Sinumerik 840D PCU 50 Action Log: Configuration, File Location, and Viewing

The Sinumerik 840D action log is the persistent, append-only audit trail recorded by the HMI (Human-Machine Interface) on the PCU (PC Unit). Unlike the alarm log buffer, the action log captures operator interventions, parameter changes, offset edits, program selections, tool commands, PLC alarms, NC alarms, and password status transitions. On a PCU 50 system, the log is written to the local hard drive as a binary stream with a .com file extension. Engineers frequently encounter two problems: the file size grows as expected, but the content appears unreadable in a standard text editor, and the configuration switches seem to have no effect. This reference consolidates the machine data, HMI menu path, file storage rules, and viewing procedure required to make the action log usable for diagnostics and FAT/SAT evidence.

1. Action Log vs. Alarm Log: Functional Split

Sinumerik distinguishes between the alarm log and the action log, and confusing the two is the most common reason operators believe the action log is empty.

The action log is the only record that captures who changed what and when, including password-protected parameter writes. For machine builders and end users subject to FDA 21 CFR Part 11, ISO 13485, or IATF 16949 audit trails, the action log is the canonical source.

2. Prerequisites

Before enabling the action log, verify the following hardware and software conditions.

For the underlying HMI architecture and the role of the PCU within the operator panel, refer to the Sinumerik 840D sl Operator Components and HMI Manual.

3. Enabling the Action Log with MD 9012

Display machine data MD 9012 is a bit-coded configuration word. It is loaded from the active CF card image and read at HMI boot. The action log categories are mapped to individual bits; setting the appropriate bits tells the HMI which event classes to record.

3.1 Bit Assignment of MD 9012

To enable all seven of the standard categories required for a complete audit trail (PLC alarm, NC alarm, parameter, offset, program, tool, password), the effective MD 9012 value is the OR of bits 1-7:

MD 9012 = 0x01 + 0x02 + 0x04 + 0x08 + 0x10 + 0x20 + 0x40 + 0x80 = 0x00FF (decimal 255)

3.2 Writing MD 9012

1. On the HMI, navigate to Startup > Machine Data > Display MD.
2. Filter by group 9xxx and locate row 9012.
3. Enter FF (hexadecimal entry is accepted when the input field is set to HEX; otherwise enter decimal 255).
4. Press Input or the Enter key to commit.
5. If a confirmation dialog appears, choose OK and acknowledge that the new value is non-volatile.

For the formal description of the display machine data concept and access control, see the Sinumerik 840D sl Base Software and HMI-Embedded Programming Manual.

4. Selecting Event Classes in HMI System Settings

MD 9012 is the global enable mask. The HMI System Settings dialog is the per-channel, per-event-class refinement layer. Both must be configured or the action log will remain empty.

1. On the HMI, press the Startup horizontal soft key.
2. Select HMI from the vertical soft-key bar.
3. Open System Settings.
4. Select the Action Log tab.
5. Tick each event class that should be recorded. The dialog presents check boxes that map one-to-one to the bits in MD 9012.
6. Press OK to apply.

If the HMI prompts for a password at this point, the current access level is below Service. Repeat the procedure after logging in with the appropriate Service or Manufacturer password.

5. Action Log File Storage on PCU 50

5.1 Default File Layout

The ring buffer size is fixed at the factory (typically 4 MB on PCU 50.5). When the file reaches the cap, the HMI renames the active file to .bak and starts a fresh .com. Operators who see a static file size for hours should check whether the .bak file exists; the buffer may have silently rotated.

5.2 File Format

The .com files are not strictly binary. They are plain ASCII or UTF-8 records with a fixed 64-byte header followed by newline-terminated entries. The .com extension is historical (COM = commissioning) and is preserved to prevent Windows from executing the file. The file can be opened in any text editor after renaming.

6. Viewing the Action Log Without Renaming

There are three reliable methods to read the log on the PCU itself or on an external PC.

6.1 Method A: Rename and Open in WordPad

1. Connect a USB keyboard and mouse to the PCU 50 front panel, or use Windows CE Remote Desktop if available.
2. Open My Computer > F: > AC_LOG.
3. Right-click actionlog.com and choose Rename.
4. Change the extension to .txt. Confirm the rename warning.
5. Double-click the file. The default Windows association may open Notepad; if entry lines are not wrapped correctly, close and open with WordPad (Start > Programs > Accessories > WordPad).

WordPad is preferred over Notepad for files larger than 64 KB because Notepad has a known buffer bug that silently truncates LF-only files. Notepad++ is the most robust choice on an external PC.

6.2 Method B: Use the HMI Built-in Viewer

For live monitoring without leaving the operator panel:

1. Navigate to Diagnostics > Logs > Action Log.
2. The viewer shows the most recent 1000 entries with timestamps and the user account responsible.
3. Use the Save soft key to export the visible range to F:\TEMP\action_export.txt.

6.3 Method C: Network Transfer to Engineering PC

1. On the engineering PC, open Windows Explorer and enter \\<PCU50_IP>\AC_LOG$ in the address bar.
2. Authenticate with the Service Windows account.
3. Copy actionlog.com to a local folder.
4. Rename locally to actionlog.txt and open in Notepad++ or Excel (use the Import > From Text wizard with the pipe character as delimiter if the entries are pipe-separated).

7. Decoding the Log Content

Each record in the action log is structured as a single line. A typical entry for a parameter change on a Sinumerik 840D with HMI-Advanced 07.06 looks like the example below (values redacted):

2024-03-14 09:21:07 | USER=Service  | ACT=PAR_WRITE | CH=1 | MD=20150[0] | OLD=2 | NEW=3 | RESULT=OK

Field meanings:

For PLC alarms the ACT field becomes ALARM_RAISE and the MD field is replaced by the alarm number, e.g. ALARM=700001. For password events, ACT=PWD_LOGIN and the USER field shows the level that was unlocked.

8. Troubleshooting Matrix

Use this matrix to triage the most common field complaints about the action log on a PCU 50 system.

9. Backup, Archival, and Long-Term Storage

The action log on a single PCU 50 typically holds 30-90 days of data before ring rotation. For audit purposes, configure a scheduled transfer to the network.

1. Create a Windows scheduled task on a domain controller that runs daily at 02:00.
2. Use robocopy \\PCU50\AC_LOG$\actionlog.com \\ARCHIVE\SINUMERIK\%COMPUTERNAME%\actionlog_%date%.com with the /COPY:DAT switch.
3. After successful copy, append a SHA-256 hash to a sidecar file (actionlog_YYYYMMDD.com.sha256) for tamper evidence.
4. Retain archives for the regulatory minimum (typically 7 years for medical devices, 15 years for energy).

For plants subject to FDA 21 CFR Part 11, the action log archive must be on a WORM (write once, read many) storage tier and must be cryptographically signed at archive time. The hash sidecar satisfies the integrity requirement; the WORM tier satisfies the immutability requirement.

10. Safety and Data Integrity Considerations

• Never edit the action log on the PCU; an editor that adds a CR character invalidates the entry CRC.
• Never copy the log via an FTP client configured for ASCII mode; always use binary transfer, then rename locally.
• Never clock the HMI backward. The action log is time-ordered; a backward clock step creates out-of-order entries that auditors will treat as evidence tampering.
• When replacing a CF card, image the old card with dd or a Windows sector tool before formatting; the log persists on the card, not in the NCK SRAM.

11. Verification Checklist

After completing the configuration, run the following verification steps in order. Each must pass before the action log is considered commissioned.

1. On the HMI, confirm MD 9012 reads back FF (or the chosen bit mask) after an HMI restart.
2. Trigger a deliberate PLC alarm from the PLC program (e.g. set DB10.DBX0.0 = TRUE with a configured alarm number). The alarm log on the HMI must display the alarm.
3. Open the HMI System Settings and confirm every required event class is ticked.
4. On the HMI, change any setting data, e.g. zero offset G54, by 0.001 mm.
5. On the HMI, log in with a Service password, perform an action, then log out.
6. Reboot the HMI. Verify that F:\AC_LOG\actionlog.com exists and is non-zero in size.
7. Stop the HMI service, copy the file to a USB stick, rename to .txt, and open in WordPad. Confirm the five test events from steps 2-5 are present in chronological order.
8. Re-start the HMI service. Confirm a new entry ("HMI started") is appended.

If step 7 returns a file that opens but is empty, the HMI is writing to a different path. Repeat step 6 with a directory search across all visible drives (C:, D:, F:, G:).

12. Field-Proven Caveats

Several behaviors observed in production cells are worth recording for future troubleshooting.

• NCU 710.x with HMI-Embedded: The action log is stored in a SQLite database on /var/log/sinumerik/actionlog.db, not as a .com file. The MD 9012 enable still applies, but the viewing method is via the embedded diagnostics web UI on port 8443.
• PCU 50.3 vs. PCU 50.5: The 50.3 uses a 2 GB CF card; the 50.5 uses a 4 GB or larger SSD. Ring buffer size scales with available free space, so log retention on a 50.5 is roughly 2x that of a 50.3 with identical configuration.
• Language switching: If the HMI language is changed after the log is written, older entries continue to display in the original language. New entries use the new language. This is by design and is the correct behavior for audit.
• Power loss during write: A power loss within the same AC cycle as a log write can truncate the most recent entry. The HMI detects the truncated tail on next boot and pads with a marker --- TRUNCATED ---. This is a known limitation, not a defect.

For the formal description of the HMI-Advanced action log subsystem and its API, consult the Sinumerik 840D sl HMI-Advanced Programming Manual and the related KB article How do you display the action log on Sinumerik 840D sl?