【正文】 ture of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops。 little electrical design is required, and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a packaged PLC is low pared to the cost of a specific custombuilt controller design. On the other hand, in the case of massproduced goods, customized control systems are economic due to the lower cost of the ponents, which can be optimally chosen instead of a generic solution, and where the nonrecurring engineering charges are spread over thousands or millions of units. For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities. A microcontrollerbased design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies and input/output hardware) can be spread over many sales, and where the enduser would not need to alter the control. Automotive applications are an example。 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。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 Interfac