Manual

Publication LOGIX-UM001B-EN-P - April 2010 57
Equipment Phase Instructions (PSC, PCMD, POVR, PFL, PCLF, PXRQ, PRNP, PPD, PATT, PDET) Appendix A
Prescan of Routines
The controller also prescans routines. Prescan is a special scan of all routines in
the controller. During prescan, the controller:
• scans all main routines.
• scans all state routines of equipment phases.
• scans all prestate routines of equipment phases.
• scans all subroutines of programs and equipment phases one time.
Once the controller prescans a subroutine, it does not prescan the
subroutine again during that prescan.
• scans all routines called by FOR instructions of a ladder diagram
routine.
• ignores jumps that could skip the execution of instructions.
• executes all instructions in prescan mode.
For details on how a specific instruction operates during prescan, see the
Execution section for the instruction.
• resets to 0 all non-retentive assignments.
• does not update input values.
• does not write output values.
The following conditions generate prescan:
• Toggle from Program to Run mode
• Automatically enter Run mode from a power-up condition
Prescan does not occur for a program or equipment phase when:
• the program or equipment phase becomes scheduled while the
controller is running.
• the program or equipment phase is unscheduled when the controller
enters Run mode.