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An HMI may still power on and display its interface while losing communication with the PLC. Typical symptoms include values not updating, control buttons not responding, or a communication error appearing on the screen.
When this happens, the HMI itself is not necessarily the problem. The communication path should be checked step by step.
Common causes
1. Communication cable or connector
A loose, damaged, or poorly connected cable can cause intermittent communication. Check the cable, connectors, and communication ports on both the HMI and PLC.
2. Incorrect communication settings
For Ethernet or serial communication, verify the settings on both devices. Depending on the system, this may include:
If the HMI settings and cable are correct, check whether the PLC or its communication module is operating normally. Network switches and other communication devices should also be checked in Ethernet-based systems.
4. HMI communication port
A damaged COM, Ethernet, or other communication interface can cause the HMI to lose communication even when the display and touch functions are still working normally.
5. Main board or internal circuit
If the external communication path has been verified, the HMI hardware itself may need further inspection, particularly the communication interface and related circuitry.
A practical troubleshooting approach
I usually recommend checking the system in this order:
Power → Cable → Connector → Communication settings → PLC/network → HMI communication port → Internal HMI hardware
It is also useful to note whether the communication failure happens immediately after startup or only when the machine begins operating. If the connection is stable when idle but fails during machine operation, power quality, electrical noise, grounding, or network conditions may also need to be investigated.
The important point is not to replace the HMI immediately just because it shows a communication error. Checking the entire communication path can help identify the actual source of the problem and avoid unnecessary replacement.
Have you experienced an HMI that displays normally but suddenly loses communication with the PLC? What was the cause in your case?
When this happens, the HMI itself is not necessarily the problem. The communication path should be checked step by step.
Common causes
1. Communication cable or connector
A loose, damaged, or poorly connected cable can cause intermittent communication. Check the cable, connectors, and communication ports on both the HMI and PLC.
2. Incorrect communication settings
For Ethernet or serial communication, verify the settings on both devices. Depending on the system, this may include:
- IP address and subnet
- Communication protocol
- Port number
- Baud rate
- Station/node address
- PLC driver settings in the HMI
If the HMI settings and cable are correct, check whether the PLC or its communication module is operating normally. Network switches and other communication devices should also be checked in Ethernet-based systems.
4. HMI communication port
A damaged COM, Ethernet, or other communication interface can cause the HMI to lose communication even when the display and touch functions are still working normally.
5. Main board or internal circuit
If the external communication path has been verified, the HMI hardware itself may need further inspection, particularly the communication interface and related circuitry.
A practical troubleshooting approach
I usually recommend checking the system in this order:
Power → Cable → Connector → Communication settings → PLC/network → HMI communication port → Internal HMI hardware
It is also useful to note whether the communication failure happens immediately after startup or only when the machine begins operating. If the connection is stable when idle but fails during machine operation, power quality, electrical noise, grounding, or network conditions may also need to be investigated.
The important point is not to replace the HMI immediately just because it shows a communication error. Checking the entire communication path can help identify the actual source of the problem and avoid unnecessary replacement.
Have you experienced an HMI that displays normally but suddenly loses communication with the PLC? What was the cause in your case?
