Many users turn pale when talking about “interference”, feeling that the interference is neither visible nor tangible, and there is no way to eliminate the faults caused by interference. Indeed, there are various types of fault phenomena caused by different interferences, including unstable communication, dropped stations, inaccurate module readings, and module burning… Now let’s take a look at what strange phenomena this user has encountered. A few days ago, I heard a user complain about the data loss of the S7-300 CPU, but after being powered on again, the CPU can run again and cannot be resolved. After communicating with the user, it can be determined that only the process data of the CPU is lost, and the CPU program stored in the MMC card is not lost. Otherwise, the CPU cannot run after being powered on again. This phenomenon is mostly caused by interference with the CPU. The CPU enters a fault mode, all LED lights flash, all communication is interrupted, and output signals are not controlled. In other words, the CPU crashes, like the blue screen of a PC, which is also a protection mode to prevent unnecessary losses caused by device misoperation. At this point, the internal data of the CPU is lost. After being powered on again (which may take some release time), the programs stored in MMC are copied to the CPU’s working memory and the CPU runs. However, all process data is initialized, and the diagnostic buffer data is maintained. The process analysis should be like this.
By default, the logic ground of the PLC is connected to the M terminal of the power supply system and PE. With good grounding and equipotential connection, the potential between the dispersed devices is the same, making it easy to interconnect. But if the grounding is not good, such as common mode interference or other interference conducted through the ground wire, this interference will affect the logical ground point of the PLC, making it unable to provide a reference point, causing confusion in CPU data, and thus causing the CPU to enter fault mode. In this case, separating the logic of PE and PLC and the so-called floating ground (CPU31X cannot be separated) will avoid interference with the data bus entering the CPU. However, when connecting multiple PLCs, attention should be paid to whether the point difference between them is within the allowable range.
Another situation also needs to be noted, such as 24V
source
Not only does it power the CPU, but it also supplies power to some intermediate relays. In some cases, the coil releases energy after being disconnected, and the high voltage generated on the power circuit will affect the logic ground of the CPU. As shown in Figure 1, high voltage interference directly affects the logic ground. Even if the logic ground of PE and PLC is separated, it does not have much effect because it is not related to PE. If there is interference, an oscilloscope can be used to directly connect the power side of the PLC for measurement. If there is interference, the power supply of the CPU can be separated from the relay control circuit, and it can also be used
diode
As a way to release high voltage, these materials can be found in Siemens manuals. Figure 2 shows the release circuit connected by diodes and Zener diodes. These protection circuits protect the contacts, and the use of Zener diodes can also serve to clamp the voltage.
I hope everyone can not only solve the same problem through this story, but also master some ideas and methods for solving interference problems.