ZCP270 P0926CP Foxboro Controller. Shanghai Sijian Intelligent Technology Co., Ltd. is engaged in technology development, technical consulting, technology transfer, and sales of electrical equipment, automation equipment, electronic products, communication products, and electromechanical products in the fields of intelligent technology, information technology, and automation technology. Industrial automation products: PLC, DCS, sensors, frequency converters, encoders, relays, servo drives, soft starters, touch screens. Brands involved in the products include: Eurotherm, Schneider, Siemens, ABB, SICK, P+F, E+H, Yokogawa, Yamakawa, Mitsubishi, Emerson, Honeywell, FANUC, GE, Eaton Electrical. The process of executing a program in a PLC is divided into three stages: input sampling stage, program execution stage, and output refresh stage. The scanning process of a PLC's operation:
(1) ZCP270 P0926CP Foxboro Controller Input Sampling Stage. In this stage, the PLC reads all input signals from the input terminals in a scanning manner and stores each input state into the corresponding input image register. At this point, the input image register is refreshed. During the program execution and output refresh stages, the input image memory is isolated from the external environment, and its content remains unchanged until the next scanning cycle's input sampling stage, where it is refreshed by the newly read input signals. It can be seen that when executing programs and processing data, the PLC does not directly use the real-time input signals from the field but uses the data input into the image area during this sampling. Generally, the width of the input signal should be greater than one scanning cycle; otherwise, signal loss may occur.
(2) Program Execution Stage. During the execution of the user program, the PLC follows the scanning principle of ladder logic programs. Generally, the PLC executes the program step by step from left to right and top to bottom. However, when encountering a program jump instruction, it determines the jump address based on whether the jump condition is met. During program execution, when an instruction involves input or output states, the PLC "reads" the corresponding input terminal state from the input image register and "reads" the current state of the corresponding component ("soft relay") from the output image register. It then performs the corresponding operation, and the result is stored back into the output image register. For the output image register, the state of each component ("soft relay") changes as the program execution progresses.
(3) Output Refresh Stage. The operation results from the program execution stage are stored in the output image area and are not sent to the output ports immediately. During the output refresh stage, the PLC sends the output variables from the output image area to the output latch, which then generates the control output for this cycle through the output module. If the state of the internal output relay is "1", the output relay contact closes, driving the external load through the output terminal. The state of all output devices is maintained for one scanning cycle.