1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

318 Using InterruptsUsing the SelectableTimed Interrupt (STI)Function FileThe Selectable Timed Interrupt (STI) provides a mechanism to solve timecritical control requirements. The STI is a trigger mechanism that allowsyou 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 aspecific time interval.• A motion application, where the motion instruction (PTO) needs tobe 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 ofthe 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, thecontroller monitors the STI value.3. When the time period has elapsed, the controller’s normal operationis 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 whereit was prior to the interrupt and continues normal operation.Publication 1766-RM001A-EN-P - October 2008