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Petruzella_ProgrammableLogicControllers__5e

A B C Y A branch within a branch D E A B C Y D C E Contact instruction C repeated Maximum 7 parallel lines 88 Chapter 5  Basics of PLC Programming limitation diagram for a typical PLC. A maximum of seven parallel lines and 10 series contacts per rung is possible. Another limitation to branch circuit programming is that the PLC will not allow for programming of vertical contacts. A typical example of this limitation is contact C of the user program drawn in Figure  5-37. To obtain the required logic, the circuit would be reprogrammed as shown in Figure 5-38. The processor examines the ladder logic rung for logic continuity from left to right only. The processor never allows for flow from right to left. This situation presents a a logically equivalent branching condition. Figure  5-34 shows an example of a circuit that contains a nested contact D. To obtain the required logic, the circuit would be programmed as shown in Figure 5-35. The duplication of contact C eliminates the nested contact D. Nested branching can be converted into non-nested branches by repeating instructions to make parallel equivalents. Some PLC manufacturers have virtually no limitations on allowable series elements, parallel branches, or outputs. For others, there may be limitations to the number of series contact instructions that can be included in one rung of a ladder diagram as well as limitations to the number of parallel branches. Also, there is an additional limitation with some PLCs: only one output per rung and the output must be located at the end of the rung. The only limitation on the number of rungs is memory size. Figure 5-36 shows the matrix Figure 5-34  Nested contact program. Figure 5-35  Program required to eliminate nested contact. C Boolean equation: Y = (AD) + (BCD) + (BE) + (ACE) Figure 5-37  Program with vertical contact. Maximum 10 contacts Figure 5-36  PLC matrix limitation diagram. A D B E Y A D Y B C D B E A C E Figure 5-38  Reprogrammed to eliminate vertical contact.


Petruzella_ProgrammableLogicControllers__5e
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