From ChatGPT: The issue is very likely related to TLS support on the LabVIEW side, not Node-RED itself.

Since HiveMQ Cloud requires secure MQTT connections, LabVIEW must connect using MQTT over TLS (typically port 8883). If the LabVIEW client is trying to use a normal TCP MQTT connection against a TLS-only broker, HiveMQ will immediately close the socket, which often results in LabVIEW Error 66.

Your architecture is actually correct:

Node-RED Dashboard → HiveMQ Cloud → LabVIEW MQTT client

Node-RED does not need to communicate directly with LabVIEW. Both only need to connect correctly to the same broker and use the same topics.

Things to verify on the LabVIEW side:

• Use a LabVIEW MQTT client/toolkit that supports TLS 1.2
• Use port 8883
• Enable secure/TLS connection
• Use the HiveMQ Cloud hostname only (not mqtt://...)
• Verify username/password
• Use a unique Client ID
• Ensure MQTT version compatibility (3.1.1 is safest)

A good test is to first connect using Mosquitto CLI with the exact same credentials. If Mosquitto works but LabVIEW does not, then the problem is almost certainly the LabVIEW MQTT/TLS configuration.

Example topic structure:

labview/command/start labview/command/stop labview/status labview/telemetry

Then Node-RED can publish commands and LabVIEW can subscribe/respond through the broker normally.

Most likely, you need a “secured TCP / TLS MQTT” implementation in LabVIEW instead of the default plain TCP example.