PLCs are based on electronic computers, but they are not the same as ordinary computers. Generally speaking, computers only consider the information itself when converting input and output information. As long as the human-machine interface is good, information input and output can be completed. PLCs, however, must also consider the reliability, real-time performance, and usability of information input and output. Special consideration should be given to how to adapt to industrial environments, such as ease of installation and anti-interference capabilities. The conversion of input and output information and reliable physical implementation are the two fundamental points of PLC control.
The conversion of input and output information is achieved by running the program stored in the PLC's memory. The PLC program includes the manufacturer's system program (which cannot be modified) and user-developed application (user) programs. The system program provides the operating platform and also performs necessary common processing for the reliable operation of the PLC program and the conversion of signals and information. The user program is designed by the user according to the control requirements. The specific control requirements dictate the type of user program required.
A reliable physical implementation primarily depends on the input and output circuits. The I/O circuits of a PLC are specifically designed. The input circuit filters the input signal to eliminate high-frequency interference. Furthermore, it is electrically isolated from the internal computer circuitry and connected via optocouplers. The output circuit is also internally and externally electrically isolated and connected via optocouplers or output relays. The output circuit also requires power amplification to drive common industrial control components such as solenoid valves and contactors.
There are many I/O circuits; each input or output point must have one I/O circuit. A PLC typically has multiple I/O points and numerous I/O circuits. However, because they are composed of highly integrated circuits, their size is not very large. The input circuit constantly monitors the input status and stores it in the input register. Each input point has a corresponding register to store its information. The output circuit transmits information from the output latch to the output point. The output latch also corresponds one-to-one with the output point.
The input registers and output latches here are actually registers of the PLC processor's I/O ports. They exchange information with the computer memory via the computer bus, primarily through the execution of the system program. Information input to the registers is read into the PLC memory; this is called input refresh. The PLC memory has a special mapped area for storing input information. Each bit in this area is called an input relay, or soft contact. If these bits are set to 1, it means the contact is connected; if set to 0, it means the contact is disconnected. Since its state is obtained through input refresh, it reflects the input status.
Output latches correspond to the output mapping area in the PLC's memory. Each output latch also has a corresponding bit, called an output relay or output coil. By running the system program, the state of the output relay is mapped onto the output latch. This mapping is also called output refresh. Output refresh is primarily achieved by running the system program. In this way, the user only needs to program the conversion from the input mapping area to the output mapping area in memory, specifically how to convert input timing to output timing. This is a data and logic processing problem. Because the PLC has a powerful instruction set, it is entirely easy to write a program to meet this requirement.
