CPU的message功能就是你需要的，详见随STEP7软件一起安装在硬盘的PDF:
C:\Program Files\Siemens\Documentation\English\STEP 7 - Programming with STEP 7.

Messages allow you to detect, localize, and remedy errors during processing on
the programmable controllers quickly, thus reducing downtimes on a plant
considerably.
Before messages can be output, they must first be configured.
With STEP 7, you can create and edit messages linked to events with assigned
message texts and message attributes. You can also compile the messages and
display them on display devices.