Ladder Diagram Rung Condition
Logix-based controllers evaluate ladder instructions based on the rung condition preceding the instruction (that is, rung-condition-in). Based on the rung-condition-in and the instruction, the controller sets the rung condition following the instruction (that is, rung-condition-out), which, in turn, affects any subsequent instruction.
If the rung-in condition to an input instruction is true, the controller evaluates the instruction and sets the rung-out condition based on the results of the instruction. If the instruction evaluates to true, the rung-out condition is true; if the instruction evaluates to false, the rung-out condition is false.
The controller also prescans instructions. Prescan is a special scan of all routines in the controller. The controller scans all main routines and subroutines during prescan, but ignores jumps that could skip the execution of instructions. The controller executes all FOR loops and subroutine calls. If a subroutine is called more than once, it is executed each time it is called. The controller uses prescan of ladder diagram instructions to reset non-retentive I/O and internal values.
During prescan, input values are not current and outputs are not written. The following conditions generate prescan:
- Toggling from Program to Run mode
- Automatically entering 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 Run mode
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