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Appendix C User Interrupts
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 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 POU.
5. When the STI POU is completed, the controller returns to where it was prior to the
interrupt and continues normal operation.
Selectable Time Interrupt
(STI) Function
Configuration and Status
This section covers the configuration and status management of the STI function.
STI Function Configuration
STI Program POU
This is the name of the Program Organizational Unit (POU) which is executed immediately
when this STI Interrupt occurs. You can choose any pre-programmed POU from the drop-down
list.
STI Auto Start (STI0.AS)
The AS (Auto Start) is a control bit that can be used in the control program. The auto start bit is
configured with the programming device and stored as part of the user program. The auto
start bit automatically sets the STI Timed Interrupt Enable (STI0.Enabled) bit when the
controller enters any executing mode.
STI Set Point Milliseconds Between Interrupts (STI0.SP)
When the controller transitions to an executing mode, the SP (set point in milliseconds) value
is loaded into the STI. If the STI is configured correctly, and enabled, the POU in the STI
configuration is executed at this interval. This value can be changed from the control program
by using the STIS instruction.
Sub-Element Description Data Format User Program Access
AS - Auto Start binary (bit) read only
Sub-Element Description Data Format Range User Program Access
SP - Set Point Msec word (INT) 0…65,535 read/write
The minimum value cannot be less than the time required to scan the
STI POUplus the Interrupt Latency.