2-1-2-1
Max Supply Voltage μC1 Exceeded
Beckhoff · EL1904 Digital Input Terminal
What does 2-1-2-1 mean?
The supply voltage to microcontroller 1 (μC1) within the EL1904 terminal has exceeded its maximum permissible limit. This overvoltage condition can stress internal components and potentially lead to permanent damage to the microcontroller.
Source: Beckhoff EL1904 Digital Input Terminal
Open Manual (Page 40)