Programmable Logic Controller

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The widespread application of the Programmable Logic Controller (PLC) has demanded the acceptance of universal standards for data format, communications, and symbolic representation to aid engineers in the design process. These standards have evolved in recent decades as industry has required greater resolution, speed , and greater numbers of variables in complex control systems.


The PLC operates by receiving information that contains the status of system components, levels, numerical counts, and sensor outputs. The PLC processes the information and returns output signals that are appropriate to control the output devices. The input signals, coming from a variety of different sources, may be in the form of analogue or digital information. Sensors such as pressure, temperature, and flow are usually analogue devices which output an analogue signal that is proportional to the parameter being measured. The standard output of 0-10 VDC can be used, though 0-20 ma has gained wider acceptance due to its immunity to loss over long cable runs. Because a PLC is a digital device, these analogue signals must be converted to digital in an analogue to digital (A/D) converter. However, simple digital information from on/off switches and limit switches may be input directly to the PLC.
When incorporating analogue sensors, the degree to which the system is able to detect a change and react to that change is known as resolution. Theoretically, an analogue sensor has infinite resolution. However, the resolution of digital information is limited by the number of bits available to express the analogue data. ...
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