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B D A C Figure 5-30  Parallel input branches. A C B D E Figure 5-31  Parallel output branches. illustrated in Figure 5-31. When there is a true logic rung path, all parallel outputs become true. In the example shown, either A or B provides a true logical path to all three output instructions: C, D, and E. Additional input logic instructions (conditions) can be programmed in the output branches to enhance conditional control of the outputs. When there is a true logic path, including extra input conditions on an output branch, that branch becomes true. In the example shown in Figure 5-32, either A and D or B and D provide a true logic path to E. Input and output branches can be nested to avoid redundant instructions and to speed up processor scan time. Figure 5-33 illustrates nested input and output branches. A nested branch starts or ends within another branch. In some PLC models, the programming of a branch circuit within a branch circuit or a nested branch cannot be done directly. It is possible, however, to program 1 2 1 L L Inputs L L PB1 LS1 I:4/5 I:4/6 R Figure 5-28  I/O connection diagram. Outputs O:2/3 O:3/6 SOL1 PL1 2 Allen-Bradley Logix 5000 controllers offer a more flexible method of addressing memory space. Instead of a fixed device with a fixed address space, tags are used for assigning and referencing memory spaces. Tags are a pure text based addressing scheme and a departure from the more conventional ways of programming PLCs. The assignment of an I/O address can be included in the I/O connection diagram, as shown in Figure 5-28. Inputs and outputs are typically represented by squares and diamonds, respectively. 5.6  Branch Instructions Branch instructions are used to create parallel paths of input condition instructions. This allows more than one combination of input conditions (OR logic) to establish logic continuity in a rung. Figure 5-29 illustrates a typical branch instruction. The rung will be true if either instruction A or B is true. Input branching by formation of parallel branches can be used in your application program to allow more than one combination of input conditions. If at least one of these parallel branches forms a true logic path, the rung logic is true and the output will be energized. If none of the parallel branches complete a logical path, logic rung continuity is not established and the output will be de-energized. In the example shown in Figure 5-30, either A and B, or C provides logical continuity and energizes output D. On most PLC models, branches can be established at both input and output portions of a rung. With output branching, you can program parallel outputs on a rung to allow a true logic path to control multiple outputs, as A B C Figure 5-29  Typical branch instruction. A C B D E Figure 5-32  Parallel output branching with conditions. Figure 5-33  Nested input and output branches. Basics of PLC Programming  Chapter 5 87


Petruzella_ProgrammableLogicControllers__5e
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