Rockwell Automation 1756-CFM ControlLogix Configurable Flowmeter Module - Manual (Page 89)

89 Rockwell Automation Publication 1756-UM010B-EN-P - December 2011
Configuring the Configurable Flowmeter Module 89
Using Message
Instructions
Ladder logic uses message instructions to change the CFM module
configuration during module operation.
Message instructions maintain the following characteristics:
 messages use unscheduled portions of system communications
bandwidth
 one service is performed per instruction
 performing module services does not impede module functionality, such
as counting incoming pulses
Processing Real-Time Control and Module Services
Because message instructions use unscheduled portions of systems
communications bandwidth, the services requested of an CFM module are not
guaranteed to occur within a specific time period. Although the module
response typically occurs in less than a second, there is no specific time
interval that reflects this response.
One Service Performed Per Instruction
Message instructions only cause a module service to be performed once per
execution. For example, if a message instruction sends new configuration data
to the CFM module, the message instruction must be reexecuted to update
send the configuration data in the future.