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2.9 HART Communication 10-0351 Option
Description:
The optional 10-0351 HART modem board, figure 2-13, supplies ability to access and
alter process variables of the OLCT 200 using a HART handheld device. This board is
stacked behind the 10-0232 Display Assembly. Addition of this option requires 3-wire
mode 4-20mA operation and thereby requires the 10-0233 I/O Power Supply board.
This is since the modem's driver circuits require much more power than 2-wire 4-20mA
loops can deliver. When installed in an OLCT 200 transmitter, power and common are
run to the I/O board and TB1 supplies 4-20 mA output. Common on the HART modem
and the I/O board are tied together.
In most applications the power is supplied from the controller that is receiving the 4-20
mA output. In these applications only three wires are required since common is shared
from the I/O board to the HART modem through the ribbon cable. Power is connected
to the I/O board and the signal connected to TB1 on the HART board.
If the 4-20 mA output is going to another device other than the one that is powering it,
or the transmitter has its own local power supply, both + and – on at TB1 on the HART
board must be utilized for the 4-20 mA loop to function.
IMPORTANT
The 4-20 mA output must be loaded with at least 250 ohms of impendence for the
HART modem to transmit process variables. Some devices receiving the 4-20 mA
output have a large enough terminating resistor installed from the factory, but others
may need additional resistance added. This is accomplished by adding a resistor in
Isolated 4-20mA Output Option
Assy: 10-0250
Figure 2-12: 10-0250 Isolated 4-20mA Output Option
OLCT 200 User Manual | 27
P1
1/2A
FUSE
U8
1
-
2
+
TB1
3
-
4
+
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