Simatic S7-300 ET200M, BF Flashing and SF On

Hello Daniel;

S7-300 CPUs use 2 diagnostic LEDs to show the user that something is wrong:

- BF means "bus fault", and indicates a problem communicating to a node in Profibus or Profinet. The faulty node will also activate  it's own BF LED to help identify it. This could be a cabling/connector problem, a missing termination on the bus (a Profibus problem) or a faulty module on a remote I/O node.

- an active SF shows a "system fault", which is more complex to decipher, could be an unreachable I/O address, a programming issue, or any number of problems. But often when it appears along with a BF LED, it means that your program is trying to access (read or write) a node that is no longer available, due to the node being declared faulty.

If you have access to Step 7, I suggest you go online and open your CPU's diagnostic buffer, where all events that matter to the CPU are logged, including the appearance of dianostic problems. On the diagnostic buffer page there is a button named Save As.., clicking on it saves all the logged messages to a text file, with all available information on possible causes of the problems.

The following link applies to Step 7 v5.x, but the diagnostic information is similar for TIAPortal. Ch. 23 is important for diagnostic tools available for S7-300 CPUs.

https://support.industry.siemens.com/cs/us/en/view/45531107

You can attach the text file to this thread so we can examine it with you, and help you find out what the CPU is complaining about.

Hope this helps,

Daniel Chartier