Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 351)

Selectable Timed Interrupt
Chapter 22
22-4
Service time (Figure 22.2b) is factory set at approximately 2/3 of the timed
interrupt period. Your subroutine execution time plus program transition
time (Figure 22.2c) should not exceed the maximum allowable service
time (Figure 22.2b). When the program transition time plus subroutine
execution time does not exceed service time, there is sufficient time for the
processor to scan the subroutine.
Figure 22.2
Time Relationship of Subroutine Execution Time versus Service Time
Time Base
Service Time
0 1 2 3 4 5 6
Subroutine
Execution
Time
Less Than
Service Time
Subroutine
Execution
Time
Greater Than
Service Time
Subroutine
Execution
Time
Greater Than
STI Period
Figure 15.2b
Figure 15.2c
Figure 15.2d
Figure 15.2e
Figure 15.2a
Should the subroutine execution time plus program transition time exceed
service time (Figure 22.2d), the selectable timed interrupt is no longer
accurate. Two things happen:
The STI too long bit (02702) is set. This bit is available for your use
and can be used to initiate annunciators or other status indicators. The
bit functions similar to a Latch instruction and must be reset with an
Unlatch instruction.
The processor completely executes the subroutine. It waits one full STI
period before executing the subroutine again (Figures 22.2d and 22.2e).
Operational Overview