What is a PLC ?


Programmable Logic Controller is a solid-state control system that has a user-programmable memory for storage of instructions to implement specific functions such as Input/output (I/O) control, logic, timing , counting, report generation, communication, arithmetic, and data file manipulation. A controller consists of a central processor, inputoutput interface, and memory. A controller is designed as an industrial control system.

Simply put, a PLC is a computer like control device that reads input and energizes electrical devices called outputs based on a user programmable memory. This controls machinery or an industrial process.

Programmable Logic Controllers or PLC are the hub of many manufacturing processes. These microprocessor based units are used in processes as simple as boxing machines or bagging equipment to controlling and tracking sophisticated manufacturing processes. They are in virtually all new manufacturing, processing and packing equipment in one form or another. Because of their popularity in industry, it becomes increasingly more important to learn skills related to these devices.

How does a PLC work ?


The microprocessor or processor module is the brain of a PLC system. It consists of the microprocessor, memory integrated circuits, and circuits necessary to store and retrieve information from memory. It also includes communications ports to other peripherals, other PLC’s or programming terminals. Today’s processors vary widely in their capabilities to control real world devices. Some control as few as 6 inputs and outputs (I/O) and others 40,000 or more. One processor can control more than one process or manufacturing line. Processors are often linked together in order to provided continuity throughout the process. The number of inputs and outputs PLCs can control are limited by the overall capacity of the PLC system hardware and memory capabilities. The job of the processor is to monitor status or state of input devices, scan and solve the logic of a user program, and ntrol on or off state of output devices.

Types of Processor Memory

RAM or Random Access Memory is a volatile memory that would lose it’s information if power were removed.. This is why some processor units incorporate a battery back up. The type of RAM normally used is CMOS or Complementary Metal Oxide Semiconductor. CMOS RAM is used for storage of the user’s program (ladder logic diagrams) and storage memory.

ROM or Read Only Memory is a nonvolatile type of memory. This means you don’t need an external power source to keep information. In this type of memory, information can be read, but not changed. For this reason the manufacture sometimes calls this firmware. It is placed there for the internal use and operation of processor units.
EEPROM or Electrically Erasable

Programmable Read Only Memory is usually an add-on memory module that is used to back up the main program in CMOS RAM of the processor. In many cases, the processor can be programmed to load the EEPOM’s program to RAM if RAM is lost or corrupted.

Input Module

There are many types of input modules to choose from. The type of input module used is dependent upon what real world input to the PLC is desired. Some examples of inputs are limit switches, electric eyes, and pushbuttons. DC inputs, such as thumbwheel switches, can be used to enter integer values to be manipulated by the PLC. DC input cards areused for this application. Since most industrial power systems are inherently noisy, electrical isolation is provided between the input and the processor. Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can cause severe problems in most solid state control systems. The component used most often to provide electrical isolation within I/O cards is called an optical isolator or optocoupler. The wiring of an input is not complex. The object is to get a voltage at a particular point on the card. Typically there are 8 to 32 input points on any one input module. Each point will be assigned a unique address by the processor. Analog input modules are special input cards that use analog to digital conversion (A to D) to sense variables such as temperature, speed, pressure, and position. The external device normally is connected to a controller (transducer) producing an electrical signal the analog input card can interpret. This signal is usually 4ma to 20 ma or a 0 to 10 volt signal.

Output Module

Output modules can be for used for ac or dc devices such as solenoids, relays, contractors, pilot lamps, and LED readouts. Output cards usually have from 6 to 32 output points on a single module. The output device within the card provides the connection from the user power supply to the load. Usually silicon controlled rectifiers (SCR), triac, or dry contact relays are use for this purpose. Individual outputs are rated most often at 2 to 3 amperes. Output cards, like input cards have electrical isolation between the load being connected and the PLC. Analog output cards are a special type of output modules that use digital to analog conversion (D to A). The analog output module can take a value stored in a 12 bit file and convert it to an analog signal. Normally this signal is 0 -10 volts dc or 4ma to 20 ma. This analog signal is often used in equipment such as motor operated valves and pneumatic position control devices.

System Power Supply

The PLC power supply is highly specialized to provide the necessary voltage levels for the PLC’s internal operation. Typically this power supply must provide current to operate the input and output modules as well as the processor. The typical output of this power supply is 5 VDC.

Programming Terminal

The most common types of programming device used today are hand held programmers, desktop IBM compatible computers, and laptop IBM compatible machines. Programming terminals are used to develop and enter the user program into memory of the PLC. The program language of choice, is ladder logic. Ladder logic is made up of a set of specialized instructions used in different combinations to control the operation sequence of the driven machinery or process. Programmers, when used offline, are simply used to construct the program. The program can be downloaded to the processor latter. When the programmer is used online, it can be used to monitor the program. This can be an extremely effective troubleshooting tool when used correctly.

Columbia Montour AVTS

Columbia Montour AVTS is a progressive Vocational Technical School that teaches many high tech skills needed to succeed in industry.

PLC Tutor

This is another great site for learning the basics of PLC’s. It is often updated and uses some nice Java scripting.


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Allen Bradley

Visit AB’s exciting web site to learn about products and download demos.

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