we are using S7 300 with HMI SIMATIC. from last few days all outputs of PLC getting OFF frequently. after that we have to ON from HMI. no any error or warning message coming. We checked all power unit & inputs no any fault found.

Pls suggest us how to solve this issue.

 

A PLC fault condition would give you at least a green LED changing to red on the output module(s) apparently at fault.

From your description however, it appears as though it has been designed to de-energise PLC outputs. The only obvious reason to do this is for Emergency Stop/Fail Safe De-energisation, this being a hard-wired trip.

What is the criteria or precondition to re-energise PLC outputs via the HMI?