Troubleshooting TP177B Alarm Event Buffer Loss After Restart

Operators lose alarm traceability on Siemens SIMATIC TP177B panels when the unit restarts, when a project update is transferred, or when the upstream PLC fault bit drops. The 256-entry ring buffer is non-adjustable, partially volatile, and the panel does not log alarms natively without removable storage. This reference explains exactly which faults are retained, why the manual contradicts itself on retentivity, and how to engineer persistent alarm tracking using WinCC flexible 2008, the MMC slot, and PLC-side bit latching on the S7-313C.

1. System Overview and Known Limitations

The TP177B is a 6" monochrome or 4" monochrome HMI in the SIMATIC 170/270/370 mid-range panel family, configured with WinCC flexible 2008 (Compact / Standard / Advanced). The 6" TP177B ships under catalog number 6AV6642-0BA01-1AX1; the 4" variant is 6AV6642-0AA11-1AX1. Both run Windows CE 5.0 and use an MMC slot on the rear for storage expansion.

Two alarm-handling objects exist in the panel firmware:

• Active alarm queue — currently triggered alarms, displayed in an AlarmView configured for "Active alarms" or "Active and acknowledged alarms". Cleared automatically when the trigger bit resets.
• Alarm event buffer — historical entries, displayed in an AlarmView configured for "Alarm events". Implemented as a ring buffer.

The frequently reported "alarm cleared when PLC fault bit goes low" is therefore not a bug. It is the active-queue semantics. Persistence requires the event buffer, external logging, or a PLC-side latch.

2. Buffer Capacity: TP170B vs TP177B vs MP277

Field note: The color variant TP177B 6AV6642-0BC01-1AX1 uses the same 256-entry buffer. WinCC flexible 2008 does not expose a register to enlarge the count — the capacity is hard-coded in the panel's CE runtime image. Cross-reference the official SIMATIC TP177B device manual for buffer size and supported MMC media.

3. The Volatility Ambiguity

Field experience surfaces two conflicting statements about TP177B retentivity. The TP177B device manual section 9.1.1 (alarm subsystem) states: "Alarm events are saved to an internal, volatile buffer." Operators who power the unit off and on report that historical entries survive the cycle. The reconciliation: the buffer is volatile with respect to (a) a project transfer, (b) a firmware / OS update, and (c) a service-menu reset, but normally persists across short power cycles because WinCC flexible 2008 flushes the buffer to the internal flash on the periodic save tick. In practice it is "best-effort retentive" and cannot be relied upon for regulatory traceability.

For a 21 CFR Part 11, ISO 9001 audit trail, or automotive AS-Interface traceability use case, the TP177B is not the correct platform. Migrate to OP277 or MP277.

4. Configuring AlarmView for the Ring Buffer

Open the WinCC flexible 2008 project, navigate to the HMI screen containing the AlarmView, and configure as follows:

1. Right-click the AlarmView, open Properties > General.
2. Set Display to "Alarm events" (NOT "Active alarms" and NOT "Active and acknowledged alarms").
3. In the same property group, set Alarm class filter to the relevant classes — typically "Errors" and "Warnings" only, to keep the buffer usable for the highest-severity events.
4. Set Column order to include: Time, Date, Alarm text, State, Acknowledgment, Class. The default ordering hides the "State" column, which is the field needed to confirm "Came In / Went Out / Acknowledged" transitions.
5. Open Properties > Layout and enable Sort order = "Descending by date". This places the newest event at the top of the view.
6. Under Properties > Appearance, set row height to 24 px and enable word wrap. Long alarm texts truncate to 80 characters otherwise.

The ring buffer is a fixed system property of the panel and cannot be enlarged, disabled, or repositioned. WinCC flexible 2008 does not expose a checkbox for it. Any attempt at true logging requires external storage (see Section 5).

5. MMC Card Logging Procedure

To enable persistent alarm logging on TP177B, an MMC card must be installed in the rear slot. The unit accepts MMC up to 128 MB (FAT16) — full-size SD cards are not supported on TP177B. Compatible Siemens part numbers:

• 6AV6671-1CB00-0AX1 — MMC 128 MB (recommended baseline)
• 6AV6671-1CC00-0AX1 — MMC 512 MB (verified on TP177B firmware > V14.01.00)

1. In WinCC flexible 2008, open Project > Device Settings > Alarm Logging.
2. Enable Log alarms and set Storage location = "Memory card".
3. Set Segment = "Day" for daily files. Set Response when segment is full = "Overwrite existing files" (ring file behavior) for continuous logging.
4. Set Format = "CSV with header" if a SCADA downstream must import the file. The default binary format is faster but opaque.
5. Transfer the project. Verify the MMC is seated BEFORE power-up — a hot-inserted MMC is not recognized by the CE 5.0 runtime until the next restart.
6. On a power cycle, the panel writes the alarm log to \Storage Card\ALARM_LOG\AlarmLog_YYYY-MM-DD.csv. Export via ProSave or pull the MMC and read on a PC.

6. S7-313C Alarm Tag Wiring

The S7-313C (6ES7313-5BF03-0AB0, firmware > V2.6) is typically configured with discrete fault bits in the process image. The TP177B polls these via MPI / PROFIBUS at the configured update rate (default 1000 ms). To prevent loss of the active alarm while waiting for the operator to acknowledge, latch the trigger in the PLC.

Example STL logic for one fault bit on input I 0.0:

// Network 1: latch fault
A     I 0.0
S     M 10.0        // Latched fault
// Network 2: clear on operator reset
A     I 0.1         // Ack/reset pushbutton
A     "HMI_Ack"     // HMI tag from TP177B button
R     M 10.0
// Network 3: send to panel
A     M 10.0
=     Q 0.0         // Or use as bit in DB for HMI tag

Configure the HMI tag "LatchedFault1" in WinCC flexible 2008 pointing to DB1.DBX0.0 (or directly to M 10.0 if using an area pointer). Tie the alarm trigger to this latched bit. The alarm remains in the panel's active queue until the operator acknowledges via the AlarmView or the pushbutton, and a record is added to the event buffer at the moment the alarm "Came In".

This is the minimum viable pattern for traceability on the TP177B platform. The PLC retains the history independently of the panel lifecycle. Refer to the SIMATIC S7-300 CPU 31xC technical data for the bit-memory and process-image limits of the 313C.

7. Cross-Platform Persistence: Why the Ring Buffer Disappears on Update

When WinCC flexible 2008 transfers a new project, the runtime stops, overwrites the project folder including the alarm buffer files, and restarts. The buffer is therefore cleared on every "Update OS" or "Download project" operation. To preserve a record across updates, do one of the following:

• Export the active buffer to a CSV file before starting the update using the ProSave function "Backup" on the HMI.
• Use the MMC ring file (Section 5) and copy the latest segment off-line before transfer.
• Archive the alarm log server-side via the S7-313C: the panel writes a "Last Alarm" record to a DB that the SCADA polls via OPC or S7 communication. This survives any panel-side loss.

The OPC approach is the most robust for SCADA. Configure a WinCC flexible 2008 area pointer "Date/Time" and a tag "LastAlarmID" mapped to DB1.DBW2. Archive using a WinCC / PCS 7 / third-party OPC server. See the WinCC flexible 2008 Communication user guide for area pointer configuration.

8. Migration to OP277 / MP277

If the application requires full alarm logging, the TP177B is undersized. The minimum drop-in replacement is the OP277 6" keys (6AV6642-0DC01-1AX1) — same 198 x 142 mm cutout as the TP177B 4", same MPI/PROFIBUS cabling, but with a 1024-entry buffer, full alarm logging, and a true event log. The MP277 8" touch (6AV6643-0CB01-1AX1) and 10" touch (6AV6643-0CD01-1AX1) variants add the WinCC flexible 2008 Alarm Logging editor and OPC DA export.

Migration path: open the TP177B project in WinCC flexible 2008, select Project > Device > Change Device, choose the OP277. The migration wizard remaps all AlarmView and tag references. Retest the alarm subsystem and re-verify the buffer behavior on the bench before production cutover. Confirm the target panel's OP277 / MP277 device manual lists the same WinCC flexible 2008 build as compatible.

9. Verification Procedure

1. From the TP177B, trigger a test fault by setting M 10.0 = 1 in the S7-313C (force via STEP 7 or a real fault). Confirm the AlarmView shows the alarm with class "Error" and the red flashing icon.
2. Acknowledge from the panel. The buffer must now contain a single "Came In / Acknowledged" pair.
3. Power-cycle the panel (pull the 24 V supply for 10 s, then re-apply). Power on, wait for the project to load.
4. Open the AlarmView. If the buffer survives, the event is still listed. If not, the volatile behavior is reproducing — Section 5 (MMC) is mandatory for this use case.
5. If MMC is fitted, navigate to \Storage Card\ALARM_LOG\ via ProSave. The CSV file should contain the test event with timestamp, state, and alarm text.
6. Trigger a project update (Device > Download > Update OS / Download project). Repeat step 4. The buffer must be confirmed cleared — this is expected behavior, not a defect.
7. Open the HMI Control Panel (on the TP177B: Start > Settings > Control Panel > OP) and confirm the current firmware version matches the WinCC flexible 2008 SP used to compile the project. A mismatch will silently reset the buffer on first power-up.

Document the verification results in the SAT (Site Acceptance Test) report with screenshots of the AlarmView before and after each test step.

10. Troubleshooting Matrix

11. Field-Proven Caveats

• Do not hot-swap the MMC on a running TP177B. Power down first. The CE 5.0 runtime does not re-enumerate the slot until restart.
• The TP177B 4" variant (6AV6642-0AA11-1AX1) has the same 256-entry buffer as the 6" variant, but only 4 MB of project memory. Large projects with many alarm texts may exhaust the project memory and force the runtime to drop buffer entries.
• WinCC flexible 2008 SP3 introduced a "Compact log" option for the alarm subsystem. This is enabled by default on new projects. Disable it (Project > Device Settings > Alarm Logging > Compact log = No) if the SCADA expects a flat CSV for import.
• When the TP177B is connected to an S7-300 / S7-400 rack, set the HMI update rate for alarm tags to 500 ms (default 1000 ms) in WinCC flexible 2008. Slower rates cause visible delay between the fault event and the panel display, leading operators to log false-clearance complaints.
• Always keep the panel firmware (image) and the WinCC flexible 2008 SP version aligned. A mismatch can corrupt the alarm buffer on first power-up after a project download. Recommended pairing: TP177B firmware V14.01.00 with WinCC flexible 2008 SP3 HF7 or later.
• The TP177B does not support the WinCC flexible "Audit Trail" option. If a GMP / FDA audit trail is required, the platform is the wrong choice — escalate to TP/OP/MP 277 or PC-based WinCC Runtime.
• When the panel boots from a cold start with no MMC and no project, the alarm buffer is reported as "empty" even if a project is later downloaded. There is no buffer to restore from in this state.
• For dual-screen applications where two TP177B share the same S7-313C, the alarm buffer is local to each panel. To merge, use the SCADA OPC layer, not the panels themselves.

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