Siemens SIMATIC IM151-1 STANDARD Manual page 42

Interrupt, error, and system messages
4.4 Diagnostics with STEP 7
Note
The length of the diagnostics frame varies depending on the parameter assignment:
• Between 6 and 62 bytes in DPV0 mode
• Between 6 and 110 bytes in DPV1 mode
You can identify the length of the last received diagnostics frame in
the RET_VAL parameter of the SFC 13.
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Interface module IM151-1 STANDARD (6ES7151-1AA06-0AB0)
Manual, 07/2015, A5E01075950-AD
STEP 7
by referring to