4-1-2-1
Min Supply Voltage μC1 Not Met
Beckhoff · EL1904 Digital Input Terminal
What does 4-1-2-1 mean?
The supply voltage to microcontroller 1 (μC1) within the EL1904 terminal has fallen below its minimum permissible limit. This undervoltage condition can cause unstable operation or prevent the microcontroller from functioning correctly, potentially leading to system errors or shutdowns.
Source: Beckhoff EL1904 Digital Input Terminal
Open Manual (Page 40)