BECKHOFF EL9xxx - Manual (Page 45)

Product overview
Fig. 36: EL9110 in the TwinCAT tree
If the Up or Us voltage is below 17 V, the corresponding diagnosis bit 'Undervoltage' has TRUE (1) status.
3.11 EL9540, EL9550
3.11.1 EL9540, EL9550 ­ Introduction and Technical Data
EL9540, EL9550
Fig. 37: EL9540
EL9xxx 45 Version 3.1