1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

308 Using InterruptsAn interrupt must be configured and enabled to execute. When any oneof the interrupts is configured (and enabled) and subsequently occurs, theuser program:1. suspends its execution2. performs a defined task based upon which interrupt occurred3. returns to the suspended operation.Interrupt Operation ExampleProgram File 2 is the main control program.Program File 10 is the interrupt routine.• An Interrupt Event occurs at rung123.• Program File 10 is executed.• Program File 2 execution resumesimmediately after program file 10 isscanned.Program File 2rung 0rung 123rung 275Program File 10Specifically, if the controller program is executing normally and aninterrupt event occurs:1. the controller stops its normal execution2. determines which interrupt occurred3. goes immediately to rung 0 of the subroutine specified for that UserInterrupt4. begins executing the User Interrupt subroutine (or set of subroutinesif the specified subroutine calls a subsequent subroutine)5. completes the subroutine(s)6. resumes normal execution from the point where the controllerprogram was interruptedWhen Can the Controller Operation be Interrupted?The Micrologix 1400 controllers only allow interrupts to be servicedduring certain periods of a program scan. They are:• At the start of a ladder rung• Anytime during End of ScanPublication 1766-RM001A-EN-P - October 2008