Â In addition to the standard logical operations that a PLC can perform, seasoned PLC programmers are aware that, by taking advantages of some of the unique features and characteristics of a PLC, some very powerful operations can be performed. Some of these are operations that would be very difficult to realize in hardwired relay logic, but are relatively simple in PLC ladder programs. Many of the program segments in this chapter are rather â€œcookbookâ€ by nature. The student should not concentrate on memorizing these programs, but instead, learn how they work and how they can be best applied to solve programming problems.
Ladder Program Execution Sequence
Â Many persons new to PLC ladder logic programming may tend to think that, because a PLC executes its program synchronously (i.e., from left to right, top to bottom), instead of asynchronously (i.e., each relay operates whenever it receives a signal) it is a hindrance to the programming task. However, after gaining some experience with programming PLCs, the programmer begins to learn how to use this to their advantage. We will see several useful program segments in this chapter that do this. Keep in mind that the order of the rungs in these programs is critical. If the rungs are rearranged in another order, it is likely that these programs will not operate properly
Â As we will see in the following several sections, a few of the circuits you learned to use in digital electronics can be developed for use in a ladder diagram. The ones we will study are the R-S, D, T and J-K Flip Flops and the One Shot. The One Shot will supply the clock pulse for the D, T and J-K Flip Flops. These are functions that can be very useful in a control system and tend to be more familiar to the student since they have been studied in previous courses.