Rockwell Automation 1756-IF4FXOF2F ControlLogix High-speed Analog I/O Module - Manual (Page 122)

122 Rockwell Automation Publication 1756-UM005B-EN-P - January 2013
Appendix C Use Message Instructions to Perform Run-time Services and Module Reconfiguration
Real-time Control and Module Services
Services sent via 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.
One Service Performed per Instruction
Message instructions cause a module service to be performed only once per
execution. You must reexecute a message instruction to perform the service a
second time.
EXAMPLE You may want to unlatch all process alarms on an input channel, but real-time
control of the process is still using data from the channel. Because this input
data is critical to your application, the high-speed analog I/O module
prioritizes the sampling of inputs ahead of the unlatch service request. After
the module has processed the input data, it can unlatch all process alarms.
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.
EXAMPLE If a message instruction sends a service to the module to unlatch the high high
alarm on input channel 0, that channel’s high high alarm unlatches, but may
be set on a subsequent channel sample.