Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 242)

Chapter 13
Using Application Specific Instructions
13–16
JSR stack depth is limited to three. You may call other subroutines to a level
three deep from an STI subroutine.
Interrupt Latency and Interrupt Occurrences
Interrupt latency is the interval between the STI timeout and the start of the
interrupt subroutine. STI interrupts can occur at any point in your program,
but not necessarily at the same point on successive interrupts. The table
below shows the interaction between an interrupt and the controller operating
cycle.
STI
Between instruction updates
Between communication packets
At start and end
Note that STI execution time adds directly to the overall scan time. During
the latency period, the controller is performing operations that cannot be
disturbed by the STI interrupt function.
Interrupt Priorities
Interrupt priorities are as follows:
1. User Fault Routine
2. High-Speed Counter
3. Selectable Timed Interrupt
An executing interrupt can only be interrupted by an interrupt having a
higher priority.
Status File Data Saved
Data in the following words is saved on entry to the STI subroutine and
re-written upon exiting the STI subroutine.
•
S0 Arithmetic flags
•
S13 and S14 Math register
•
S24 Index register
Controller Overhead
Communication
Output Scan
Program Scan
Input Scan
Events in the controller operating cycle