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Sensors |
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The second most important devices in control applications after actuators are sensors which measure a physical quantity and transmit this value over the PLC network. Transmission conditions may differ from one sensor or one application to the next. Some sensors will not transmit without first being polled; some transmit only when their input changes beyond a given threshold, others transmit at every t time unit, etc. The most common sensors are the binary sensor, i.e., a switch, and the general-purpose analog sensor used for temperature, light level, pressure, and other such applications.
PLM-1 chip is a general-purpose data transportation IC and, unlike other PLC technologies, can convey sensor-type information. Typical system architectures implement sensors and actuators as simple input/output devices. However, a thermostat, for example, can also act as a sensor and broadcast a temperature measurement that other controllers or actuators will then use. Passive Infra-Red (IR) sensors can also trigger lights in a given room and activate the alarm system if the house system is set in the unoccupied mode.
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