Publication 1763-RM001C-EN-P - October 2009
268 Using Interrupts
Using the Selectable
Timed Interrupt (STI)
Function File
The Selectable Timed Interrupt (STI) provides a mechanism to solve time
critical control requirements. The STI is a trigger mechanism that allows
you to scan or solve control program logic that is time sensitive.
Example of where you would use the STI are:
• PID type applications, where a calculation must be performed at a
specific time interval.
• A motion application, where the motion instruction (PTO) needs to
be scanned at a specific rate to guarantee a consistent acceleration/
deceleration profile.
• A block of logic that needs to be scanned more often.
How an STI is used is typically driven by the demands/requirements of
the application. It operates using the following sequence:
1. The user selects a time interval.
2. When a valid interval is set and the STI is properly configured, the
controller monitors the STI value.
3. When the time period has elapsed, the controller’s normal operation
is interrupted.
4. The controller then scans the logic in the STI program file.
5. When the STI file scan is completed, the controller returns to where
it was prior to the interrupt and continues normal operation.
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