Industrialtext & Video Company www.industrialtext.com 1-800-752-8398APLCPrimerTABLE OF CONTENTSWhat Is a PLC? pg 3Why Use PLCs? pg 4But What Exactly Is a PLC? pg 5A Little More About Inputs and Outputs pg 7And a Little More About the Control Program pg 10So How Does a PLC Keep All This Straight? pg 13To Sum It All Up pg 14Want To Learn More? pg 15©1999 by Industrial Text & Video Company. All rights reserved.A PLC PrimerIndustrial Text & Video Company 1-800-752-8398 www.industrialtext.com 3WHATIS APLC?…In a PLC system, all control devices arewired to the PLC.In a traditional system, all control devicesare wired directly to each other…A programmable logic controller, also called a PLC or program-mable controller, is a computer-type device used to control equipmentin an industrial facility. The kinds of equipment that PLCs can controlare as varied as industrial facilities themselves. Conveyor systems,food processing machinery, auto assembly lines…you name it andthere’s probably a PLC out there controlling it.In a traditional industrial control system, all control devices are wireddirectly to each other according to how the system is supposed tooperate. In a PLC system, however, the PLC replaces the wiringbetween the devices. Thus, instead of being wired directly to eachother, all equipment is wired to the PLC. Then, the control programinside the PLC provides the “wiring” connection between the devices.The control program is the computer program stored in the PLC’smemory that tells the PLC what’s supposed to be going on in thesystem. The use of a PLC to provide the wiring connections betweensystem devices is called softwiring.Let’s say that a push button is supposed to control the operation of amotor. In a traditional control system, the push button would be wireddirectly to the motor. In a PLC system, however, both the push buttonand the motor would be wired to the PLC instead. Then, the PLC’scontrol program would complete the electrical circuit between the two,allowing the button to control the motor.EXAMPLEPLCA PLC PrimerIndustrial Text & Video Company 1-800-752-8398 www.industrialtext.com4WHYUSEPLCS?The softwiring advantage provided by programmable controllers istremendous. In fact, it is one of the most important features of PLCs.Softwiring makes changes in the control system easy and cheap. Ifyou want a device in a PLC system to behave differently or to controla different process element, all you have to do is change the controlprogram. In a traditional system, making this type of change wouldinvolve physically changing the wiring between the devices, a costlyand time-consuming endeavor.Let’s say that two push buttons, PB1 and PB2, are connected to a PLC.Two pilot lights, PL1 and PL2, are also connected to the PLC. The waythese devices are connected now pressing push button PB1 turns onpilot light PL1 and pressing push button PB2 turns on pilot light PL2.Let’s say that you want to change this around so that PB1 controls PL2and PB2 controls PL1. In a traditional system, you would have to rewirethe circuit so that the wiring from the first push button goes to the sec-ond pilot light and vice versa. However, because these devices are con-nected to a PLC, making this change is as simple as making a smallchange in the control program.EXAMPLEIn addition to the programming flexibility we just mentioned, PLCsoffer other advantages over traditional control systems. These advan-tages include:• high reliability• small space requirements• computing capabilities• reduced costs• ability to withstand harsh environments• expandabilityA PLC PrimerIndustrial Text & Video Company 1-800-752-8398 www.industrialtext.com 5BUTWHATEXACTLYIS APLC?A PLC basically consists of two elements:• the central processing unit• the input/output systemThe Central Processing UnitThe central processing unit (CPU) is the part of a programmablecontroller that retrieves, decodes, stores, and processes information.It also executes the control program stored in the PLC’s memory. Inessence, the CPU is the “brains” of a programmable controller. Itfunctions much the same way the CPU of a regular computer does,except that it uses special instructions and coding to perform its func-tions. The CPU has three parts:• the processor• the memory system• the power supplyThe processor is the section of the CPU that codes, decodes, andcomputes data. The memory system is the section of the CPU thatstores both the control program and data from the equipment con-nected to the PLC. The power supply is the section that provides thePLC with the voltage and current it needs to operate.The CPU has three main parts……the processor……the memory system……and the power supply.PowerSystemMemoryProcessorCPUSupplyA PLC PrimerIndustrial Text & Video Company 1-800-752-8398 www.industrialtext.com6The Input/Output SystemThe input/output (I/O) system is the section of a PLC to which allof the field devices are connected. If the CPU can be thought of as thebrains of a PLC, then the I/O system can be thought of as the armsand legs. The I/O system is what actually physically carries out thecontrol commands from the program stored in the PLC’s memory.The I/O system consists of two main parts:• the rack• I/O modulesThe rack is an enclosure with slots in it that is connected to the CPU.I/O modules are devices with connection terminals to which thefield devices are wired. Together, the rack and the I/O modules formthe interface between the field devices and the PLC. When set upproperly, each I/O module is both securely wired to its correspond-ing field devices and securely installed in a slot in the rack. Thiscreates the physical connection between the field equipment and thePLC. In some small PLCs, the rack and the I/O modules come pre-packaged as one unit.A rack is an enclosure with slots……into which I/O modules……are installed.21032103A PLC PrimerIndustrial Text & Video Company 1-800-752-8398 www.industrialtext.com 7A LITTLEMOREABOUTINPUTSANDOUTPUTSAll of the field devices connected to a PLC can be classified in one oftwo categories:• inputs• outputsInputs are devices that supply a signal/data to a PLC. Typical ex-amples of inputs are push buttons, switches, and measurement de-vices. Basically, an input device tells the PLC, “Hey, something’s hap-pening out here…you need to