In recent years, the author has summarized some experiences in quickly finding the cause of PLC system failures in the maintenance and repair of the plc system. Now I am communicating with you as follows.
A typical PLC system consists of a field PLC station, and a host computer connected to it via a high-speed data line, and an analog screen PLC station. The host computer is used to display various graphics and data. The analog screen PLC station is used to drive the analog screen. led. The entire PLC system is connected to the external equipment to form an automatic control system.
Usually the PLC is treated as a black box, and we can simply determine the location of the fault based on the I/O signal. There are two kinds of situations for judging the fault, that is, the flashing fault signal on the analog screen and the running device are not displayed on the analog screen.
1. The flashing fault signal on the analog screen is based on the drawing of the PLC control station. First check the display status of the equipment in the analog screen PLC cabinet. If the display is in compliance with the display status of the field PLC cabinet, continue to check the PLC cabinet. The I/O terminal, the I/O terminal of the external device, and whether it is a device fault or a PLC fault. The above process can be represented by the following block diagram (Figure 1). 
figure 1
It is very simple to judge whether the I/O terminal of the PLC cabinet and the I/O terminal of the external device matches the status signal. As long as the DC voltage of the multimeter is used to measure the voltage value of the terminal number and the common terminal, it is disconnected for 24V, no signal. ; 0V means ON, there is a signal.
2. The running equipment has no display on the analog screen. At this time, it should be judged that the PLC does not give the running signal, or the running signal is given and the equipment is faulty and cannot be operated.
We can observe whether there is signal display from the output module address of the PLC cabinet on the site, and then check whether the output relay of the PLC station is connected or not, and then see if there is any drive signal in the electrical cabinet of the external equipment. If there is no operation of the device, the device is faulty. If the device is running normally, it should be checked back from the input of the external device. The process is exactly the reverse of the first failure check process.
The above process can be represented by the following block diagram (Figure 2): 
figure 2
If the device is running, check the block diagram below (Figure 3).
For the detection of analog signals, because the instrument uses 4~20mA input, a multimeter is connected in series with the analog signal input terminal to detect the current value of the analog signal, and compare with the output value of the PLC to know whether the value is correct. 
image 3
There is a simple way to quickly judge whether it is a PLC fault or an electrical equipment fault, that is, to use the short circuit method: disconnect the external equipment status input line, and connect the input port to the common line with a wire, which means that the PLC is connected. The signal of the pass, if the PLC has a display, the PLC is normal; otherwise, it is a PLC fault.
After the fault point is found, it should be handled accordingly. Generally speaking, the possibility of PLC failure is small. Most of the faults are caused by loose wiring, or the wiring is wrong, or the relay is faulty. There is also a PLC template burned. At this time, the PLC template can only be replaced. . Remember to turn off the power, otherwise it will be easy to burn a good template, it may also be implicated in the PLC processor.
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