Hello 

following your enqury the reason is as follow:

Acknowledge: IMMEDIATELY (POWER ON) Cause: Power supply fault in the power unit. - the voltage monitor signals an undervoltage fault on the module. The following applies for CU31x: - the voltage monitoring on the DAC board signals an undervoltage fault on the module. Remedy: - check the power supply of the power unit. - carry out a POWER ON (switch-off/switch-on) for the component. - replace the module if necessary.

Detail about this and errormessages ::

https://support.industry.siemens.com/cs/ww/en/view/109781807