The user program through the programming sequence entered into the user memory, CPU and the user program cycle scan order, which is the basic working process of the PLC.
When the PLC is running, the user program has a large number of operations required to perform, but the CPU is not to perform multiple operations simultaneously, it can only operate according to the principle of sharing, every time an operation is performed. However, due to the high CPU processing speed, so that the results emerging from the external macro point of view seems to be accomplished simultaneously. This timesharing process, referred to as CPU program scan (average CPU processing execution time of each instruction: Small PLC as OMRON-P series of 10μs, such as medium-sized PLC FANUC-PLC-B is 7μs).
After the PLC is powered up and running, begin immediately self-diagnosis, self-diagnosis of the length of time varies with the length of user program changes. Since the diagnosis through, CPU on the user program scan. Scanning from 0000H address stored in the first user program, order, until the last address of the user program until the possession, forming a scan cycle again and again. Sequential scanning simple and intuitive way of working, which simplifies the design process and provide a guarantee for the reliable operation of the PLC. This latter aspect of the scanned instruction is executed, the results can immediately be scanned to be utilized by the instruction. On the other hand can also be set by the CPU scan time watchdog timer to monitor each scan exceeds the specified time, thereby avoiding CPU internal failures of the program execution into an infinite loop caused by the fault.
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