Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicNew Applications• Understand the desired function ofthe system.• Review possible control methodsand optimize the process operation.• Flowchart the process operation.• Implement the flowchart by usinglogic diagrams or relay logicsymbology.• Assign real I/O addresses andinternal addresses to inputs andoutputs.• Translate the logic implementationinto PLC coding.Modernizations• Understand the actual process ormachine function.• Review machine logic of operationand optimize when possible.• AssignrealI/Oandinternaladdressesto inputs and outputs.• Translate relay ladder diagram intoPLC coding.Table 1. Programming guidelines.As mentioned previously, understanding the process or machine operationis the first step in a systematic approach to solving the control problem. Fornew applications, the strategy should follow the problem definition. Reviewingstrategies for new applications, as well as revising the actual method ofcontrol for a modernization project, will help detect errors that were introducedduring the planning stages.The programming stage reveals the difference between new and modernizationprojects. In a modernization project, the user already understands theoperation of the machine or process, along with the control task. An existingrelay ladder diagram, like the one shown in Figure1, usually defines thesequence of events in the control program. This ladder diagram can be almostdirectly translated into PLC ladder diagrams.New applications usually begin with specifications given to the person whowill design and install the control system. The designer translates thesespecifications into a written description that explains the possible controlstrategies. The written explanation should be simple to avoid confusion. Thedesigner then uses this explanation to develop the control program.4 PROGRAM ORGANIZATION AND IMPLEMENTATIONOrganization is a key word when programming and implementing a controlsolution. The larger the project, the more organization is needed, especiallywhen a group of people is involved.In addition to organization, a successful control solution also depends on theability to implement it. The programmer must understand the PLC controltask and controlled devices, choose the correct equipment for the jobIndustrial Text & Video Company www.industrialtext.com 1-800-752-83986
Introduction to PLC Programming and Implementation—from relay logic to PLC logicL1L2PB14LS7CR1CR1PL3PS7CR1SOLSOL3 UPCR2LS8PS7LS9CR2SOL4 FWDStartResetCR2SOL5 DWNLS8CR3CR3PL4Figure 1. Electromechanical relay circuit diagram.(hardware and software), and understand the PLC system. Once thesepreliminary details are understood, the programmer can begin sketching thecontrol program solution. The work performed during this time forms animportant part of the system or project documentation. Documenting a systemonce it is installed and working is difficult, especially if you do notremember how you got it to work in the first place. Therefore, documentingthe system throughout its development will pay off in the end.CREATING FLOWCHARTS AND OUTPUT SEQUENCESFlowcharting is a technique often used when planning a program after awritten description has been developed. A flowchart is a pictorial representationthat records, analyzes, and communicates information, as well asdescribes the operational process in a sequential manner. Figure 2 illustratesa simple flowchart. Each step in the chart performs an operation, whether itis an input/output, decision, or data process.In a flowchart, broad concepts and minor details, along with their relationshipto each other, are readily apparent. Sequences and relationships that are hardto extract from general descriptions also become obvious when expressedIndustrial Text & Video Company www.industrialtext.com 1-800-752-83987
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