Logix5000 Controllers General Instructions - SLAC Confluence

20 PrefaceRelay ladder rung conditionThe controller evaluates ladder instructions based on the rungcondition preceding the instruction (rung-condition-in). Based on therung-condition-in and the instruction, the controller sets the rungcondition following the instruction (rung-condition-out), which inturn, affects any subsequent instruction.input instructionoutput instructionrung-inconditionrung-outconditionIf the rung-in condition to an input instruction is true, the controllerevaluates the instruction and sets the rung-out condition based on theresults of the instruction. If the instruction evaluates to true, therung-out condition is true; if the instruction evaluates to false, therung-out condition is false.The controller also prescans instructions. Prescan is a special scan ofall routines in the controller. The controller scans all main routinesand subroutines during prescan, but ignores jumps that could skip theexecution of instructions. The controller executes all FOR loops andsubroutine calls. If a subroutine is called more than once, it isexecuted each time it is called. The controller uses prescan of relayladder instructions to reset non-retentive I/O and internal values.During prescan, input values are not current and outputs are notwritten. 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 when:• The program becomes scheduled while the controller is running.• The program is unscheduled when the controller enters Runmode.Publication 1756-RM003I-EN-P - January 2007