Applications
Data transfer between equipment
which employs current loop
circuits can be accomplished via
one of three configurations:
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 multi-
drop, gives unidirectional data
flow from transmitter to trans-
mitter(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 11 for the HCPL-4100
transmitter optocoupler.
Major factors which limit maxi-
mum data rate performance for a
simplex loop are the location and
compliance voltage 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) during the MARK state
in multidrop applications for long
line lengths.
In a simplex multidrop applica-
tion 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 12. 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 capac-
itance 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
CC
,
of the circuit in Figure 12 is
graphically illustrated in Figure
13. Multidrop configurations will
require larger V
CC
than Figure 13
predicts in order to account for
additional station terminal
voltage drops.
Typical data rate performance
versus distance is illustrated in
Figure 14 for the combination of
a non-isolated active receiver and
HCPL-4100 optically coupled
current loop transmitter shown in
Figure 12. Curves are shown for
1-369
Figure 11. Simplex Point to Point Current Loop System Configuration.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
