Applicable Systems
cMT Series
Symptoms
MQTT client shows 'Broker Unreachable' and publish messages queue indefinitely.
Possible Causes
- MQTT broker IP or DNS name changed and not updated in the HMI client settings.
- TLS/SSL certificate expired on the broker, causing the HMI to reject the connection.
- Corporate firewall blocking outbound TCP 8883 (MQTT over TLS) from the OT network.
Troubleshooting Steps
- Step 1: Verify the broker IP/DNS and port in EasyBuilder Pro MQTT client settings.
Address resolves correctly and the port matches the broker listener configuration. - Step 2: Check the broker TLS certificate expiration date with openssl s_client or browser.
Certificate is valid and the HMI CA trust store includes the signing root. - Step 3: Test TCP 8883 connectivity from a laptop on the same VLAN using telnet/nc.
Connection to broker:8883 succeeds and TLS handshake completes.
How to Reset
Update the broker address, renew or import the TLS certificate, and open firewall port 8883.
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Source: Weintek MT8071iE MQTT Communication Manual
Category: HMI
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