PLC controller works
Although the ladder program used by many PLC often use such as relays, timers and counters name, but not the internal PLC entities have the hardware , but in the memory and logic control programs do programmatically edit and output elements borrowed from the connect an external china industrial automation plc entity controlled mechanical device to do . So can greatly reduce the space required for hardware controllers . PLC ladder program execution actually works is the first progressive ladder code to scan and read into the CPU and finally perform control operations. Throughout the scanning process includes three steps , "Input status check ", " run " , "Output status updates " as follows:Step one " input status check ": PLC First check the status of each point switch or sensor inputs connected components of ( 1 or 0 for on or off ) , and its status is written in the corresponding memory location Xn. Step two " running " : progressive ladder program will take into the CPU operation , if the program is running you need to enter the contact state , CPU direct inquiries removed from memory. Operation result is stored in the output coil plc analog output modulesmemory location corresponding to withhold response to the output Yn. Step Three "Output status update " : the step two of the output status update to the PLC output unit contacts , and return to step one . This third step is called the PLC scan cycle , and the time required to complete the PLC called the reaction time ,the time of the PLC input signal if the reaction time is less than this , there is the possibility of misreading . After each run the next time the program before execution , output and input status will be updated once, so this mode of operation as an output , said input of " the end of the regeneration program ."