How to Create Flowcharts and Output Sequences for PLC Programming

Flowcharting is an approach used to plan a program after a detailed written description has been created. A flowchart is an illustration that documents, analyses, and communicates information. It also describes the operational process in a sequence. The flowchart shown in Figure 2 is a simple example. The chart shows a simple flowchart. Each step performs an operation.
Simple flowchart

Figure 2: Simple flowchart

A flowchart makes it easy to see the relationship between minor details and broad concepts. A flowchart makes it easy to see relationships and sequences that are difficult to deduce from general descriptions. The flowchart symbols have specific meanings that aid in understanding the solution algorithm. Figure 3 shows the meanings of the most commonly used flowchart symbols.

Flowchart symbols
Figure 3: Flowchart symbols

The main flowchart should not be lengthy and complicated. Instead, it should highlight the most important functions that must be performed (e.g. compute engineering units from analogue input counts). To further explain the functions in the main flowchart, you can use several flowcharts.
After the flowchart has been completed, the user may use logic gates or contact symbology in order to implement the logic sequences. PLC contact symbology implements the logic required to program an output step. Logic gates create a logical output sequence based on specific input conditions.
Figure 4 shows both these programming methods. The user should choose the method that is most familiar to them, or a combination of both (see figure 5). However, logic gate diagrams may not be as appropriate for controllers that use Boolean instruction set.
PLC contact symbology and (b) logic gate representation of a logic

Figure 4: (a) PLC contact symbology and (b) logic gate representation of a logic

A combination of logic gates and contact symbology.

Figure 5: A combination of logic gates and contact symbology.

As shown in Figure 4b (Logic gate diagram), inputs and outputs marked by an X are real I/O. An internal is an I/O point if there is no mark. You can use the actual device names, such as LS1, PB10 or AUTO, to label actual input signals. The labels used for actual input signals can be either the actual device names (e.g. LS1, PB10, AUTO), or symbols letters and numbers that are associated each of the field elements. The user should provide a brief description of the logic sequence during this stage.

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