【正文】 ation. 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。 millions of units are built each year, and very few endusers alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of customdesigned controls, because the volumes are low and the development cost would be uneconomic. Very plex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even highperformance PLCs. Very highspeed or precision controls may also require customized solutions。 for example, aircraft flight controls. Programmable controllers are widely used in motion control, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that Gcode (involving a CNC machine) can be used to instruct machine movements. PLCs may include logic for singlevariable feedback analog control loop, a proportional, integral, derivative or PID controller. A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops。 where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLCs have bee more powerful, the boundary between DCS and PLC applications has bee less distinct. PLCs have similar functionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly bees more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLClike features and vice versa. The industry has standardized on the IEC 611313 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments. Digital and analog signals Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O. Analog signals are like volume controls, with a range of values between zero and fullscale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16bit signed binary processors, the integer values are limited between 32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 420 mA or 0 10 V input would be converted into an integer value of 0 32767. Current inputs are less sensitive to electrical noise (. from welders or electric motor starts) than voltage inputs. E