S7-300 SF Fault

Hello plynchy;

Whenever a diagnostic LED (SF, BF...) becomes active on the CPU, information on the fault can be found in the diagnostic buffer of the PLC.

Please open the diagnostic buffer and study the messages there (every event that is important to the CPU is registered there). If you cannot understanmd the messages please save them ("AsText") and post them here on the forum. we may be able to help you with the source of your problem.

Hope this helps,

Daniel Chartier

Hello;

A diagnostic LED lights up when the CPU's OS detects an event that does not correspond to its normal operational settings. This could be related to a programming issue, a hardware defect or a lost communication with a remote Partner.

The appearance of a diagnostic LED is combined with a message entry in the diagnostic buffer of the CPU, and possibly in the diagnostic buffer of the offending module or remote interface, in the case of a Profibus/Profinet error. These messages will log the events and give information on the source of the faults.

Basically BF1 and BF2 (Bus Fault 1 and 2) indicate the loss of a remote IO station on Profibus or Profinet, the number indficates the CPU interface that is affected (no. 1 or no. 2). You should have a diagnostic LED light up on the lost module at the same time, which could help detect the source of the fault..

The SF (system fault) LED active means that a fault has been located on the CPU hardware or software. When it appears along with a BFx fault, it often means that your program is trying to access data from one of the faulty remote modules on the communications network, so this data is not available to the CPU.

A (brief) descritpiton of the LEDs is available in the attached picture. This was extracted from the following manual:

SIMATIC S7-300 CPU 31xC and CPU 31x: Technical specifications

If you can have access to an online connection to the CPU (through Step 7, either V.5 or TIA Portal), you will be able to open the diagnostic buffer of the CPU to collect the messages logged there. These will be invaluable in determining the location of a fault, and in correcting it. See chapter 23 in the following manual:

SIMATIC Programming with STEP 7 V5.5

Hope this helps,

Daniel Chartier

Hello Shahjee;

Please open the diagnostic buffer on the CPU and locate messages that correspond to the fault on the SMC module.

Post the content of those messages here so we can help you understand them.

Hope this helps,

Daniel Chartier