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Access Diagnostics menu on SINUMERIK 840D: Press [Diagnostics] → [Machine Data] → [Safety] or navigate via HMI to find PROFIsafe-related MD parameters.

Locate the bus segment MD parameter referenced in alarm message %1[%2]. Cross-reference with machine documentation to identify the exact MD number (typically MD 37xxx or MD 38xxx range for safety parameters).

Note the complete MD parameter name and index from the alarm display (%1 and %2 fields).

Verify current value and change it to 5. Edit the MD parameter using [Edit] or [Modify] function, then press [Enter] to confirm the change to value 5.

Do not modify other safety-related MD parameters simultaneously; change only the bus segment value.

Execute NCK reset: Press [Reset] on control panel or use HMI Reset function to reinitialize the system and validate the new MD value.

Review the current PROFIsafe network configuration in TIA Portal or SIMATIC engineering tool. Verify which bus segment the safety I/O modules are actually assigned to in the hardware topology.

Ensure the PROFINET network diagram matches the actual installation.

Compare the hardware configuration bus segment assignment with the MD value in the 840D. If the hardware uses a different bus segment, update the MD parameter to match the actual topology.

Bus segment 5 is the Siemens default for 840D PROFIsafe systems; confirm this is correct for your specific installation.

If hardware changes were made (new safety modules added, removed, or replaced), regenerate or re-import the PROFIsafe communication database (.gsd or .xml) into the NCK.

Perform a full system check: Press [Diagnostics] → [Communication] → [PROFIsafe] to verify handshake status and confirm all safety modules respond correctly after the MD change.

Check if MD data was recently modified by reviewing control panel logs or engineering system change history. Identify when the bus segment MD parameter was last altered.

If a backup was recently restored, re-download the correct machine data from the commissioning engineering tool (TIA Portal or equivalent) to ensure all safety parameters, including bus segment value of 5, are properly configured.

Verify the backup/source file is from a known-good configuration before restoring.

Manually verify and set the bus segment MD to 5 using the HMI edit function as detailed in Cause Block 1, Step 3.

Create a new validated backup of the corrected machine data via [Maintenance] → [Backup] function to prevent recurrence of this issue.