Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicthrough a flowchart. Even the flowchart symbols themselves have specificmeanings, which aid in the interpretation of the solution algorithm. Figure 3illustrates the most common flowchart symbols and their meanings.The main flowchart itself should not be long and complex; instead, it shouldpoint out the major functions to be performed (e.g., compute engineeringunits from analog input counts). Several smaller flowcharts can be used tofurther describe the functions specified in the main flowchart.Once the flowchart is completed, the user can employ either logic gates orcontact symbology to implement the logic sequences. Logic gates implementa logical output sequence given specific real and/or internal input conditions,STARTSet PresetValuesIs PBPressed?Read AnalogInputStore InTemp. Reg.Is Temp.> 100˚CTurn HeaterCoil ONENDNoNOYesGo ToSubroutineProcessA group of one or moreinstructions that performa processing functionInput/OutputAny function involvingan input /output deviceDecisionA point in the programwhere a branch to alternatepaths is possiblePreparationA group of one or moreinstructions that setsthe stage for subsequentprocessingPredefined ProcessA group of operationsnot detailed in theflowchart (often alibrary subroutine)TerminalBeginning, end, or pointof interruption in aprogramConnectorEntry from, or exit to,another part of theflowchartFlowlineDirection of processingor data flowAnnotationDescriptive commentsor explanatory notesprovided for clarificationFigure 2. Simple flowchart.Figure 3. Flowchart symbols.Industrial Text & Video Company www.industrialtext.com 1-800-752-83988
Introduction to PLC Programming and Implementation—from relay logic to PLC logicwhile PLC contact symbology directly implements the logic necessary toprogram an output rung. Figure 4 illustrates both of these programmingmethods. Users should employ whichever method they feel most comfortablewith or, perhaps, a combination of both (see Figure 5). Logic gate diagrams,however, may be more appropriate in controllers that use Boolean instructionsets.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 aninternal. The labels used for actual input signals can be either the actualdevice names (e.g., LS1, PB10, AUTO, etc.) or symbolic letters and numbersthat are associated with each of the field elements. During this stage, the usershould prepare a short description of the logic sequence.Counter 2330 gallons of BReset B(Reset SOL2)B Finished(Start of pump back B)(a)MB FinishedCounter 2330 gallons of B(b)Reset B(Reset SOL2)B Finished(Start of pumpback B)Figure 4. (a) PLC contact symbology and (b) logic gate representation of a logicsequence.MeterSOL1Count A GallonUpC1500 Gal. of APV = 500 Gal.Clear C1A FinishedResetFigure 5. A combination of logic gates and contact symbology.Industrial Text & Video Company www.industrialtext.com 1-800-752-83989
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Programmable Controllers: Theory and Implementation

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