Programmable controllers — operation and application
1989; Elsevier BV; Volume: 13; Issue: 10 Linguagem: Inglês
10.1016/0141-9331(89)90077-x
ISSN1872-9436
Autores Tópico(s)Sensor Technology and Measurement Systems
ResumoThis chapter discusses the processing of the signals from input and output devices. The input/output (I/O) unit provides the interface between the PLC controller and the outside world and must therefore provide the necessary signal conditioning to get the signal to the required level and also to isolate it from possible electrical hazards such as high voltages. The input/output units of PLCs are designed so that a range of input signals can be changed into 5 V digital signals and a range of output signals are available. The chapter includes the forms of typical input/output modules and, in an installation where sensors are some distance from the PLC processing unit, their communication links to the PLC. Serial communication is when data is transmitted one bit at a time. Parallel communication occurs when a data word is separated into its constituent bits and each bit is simultaneously transmitted along parallel cables. The most common serial standard is RS232; other standards are RS422 and RS423. The 20 mA loop can be used for serial communication. The most common parallel standard interface is IEEE-488. Protocols are necessary to exercise control of the flow of data between devices. The most commonly used code for the transmission of characters is ASCII.
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