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INTRODUCTION: A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.


HISTORY: Before the PLC, control, sequencing and safety interlock logic for manufacturing automobiles was accomplished using hundred or thousands of relays, cam timers, drum sequencers and dedicated closed loop controllers. Early computers required specialist programmers, and stringent operating environmental control for temperature, cleanliness and power quality.

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INPUT OUTPUT Sensing Devices Valves Switches and Pushbuttons Solenoids Proximity Sensors Motor Limit Switches Actuators Pressure Switches Pumps


Programming: PLC programs are typically written in a special application on a pc, then downloaded by a direct-connection cable or over a network to the PLC. The computer is connected to the PLC through Ethernet, RS-232, RS-485 or RS-422 cabling.


FEATURES: High Reliability: Strong anti- interference quality and very high reliability are the most important features of PLC. Good Flexibility: Based on engineering practice, capacity and function can be expanded by expanding number of module, so PLC has a good flexibility. Quality of Strong Easy-Operating: It is very easy to edit and modify program for PLC by computer offline or online.

User interface::

User interface: PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interface (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.


COMMUNICation : PLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-485 or Ethernet. Modbus , BACnet or DF1 is usually included as one of the communications protocols. Other options include various fieldbuses such asDeviceNet or Profibus . Other communications protocols that may be used are listed in the List of automation protocols.


APPLICATION: Programmable controllers are widely used in motion control, positioning control and torque control.

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Rockwell Automation AB - SLC 500 Programmable Logic Control

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PLC-5 / 1771 Controller System ller

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Siemens Simatic PLC's - Simatic S7

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Simatic ET200S

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