Symbol Output energize Output data 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Program Input data I:1/1 1 Output module I:1/4 O:2/1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 I:1/1 I:1/4 1 1 Input module Figure 5-23 Output Energize (OTE) instruction. O:2/1 ON the concept. A better approach is to separate the action of the field device from the action of the PLC bits as illustrated in Figure 5-24. A signal present makes the NO bit (1) true; a signal absent makes the NO bit (0) false. The reverse is true for an NC bit. A signal present makes the NC bit (1) false; a signal absent makes the NC bit (0) true. The main function of the ladder logic diagram program is to control outputs based on input conditions, as illustrated in Figure 5-25. This control is accomplished through the use of what is referred to as a ladder rung. In general, a rung consists of a set of input conditions, represented by contact instructions, and an output instruction at the end of the rung, represented by the coil symbol. Sometimes beginner programmers who are used to thinking in terms of hardwired relay control circuits tend to use the same type of contact (NO or NC) in the ladder logic program that corresponds to the type of field switch wired to the discrete input. While this is true in many instances, it is not the best way to think of Input module A A Bit status Ladder logic program FALSE A A TRUE OUTPUT OFF ON Button not actuated 0 Bit status Ladder logic program TRUE A A FALSE Input module OUTPUT ON OFF Button actuated 1 Figure 5-24 Separating the action of the field device and PLC bit. Inputs Outputs I/1 I/2 I/3 O/1 Rung 0 Rung 1 O/1 I/2 I/4 O/2 Figure 5-25 Ladder logic diagram rungs. Basics of PLC Programming Chapter 5 85
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