The working principle of the PLC scan

When the operation of the PLC controller, reflects by executing control requirements of the user program to complete the control task, the need to perform many operations, but the CPU is not possible to perform multiple operations at the same time, it can only according to time-sharing operation ( Serial work) Way, each performing an operation, execute them one by one in sequence. Due to the fast processing speed of the CPU, so from a macro point of view, the result of the PLC external appearance seems to be at the same time ( Parallel) The finished. This serial process called PLC scanning way. In scanning mode, when performing a user program scan begins with the first program, without interruption or jump control, according to the program storage order successively, to perform a detailed user program, until the end of the program. Then scan performed from scratch, repeat operation cycle. PLC controller scanning way and the working principle of electric control are obviously different. Electric control device adopts parallel work hard logic method, if a relay coil electricity or without electricity, and all of the relay is often opened normally closed contact no matter in which position of the control circuit, will immediately action at the same time; The PLC controller using scanning works ( The serial works) , if a soft relay coil is on or off, all of their contact no immediate action, must wait for scanning to the action. But as a result of PLC scan speed, usually PLC and electrical control device on the I/O processing result and there is no difference.
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