Slower data rates will occur with larger
compliance voltage levels. The maximum compliance level
is determined by the transmitter breakdown characteristic.
In addition, adequate compliance of the current source
must be available for voltage drops across station(s) dur-
ing the MARK state in multidrop applications for long
line lengths.
In a simplex multidrop application with multiple HCPL-
4100 transmitters and one non-isolated active receiver,
priority of transmitters must be established.
A recommended non-isolated active receiver circuit which
can be used with the HCPL-4100 in point-to-point or in
multidrop 20 mA current loop applications is given in Figure
13. This non-isolated active receiver current threshold must
be chosen properly in order to provide adequate noise
immunity as well as not to detect SPACE state current (bias
current) of the HCPL-4100 transmitter. The receiver input
threshold current is Vth/Rth | 10 mA. A simple transistor
current source provides a nominal 20 mA loop current over
a V_CC compliance range of 6 V dc to 27 V dc. A resistor can
be used in place of the constant current source for simple
applications where the wire loop distance and number
of stations on the loop are fixed. A minimum transmitter
output load capacitance of 1000 pF is required between
pins 3 and 4 to ensure absolute stability.
Length of current loop (one direction) versus minimum
required DC supply voltage, V , of the circuit in Figure 13
CC
is graphically illustrated in Figure 14. Multidrop configura-
tions will require largerV than Figure 14 predicts in order
CC
to account for additional station terminal voltage drops.
Typical data rate performance versus distance is illustrated
in Figure 15 for the combination of a non-isolated active
receiver and HCPL-4100 optically coupled current loop
transmitter shown in Figure 13. Curves are shown for
Figure 12. Simplex Point to Point Current Loop System Configuration.
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