Beckhoff · EL6900 Operating Instructions
This LED combination (Diag 3 lit, Diag 4 flashing) indicates an internal error related to microcontroller 1 (μC1) of the terminal. This is a global fault, leading to the shutdown of the terminal. Such errors typically necessitate inspection by the manufacturer.
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Record the exact LED states (Diag 3 lit, Diag 4 flashing) for documentation.
Power cycle the EL6900 terminal and observe if the fault clears or reoccurs immediately upon restart.
Access the TwinCAT System Manager to check for any additional diagnostic messages or system events logged by the EL6900 prior to the shutdown.