Internal Loop Power Source - Allen-Bradley 1756-IF8I User Manual

Chapter 3 1756-IF8I Isolated Analog Input Module
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Internal Loop Power Source

The 1756-IF8I module offers a software user-configurable selection for an
internal loop power source on each channel. You must use the Current input type
and enable Source Loop Current to use an internal power source on the channel.
The source is current limited to ~45 mA and lets the module power a two-wire
transmitter directly without the need for an external power supply.
A sourcing overcurrent condition typically occurs due to a short between
terminals on the module. With this module, the short is between terminals
IN_x/I/SRC and RTN_x (where x is the channel number).
If a sourcing overcurrent condition exists, the 1756-IF8I module sets the input to
24 mA, that is, the equivalent engineering unit value. This value indicates a
special error condition beyond the normal overrange value, that is, 21 mA:
• For one second, the overcurrent condition self-corrects if the condition
trigger is removed.
• After one second, the condition latches, the channel disables Source Loop
Current and continues to send 24 mA with an Overrange indication.
The following are examples of events that unlatch the condition:
– Power is cycled to the module.
– The module is reset.
– The controller connection to the module is inhibited and
then uninhibited.
– New configuration is sent from the controller.
The transmitter varies the current to the analog input in proportion to the
process variable being measured. The inclusion of an internal onboard loop
power source saves you the expense of extra power supplies and greatly simplifies
the interface wiring to field devices. Each channel on the module provides
independent, isolated, current-limited power to its current transmitter.
In addition to supplying loop power to two-wire transmitters, the module can
also accommodate current transmitters that are powered by an external supply.
The module accommodates two-wire and four-wire transmitters when
configured for Current input type and Source Loop Current is disabled.
Rockwell Automation Publication 1756-UM540E-EN-P - December 2017