Applicable Systems
cMT Series
Symptoms
The HMI cannot establish a connection to the remote SCADA server via the gateway. The OPC UA client reports a connection timeout after 30 seconds.
Possible Causes
- Firewall blocking the OPC UA port (default 4840) between HMI and server
- OPC UA certificate expired or not trusted by the server
- DNS resolution failure for the SCADA server hostname
- Network routing or VLAN configuration preventing end-to-end connectivity
Troubleshooting Steps
- Step 1: Verify network connectivity by pinging the SCADA server IP from the HMI
If ping fails, check routing and firewall rules - Step 2: Check the OPC UA certificate status in the HMI security settings
Renew or import a trusted certificate if the current one is expired - Step 3: If using a hostname, verify DNS resolution by checking the resolved IP
Configure a static IP in the HMI hosts table if DNS is unreliable - Step 4: Inspect the firewall rules on the SCADA server for port 4840
Add an inbound allow rule for the HMI IP address
How to Reset
Reset: The OPC UA connection restores automatically once the network and certificate issues are resolved. Clear the alarm on the HMI.
Common Replacement Parts
- OPC UA certificate: X.509 v3, 2048-bit RSA (Generate from the HMI or import from a CA)
- Network cable: Cat 6, industrial shielded (Verify continuity with a cable tester)
Frequently Asked Questions
- Q: What is OPC UA?
OPC Unified Architecture is a machine-to-machine communication protocol for industrial automation with built-in security.
- Q: How do I install an OPC UA certificate?
Export the HMI's public certificate, send it to the SCADA administrator to add to the trusted list, and import the server certificate on the HMI.
- Q: Can I use OPC UA over the internet?
Not recommended without a VPN tunnel. Use a site-to-site VPN or dedicated cellular connection for remote access.
Source: OEM Technical Manual
Category: HMI