Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 339)

Publication 1756-UM009C-EN-P - December 2010 339
Appendix
C
Use Ladder Logic To Perform Run Time
Services and Reconfiguration
You can use ladder logic to perform run time services on your module. For
example,
page 212 explains how to unlatch alarms on the 1756-IF6I module by
using RSLogix 5000 software. This appendix provides an example of how to
unlatch those same alarms without using RSLogix 5000 software.
In addition to performing run time services, you can use ladder logic to change
configuration.
Chapter 10 explains how to use the RSLogix 5000 software to set
configuration parameters in your ControlLogix analog I/O module. Some of
those parameters also may be changed through ladder logic.
Using Message Instructions
In ladder logic, you can use message instructions to send occasional services to
any ControlLogix I/O module. Message instructions send an explicit service
to the module, causing specific behavior to occur. For example, unlatching a
high alarm can be performed by a message instruction.
Message instructions maintain the following characteristics:
• Messages use unscheduled portions of system communication
bandwidth
• One service is performed per instruction
• Performing module services does not impede module functionality, such
as sampling inputs or applying new outputs
Processing Real-time Control and Module Services
Services sent through message instructions are not as time critical as the
module behavior defined during configuration and maintained by a real-time
connection. Therefore, the module processes messaging services only after the
needs of the I/O connection have been met.
For example, you may want to unlatch all process alarms on the module, but
real-time control of your process still is occurring by using the input value
from that same channel. Because the input value is critical to your application,
the module prioritizes the sampling of inputs ahead of the unlatch
service request.