Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 61)

Rockwell Automation Publication 1756-UM533C-EN-P - February 2011 61
1756-IF8H HART Analog Input Module Chapter 4
The 1756-IF8H is a differential input module. However, there are limitations on
its use in Differential mode. Any time the low ends of the terminal block pins are
connected together, they must also be jumpered to the RTN pin on the terminal
block. There are two scenarios in which this happens.
First, if a single power supply is used for multiple devices, then the low end from
each channel is connected together and connected to the ground return of the
power supply. See the following diagram.
Figure 6 - Single Power Supply with Multiple HART Devices
The second way for channels to share a ground is to have multiple power supplies
hooked to the same ground. In this case the low ends of the channels are
effectively connected together by the common grounds of the power supplies.
Figure 7 - Multiple Power Supplies with a Common Ground
Power Supply 1
HART
Device 1
HART
Device 2
IN0+ IRTN-0
IRTN-1
RTN
IN1+
IN0-
IN 1-
+
-
32138-M
Power Supply 1
HART
Device 1
HART
Device 2
Power Supply 2
IN0+ IRTN-0
IRTN-1
RTN
IN1+
IN0-
IN1-
+
-
+
-
Ground wire from AC
source common to both
DC power supplies
32139-M