“Both Input_1 and Input_2 must be false for Output_1 to remain True.”įigure 4. The NOR is more typical for an emergency stop (e-stop) situation (as shown in figure 4). real-world sensors, like an E-stop circuit, but it can also easily be accomplished with existing N.O. Now, this could be accomplished with N.C. If either sensor goals false, an alarm must sound. Regarding the NAND: “Output_1 will be True if either Input_1 or Input_2 are False”.Īs an example, two liquid tanks must be full, each monitored by a sensor. Whatever combination made the original logic True is now the exact combination making the output false. To best consider their effect in ladder logic, think of the AND/OR logic, respectively the result is the exact opposite. The next two logical combinations are NAND as well as NOR. Ladder diagram showing an inverter boolean logic structure. The inverter simply causes a True input condition to create a False output condition and, therefore, is the only logical structure with only one input.įigure 3. The first three are the Inverter, AND, as well as OR. AND is one of the simplest.īoolean Logic Structures: Inverter, AND, OR There are seven primary boolean logic structures. Same logic as figure 1 but shown in C++ programming language.īoth of these code examples follow the rules of boolean AND logic. If (Input_1 = true & Input_2 = true) Figure 2. To turn the comparison to a setting a computer programmer would better recognize, the C++ code would be as shown in figure 2. In this statement, the use of the term AND indicates the logical instruction. “Input_1 and Input_2 must both be TRUE in order for Output_1 to become TRUE”. Since ladder logic, as indicated by name, is logical, we should be able to express the statement in plain English. In this diagram, two inputs are placed in series leading to the output. An example ladder diagram showing two inputs in a series leading to the output. Without some understanding, this can be confusing.įirst, and most simply, we can place simple discrete instructions onto a ladder logic rung, creating a set of true/false combinations to activate output coils.Ĭonsider the following example ladder diagram in figure 1.įigure 1. This is the difference in programmable logic controller (PLC) instructions between discrete structures, like two Normally Open contacts in series AND logic, versus two integers matched together with a Bitwise AND command. So that means that we can work with bits by themselves one at a time, or we can deal with chunks of bits altogether. All other integers, strings, and various types can be simplified into the individual bits of which they are made (e.g., a 16-bit integer really is just 16 bits). The bool-also called “bit”-is unique in data structures in that it cannot be broken down into smaller pieces. But this concept applies to far more than simply the tag listing, and the ability to execute instructions based on boolean logic combinations is an important concept to understand. This statement is commonly repeated through many fields of computer science and electronics, yet it can be difficult to understand in a meaningful way for engineers and technicians who work with digital devices on a daily basis.įor those familiar with any brand of PLC, the “boolean” or “BOOL” data type is the core atomic data type used to generate tags in a database. Every device controlled by a central processing unit (CPU) understands instructions in the form of 1’s and 0’s.
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