258 Rockwell Automation Publication 2080-UM002L-EN-E - November 2021
Appendix D User Interrupts
• 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.
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