Siemens SIMATIC Manual page 60

Digital modules
3.4 Diagnostics of digital modules
Diagnostics message using the SF LED
Digital modules with diagnostics function indicate errors at their SF LED (group error LED.)
The SF LED lights up when the digital module generates a diagnostics message. It goes
dark after all error states are cleared.
The SF LED also lights up to indicate external errors (short-circuit at the sensor supply),
regardless of the CPU operating state (at POWER ON.)
Diagnostics messages and interrupt processing of digital modules
For information on diagnostics messages, their possible causes, troubleshooting measures,
and possible interrupts refer to the specific module chapter.
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S7-300 Module data
Manual, 06/2017, A5E00105505-AJ