Allen-Bradley MicroLogix 1200 Reference Manual page 223

Specifically, if the controller program is executing normally and an
interrupt event occurs:
1. the controller stops its normal execution
2. determines which interrupt occurred
3. goes immediately to rung 0 of the subroutine specified for that User
Interrupt
4. begins executing the User Interrupt subroutine (or set of subroutines if
the specified subroutine calls a subsequent subroutine)
5. completes the subroutine(s)
6. resumes normal execution from the point where the controller
program was interrupted
When Can the Controller Operation be Interrupted?
The Micrologix 1200 and 1500 controllers only allow interrupts to be
serviced during certain periods of a program scan. They are:
• At the start of a ladder rung
• Anytime during End of Scan
• Between data words in an expansion I/O scan
The interrupt is only serviced by the controller at these opportunities. If
the interrupt is disabled, the pending bit is set at the next occurrence of
one of the three occasions listed above.
If you enable interrupts during the program scan via an
ATTENTION
OTL, OTE, or UIE, this instruction (OTL, OTE, or UIE)
must be the last instruction executed on the rung (last
!
instruction on last branch). It is recommended this be the
only output instruction on the rung.
Using Interrupts 18-3
Publication 1762-RM001C-EN-P