In addition to these basic elements, a PLC system may also incorporate an operator interface device of some sort to simplify monitoring of the machine or process. Once entered, the program and associated variables are stored in the CPU. The programming device is used to enter and change the PLC’s program, to monitor and change the stored values. The output modules convert the control signals from the CPU into digital or analog values that can be used to control various output devices. The CPU then sends signals to update the status of the outputs. The CPU evaluates the statuses of the inputs, outputs, and other variables as it executes a stored program. The primary function of a PLC’s input circuitry is to convert the signals provided by these various switches and sensors into logic signals that can be used by the CPU. 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 wide range of modules can be used to expand the system centrally or to create decentralized structures according to the task at hand, and facilitates a cost-effective stock of spare parts.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. The SIMATIC S7– 300 universal Controllers basically saves on installation space and features a modular design. The SIMATIC S7-300 also called The Legendary universal PLC by siemens and is used worldwide in many successful operations.
The latest generation of SIMATIC available now is S7 and is programmed in STEP 7 (V5.x or TIA Portal).ġ. The programs on the SIMATIC devices respond in real time to inputs from sensors on the controlled machines, and send output signals to actuators on the machines that direct their subsequent operation. SIMATIC devices are intended to separate the control of a machine from direct operation, in a versatile manner than controls hard-wired for a specific machine. Siemens developed a series of PLCs, Automation system and production called SIMATIC.
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