350005-02-04-00 BENTLY
In the input sampling stage, the programmable logic controller sequentially reads in all input states and data in a scanning manner, and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it enters the user program execution and output refresh phase. In these two stages, even if the input state and data change, the state and data of the corresponding units in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scanning cycle to ensure that the input can be read in any case.
User program execution phase
During the execution phase of the user program, the programmable logic controller always scans the user program (ladder diagram) in a top-down order. When scanning each ladder diagram, always scan the control circuit composed of each contact on the left side of the ladder diagram first, and perform logical operations on the control circuit composed of the contacts in the order of left to right, top to bottom. Then, based on the results of the logical operations, refresh the corresponding bit status of the logical coil in the system RAM storage area; Or refresh the state of the corresponding bit of the output coil in the I/O image area; Or determine whether to execute the special functional instructions specified in the ladder diagram.
During the execution of user programs, only the state and data of the input points in the I/O image area will not change, while the state and data of other output points and software devices in the I/O image area or system RAM storage area may change. Moreover, the program execution results of the ladder diagram listed above will affect the ladder diagram that uses these coils or data listed below; On the contrary, in the ladder diagram below, the status or data of the refreshed logic coil can only be applied to the program above it in the next scanning cycle.
If immediate I/O instructions are used during program execution, I/O points can be directly accessed. If I/O instructions are used, the value of the input process image register will not be updated. The program takes values directly from the I/O module, and the output process image register will be immediately updated, which is somewhat different from immediate input.
