Introduction to PLC Programming and Implementation (5)


Flowcharting is a technique often used when planning a program after a written description has been developed. A flowchart is a pictorial representation that records, analyzes, and communicates information, as well as describes the operational process in a sequential manner. Figure 2 illustrates a simple flowchart. Each step in the chart performs an operation, whether it is an input/output, decision, or data process.

In a flowchart, broad concepts and minor details, along with their relationship to each other, are readily apparent. Sequences and relationships that are hard to extract from general descriptions also become obvious when expressed through a flowchart. Even the flowchart symbols themselves have specific meanings, which aid in the interpretation of the solution algorithm. Figure 3 illustrates the most common flowchart symbols and their meanings.
The main flowchart itself should not be long and complex; instead, it should point out the major functions to be performed (e.g., compute engineering units from analog input counts). Several smaller flowcharts can be used to further describe the functions specified in the main flowchart.
Once the flowchart is completed, the user can employ either logic gates or contact symbology to implement the logic sequences. Logic gates implement a logical output sequence given specific real and/or internal input conditions,while PLC contact symbology directly implements the logic necessary to program an output rung.

Figure 4 illustrates both of these programming methods. Users should employ whichever method they feel most comfortable with or, perhaps, a combination of both (see Figure 5). Logic gate diagrams, however, may be more appropriate in controllers that use Boolean instruction sets. Inputs and outputs marked with an X on a logic gate diagram, as in Figure 4b, represent real I/O in the system. If no mark is present, an I/O point is an internal. The labels used for actual input signals can be either the actual device names (e.g., LS1, PB10, AUTO, etc.) or symbolic letters and numbers that are associated with each of the field elements. During this stage, the user should prepare a short description of the logic sequence.


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