These analog signals represent machine or process conditions as a range of voltage or current values. Other modules or inputs respond to analog signals. Some input modules or points respond to digital inputs, also called discrete inputs, which are either on or off. The type of the input modules or points used by a PLC depends upon the types of the input devices used. The basic elements of a PLC include input modules or points, a central processing unit (CPU), output modules or points, and a programming device. While the specific applications vary widely, all PLCs monitor inputs and other variable values, make decisions based on a stored program, and control outputs to automate a process or machine. PLCs differ from office computers in the types of tasks that they perform, and the hardware and software they require to perform these tasks. A programmable logic controller (PLC), also referred to as a programmable controller, is the name given to a type of computer commonly used in commercial and industrial control applications.
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