Hi,

here there is some information about diagnose in S7-1200 user program (STEP 7 Basic user program):

There are two types the errors that could be detected by the user program:

1 - Detection and report of peripheral device errors from diagnostics-capable devices.
In this case an external factor affects the S7-1200 hardware.
For example:
- No user power (on the signal module / board)
- High limit exceeded (of the value on an analog input or output)
- Low limit exceeded (of the value on an analog input or output)
- Wire break (on an analog Current output)
- Short circuit (on an analog Voltage output)

This type of error don’t leave the CPU to STOP mode.
The diagnose information could be extracted by evaluation of OB82.

Related FAQ: “How can you manage peripheral device errors detected by the S7-1200 PLC?”
http://support.automation.siemens.com/BR/view/en/42781105

2 – Block execution errors (“program errors”)

In this case an error occurs while the PLC runs the user program.
For example:
- Addressing a I/O that does not exist.

If not treated by the user program, this type of error could leave the CPU to STOP mode.
To avoid this (and get informations about the cause), the program should call at least once one of this two block: GetError / GetErrorID.

Extra Tips:
- GetError / GetErrorID get just the FIRST error that occurs from the being of the block (e.g. MAIN) to the call of them (GetError / GetErrorID).
- So, if you need to monitor all OB, for example, place it at end of this block.
- On another hand, if you need to monitor more than one error, you need to place more calls of GetError / GetErrorID, in strategically points of your program.
- GetError / GetErrorID inhibit the registry of the error on the Diagnostic Buffer, even if the fault occurs after the block call. In this case a fault could happens without diagnose. So, it is important to put the GetError / GetErrorID at end of the blocks.

Related FAQ:
“How can you manage block execution errors detected by the S7-1200 PLC?”
http://support.automation.siemens.com/BR/view/en/42908460

How can you query the occurrence of errors in a block in STEP 7 Basic V10.5 with local error handling?
http://support.automation.siemens.com/BR/view/en/43713352