PLC is based on electronic computers, but it is not the same as ordinary computers. Generally speaking, computers only consider the information itself when translating input and output information. As long as the human-machine interface is good, the input and output of information can be completed. The PLC should also consider the reliability, real-time and use of information in and out. Special consideration should be given to how to adapt to the industrial environment, such as ease of installation, anti-interference, etc. The transformation of input and output information and the reliable physical completion are the two basic points of AD PLC control.
The conversion of input and output information is done by running a program stored in the PLC memory. PLC programs include the manufacturer's system program (which cannot be changed) and the application program developed by the user (user). The system program provides the operating platform, and at the same time, it also carries out the necessary public processing for the reliable operation of the PLC program and the transformation of signals and information. User programs are planned by the user according to the control requirements. What kind of control requirements there are, what kind of user programs should be there.
Reliable physical completion depends primarily on the input and output circuitry. The I/O circuits of the PLC are specially planned. The input circuit should filter the input signal to eliminate high-frequency interference. In addition, it is electrically isolated from the computer's internal circuitry and is articulated by optocoupler elements. The internal and external output circuits are also galvanically isolated and connected by optocouplers or output relays. The output circuit also needs to be amplified to drive general industrial control components, such as solenoid valves, contactors, etc.
There are many I/O circuits, and each input or output point must have an I/O circuit. PLCs have multiple I/O points, and generally have many I/O circuits. However, because they are made up of highly integrated circuits, they do not occupy a large volume. The input circuit monitors the input condition at all times and stores it in the input registers. Each input point has a corresponding register to store its information. The output circuit passes information from the output latch to the output point. The output latch also corresponds to the output point 1-to-1.
The input registers and output latches here are actually the registers for the I/O ports of the PLC processor. They exchange information with computer memory through the computer bus, and are mainly completed through the operation system program. The information entered into the scratchpad is read into the PLC memory, which is known as input override. The PLC memory has a special mapping area for storing input information. Each bit in this area is called an input relay, or soft contact. If these positions are set to 1, it means that the contact point is connected, and if it is set to 0, it means that the contact point is disconnected. Because the status is obtained through input rewriting, it reflects the status of the input.
The output latch corresponds to the output mapping area in the AD PLC memory. An output latch also has a bit to correspond to, which is called an output relay or output coil. After the operation system program, the condition of the output relay is mapped to the output latch. This mapping is also known as output overrides. The output rewriting is mainly done through the operating system program. In this way, the user only needs to program the conversion from the input map to the output map in memory, and in particular how to convert the input timing to the output timing. It's a matter of data and logic. Because PLCs have a robust instruction system, it is completely easy to program to meet this requirement.
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