Hello everyone, I am Xiao Zhi. Recently, I have received many messages from friends asking about communication issues, so today we will discuss PLC network faults. A few months ago, I went to a beverage factory to resolve a communication fault, and the project manager was so anxious that he was wringing his hands: “Lao Liu, hurry up and find a solution, the production line has been down for two days, and the management is about to explode!”
To be honest, PLC communication issues are among the most troublesome faults because they are not visible to the naked eye and are related to the network environment. However, as long as you master the methods, there is no problem that cannot be solved. Below, I will share with you the troubleshooting tips I have summarized over the years!
Tip One: Clarify the Fault Symptoms First
Don’t start guessing blindly; first clarify the fault symptoms:
- No communication at all? – This may be a hardware, address, or communication parameter issue.
- Occasional packet loss? – This is often due to interference or load issues.
- Data corruption? – There may be errors in the protocol, byte order, or data type.
At the end of last year, I resolved a communication issue at a coking plant where two Siemens PLCs occasionally lost communication. The on-site engineer checked the program multiple times but found no issues. My first step was to observe the fault pattern and found that each interruption was accompanied by the startup of a certain device, immediately identifying it as an electromagnetic interference issue.
Tip Two: Layered Troubleshooting Method
I prefer to troubleshoot from the bottom up:
// Communication fault troubleshooting order
Layer 1: Physical Layer (wiring, power supply, grounding)
Layer 2: Data Link Layer (baud rate, parity, station number)
Layer 3: Application Layer (protocol, address mapping, data type)
Layer 4: Program Logic (communication trigger conditions, timeout handling)This method has proven effective time and again. Last month, I went to a paper mill where Modbus communication was frequently problematic. I followed this order of troubleshooting and found the issue at Layer 2—mismatched baud rates (one side was 9600, the other was 19200). After modifying it, everything worked perfectly, and the customer gave me a thumbs up!
Tip Three: Utilize Diagnostic Tools
Now, various programming software comes with diagnostic functions, and you must know how to use them:
// Siemens S7 series network diagnostics
1. Enter NetPro configuration
2. Select ProfiNet or Profibus network
3. Right-click to select "Diagnostics"
4. Check communication status, packet loss rate, etc.
// Mitsubishi GX Works network diagnostics
1. Online menu -> Diagnostics -> Network Diagnostics
2. Check network configuration, communication status
3. Red indicates communication abnormalityYou can also use third-party tools like Modbus Poll and Wireshark for packet analysis. Once, I used Wireshark at a cement plant and found that every communication had CRC checksum errors. It turned out to be due to the cable routing being too close to the frequency converter; after replacing it with a shielded cable, the problem was immediately resolved.
Tip Four: Address Both Software and Hardware
Many people focus only on software when troubleshooting communication issues, but hardware problems are actually more common:
// Common hardware troubleshooting points
1. Check if the communication cable is intact (test with a multimeter)
2. Check if the interface is loose
3. Ensure network termination resistors are correctly installed (must check RS485!)
4. Check grounding (test with a grounding resistance tester)
5. Check power quality (isolation may be needed in high interference areas)Only after confirming that the hardware is fine should you check the software:
// Software troubleshooting key points
1. Check communication parameter matching
LD SM0.0 // First scan
MOVW K9600, D0 // Baud rate setting
MOVW K1, D1 // Station number setting
MOVB K0, D2 // Parity (0=no)
2. Communication timeout handling
LD M100.0 // Communication start flag
TON T0, K50 // 5 seconds timeout
LD T0 // Timeout detection
S M100.1 // Set communication fault flagTip Five: Add Fault Tolerance Design
After resolving the fault, don’t rush to finish; add some fault tolerance mechanisms:
// Siemens S7 communication fault tolerance example
// Heartbeat detection mechanism
L #HeartBeat // Heartbeat counter
L 1
+I // Add 1
T #HeartBeat // Save back to counter
// Respond after the other party receives it
U #CommTimeout // Communication timeout?
SPBN SEND // If not timed out, send
CALL "CommReInit" // Communication re-initialization
SET
= #ReInitFlag // Set re-initialization flag
SEND: CALL "TSEND" // Send data block
ID := W#16#1
LEN := 10
DATA := P#DB10.DBX0.0This fault tolerance mechanism is particularly useful on-site. Last week, I went to a clothing factory where the PLC and printer occasionally lost communication. I added a heartbeat mechanism and automatic reconnection, and the customer reported that there have been no issues since.
Summary of Practical Skills
Based on years of practical experience, here are some key takeaways:
- 80% of communication issues occur at the physical layer – Start by checking wiring and grounding.
- Keep communication cables away from high voltage – Maintain at least a 20cm distance during wiring.
- Test complex networks in segments – Troubleshoot one segment at a time.
- Use fiber optics in high interference environments – This is the most effective anti-interference measure.
- Regularly check network health – Don’t wait until problems arise to check.
I remember a couple of months ago, the PLC and upper computer communication at a water plant always disconnected at dawn. I stayed up until 3 AM and found that it was due to the server’s CPU usage spiking during automatic backups, causing the communication program to time out. I adjusted the backup time and communication timeout parameters, and the problem was easily resolved.
Communication issues can indeed be a headache, but the fundamentals remain the same. Mastering these tips will help you handle communication problems with confidence next time!
Friends, if you have any communication issues, feel free to discuss them in the comments section, and let’s explore together. I have been in this industry for over twenty years, and experience is meant to be shared!