【正文】 Programmable logic controller A programmable logic controller (PLC) or programmable controller is a digital puter used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures. PLCs are used in many industries and machines, such as packaging and semiconductor machines. Unlike generalpurpose puters, 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 batterybacked or nonvolatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result. Features The main difference from other puters is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of plex positioning systems. Some use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a puter work that plugs into the PLC. System scale A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O. Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants. User interface See also: List of humanputer interaction topics。 PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday HumanMachine Interface (HMI) is employed for this purpose. HMIs are also referred to as MMIs (Man Machine Interface) and GUI (Graphical User Interface). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More plex systems use a programming and monitoring software installed on a puter, with the PLC connected via a munication interface. Communications PLCs have built in munications ports usually 9Pin RS232, and optionally for RS485 and Ether. Modbus, BAC or DF1 is usually included as one of the munications protocols. Others39。 options include various fieldbuses such as DeviceNet or Profibus. Other munications protocols that may be used are listed in the List of automation protocols. Most modern PLCs can municate over a work to some other system, such as a puter running a SCADA (Supervisory Control And Data Acquisition) system or web browser. PLCs used in larger I/O systems may have peertopeer (P2P) munication between processors. This allows separate parts of a plex process to have individual control while allowing the subsystems to coordinate over the munication link. These munication links are also often used for HMI devices such as keypads or PCtype workstations. Some of today39。s PLCs can municate over a wide range of media including RS485, Coaxial, and even Ether. PLC pared with other control systems PLCs are welladapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices patible with industrial pilot devices and controls。 little electrical design is required, and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) not