PLC analog IO module user program execution stage works

PLC works When the PLC into operation, their work is generally divided into three stages, namely the input sample, a user program execution and output refresh three stages. Completion of the three phases is called a scan cycle. During the entire operation, PLC's plc analog input output module to a scanning speed of repeating the above three phases. User program execution stage In the user program execution stage, PLC always sequentially from top to bottom in order to scan the user program (ladder). When scanning each ladder, and always scan the ladder to the left of the control line by the contact form, and then first left and right, the first order under the control of the line formed by the contact logic operation , then according to the results of logic operations, refresh the logic state of the corresponding bit in the coil system RAM storage area; or refresh state corresponds to the plc input output modules coil bit plc analog io module image area; or to determine whether to execute the ladder by Special functions prescribed instructions. That is, during the execution of the user program, the only entry point in the I / O status and data mapping in the region will not change, while the other output point and soft devices I / O image area or system RAM storage area status and data are likely to change, and came in above the ladder, the results of its programs ranked in the following who will use these data ladder coil or work; contrast, came in below the ladder, its be refreshed logic coil state or data can only scan to the next ranked above it on the program work.