Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 214)

214 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Appendix C Use Ladder Logic To Perform Run Time Services and Reconfiguration
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 is still 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.
This prioritization allows input channels to be sampled at the same frequency
and the process alarms to be unlatched in the time between sampling and
producing the real-time input data.
One Service Performed Per
Instruction Message instructions will only cause a module service to be performed once per
execution. For example, if a message instruction sends a service to the module to
unlatch the high high alarm on a particular channel, that channel’s high high
alarm will unlatch, but may be set on a subsequent channel sample. The message
instruction must then be re-executed to unlatch the alarm a second time.
Create a New Tag This section shows how to create a tag in ladder logic when adding a message
instruction. Ladder logic is located in the main routine within
RSLogix 5000 software.
Follow these steps to create a tag.
1. Start the RSLogix 5000 software and open an existing I/O project or
create a new one.
2. On the Controller Organizer, double-click MainRoutine.
Expand MainProgram to see Main Routine as a sub-menu item.