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PLC (11 main ideas about PLC (Process of a scan cycle: (Reading inputs,…
PLC
11 main ideas about PLC
A PLC has 4 steps from operations: INPUT SCAN, OUTPUT SCAN, PROGRAM SCAN and HOUSEKEEPING.
Programmable logic relay: The most basic function of a programmable controller is to emulate the functions of electro-mechanical relays.
Scan time: The program then runs from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs.
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System scale: The processor and selection of I/O modules are customized for the particular application.
Security: Modern PLCs generally contain a real-time operating system such as OS-9 or VxWorks, and exploits for these systems exist much as they do for desktop computer operating systems such as Microsoft Windows.
Simulation: PLC systems are inherently expensive, and down-time is often very costly. In addition, if a PLC is programmed incorrectly it can result in lost productivity and dangerous conditions.
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Discrete (digital) and analog signals: Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off.
4 main ideas aboun PLC
Features: The main difference from most other computing devices is that PLCs are intended-for and therefore tolerant-of more severe conditions (such as dust, moisture, heat, cold), while offering extensive input/output (I/O) to connect the PLC to sensors and actuators.
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