s7_fault

Hello Ted;

The first tool would be the diagnostic LEDs on the CPU, the distributed I/O interface modules and the signal modules. They will tell you that there is an existing problem in the hardware or the program of the CPU.

Next, the CPU logs all status and fault information in the diagnostic buffer, where you will find timestamped messages giving details on existing faults and hints on how to correct them. So opening the diagnostic buffer is always recommended if you need to examine the status of hardware and software.

Use the Save As button on the diagnostic page to copy the logged messages to a text file that contains detailed information on the existing errors on the PLC.

There are also diagnostic OBs that are called by the CPU's operating system on the appearance of special classes of faults: OB 86 is called when there is a remote I/O fault, OB 121 when there are programming errors... Open the Online help on OBs over OB 80 for more details. These diagnostic OBs act in 2 ways: when called for a specific type of error. They log more precise information on the fault inside the diagnostic buffer, and they allow the PLC to continue running in the presence of one fault.

https://support.industry.siemens.com/cs/ww/en/view/19350076

Hope this helps,

Daniel Chartier