The use of PROFINET network communication in industrial sites is becoming increasingly common. Most users are only familiar with the usage of PROFINET network, and know very little about the underlying working mechanism of PROFINET.

With the increasing number of PROFINET device nodes, the scale of the network is becoming larger and more complex than before. Many users have encountered various problems due to improper use during the usage process. Below, I will introduce a case of network failure caused by improper use.

The control network of a certain tobacco factory in the silk processing workshop uses PROFINET network. On site reports indicate that PROFINET control networks often experience network interruptions. The network topology on site is shown in Figure 1.

From Figure 1, it can be seen that the PLC of S7-400 is connected to the switch of SCLANCE XM414, and then the switch of SCLANCE XM414 is cascaded with the switch of X200 again, and then the switch of X200 is connected to the distributed IO station; Some secondary X200 switches are connected to distributed IO stations. And there are S7-300 PLCs connected to the terminals on some branch lines. Figure 2 is an enlarged view of Figure 1.

Figure 2: Partial enlarged view of the topology diagram
When performing packet capture analysis on the network cable connecting S7-400 and SCALANCE XM414, it can be seen that there are both PNIO and S7 communication packets passing through this line, and at some times, the frequency of PNIO communication is similar to that of S7 communication, as shown in Figure 3.

Figure 3: Data packet capture situation of PLC connected network
Analyze the data packet in Figure 3, as shown in Figure 4. From Figure 4, it can be seen that sometimes the communication load of S7 is higher than that of PNIO.

Figure 4: The situation where the communication load of S7 is higher than that of PNIO

When the situation in Figure 4 occurs and lasts for a long time, it will cause the XM400 switch to change the priority of PNIO data frames from 6 to 0 when both PNIO data and S7 communication data reach SCALANCE XM414, meaning that PNIO data no longer has priority. At this time, when a large amount of S7 communication data occurs, it will cause communication delay in PNIO. This will cause PNIO station failure.

So although the XM400 switch is expensive, it is mainly used as a switch for backbone networks rather than switches on PNIO communication links. So the XM400 switch cannot be cascaded in the PNIO communication link. For PNIO communication, it is recommended to use PNIO's dedicated switch X200 series switch. The X200 switch can ensure that the priority of PNIO remains unchanged when entering or leaving the X200 switch. The special feature of this case is that due to the unreasonable network architecture, S7 communication and PNIO communication share the same logical link, and PNIO communication failure occurs when the communication volume is large.