MP 377 12" Touch Unresponsive: Calibration and Hardware Diagnosis

1. Problem Definition

The Siemens SIMATIC MP 377 12" Multi Panel occasionally reports a fault where runtime values (process variables, status indicators, animated graphics) continue to refresh correctly, but a subset of soft buttons — typically the window-change, screen-swap, or navigation buttons — stop responding to finger presses. The panel is reachable over Ethernet, the HMI-to-PLC tag poll continues, and alarms can still be acknowledged by other input objects. The fault is therefore not a communication loss, a project-load failure, or a PLC tag failure; it is a local input-layer fault on the HMI itself.

Two distinct physical variants are affected by this class of symptom, and the diagnostic path is materially different for each:

• MP 377 12" Key (function-key variant) — front membrane keys stop registering.
• MP 377 12" Touch (resistive touch variant) — touch input from a graphics-design button is not accepted.

2. Affected Hardware Variants and MLFBs

The MP 377 12" family is sold in several MLFB (Maschinenlesbare Fabrikate-Bezeichnung) configurations. The Touch and Key variants share the 12.1" TFT chassis but differ in the front overlay, the input controller, and the firmware build identification. Use the table below to locate the part in service:

Verify the MLFB on the rating plate on the rear of the unit, the label inside the SD/CF card slot cover, or by reading Start → Settings → Control Panel → OP (Operator Panel) → Device on the panel itself. The exact MLFB is required when ordering a replacement or a transfer license.

Reference: SIMATIC HMI Multi Panel MP 377 device manual (Siemens Online Support entry ID 21663399).

3. Differentiating Key vs Touch Failure

The first triage decision is to decide whether the failed input element is a physical function key on the front bezel or a soft button drawn inside the WinCC flexible / TIA project.

Indicators of a Key variant failure (hardware):

• The fault only affects the system function keys (F1–F32, K1–K36 depending on layout), not soft buttons inside the screen.
• Pressing the key produces no audible click, no LED change, and the configured F-key action is not triggered.
• Touching the screen with a USB mouse plugged in still works for soft buttons — only the membrane keys are dead.

Indicators of a Touch variant failure (touch layer / calibration):

• All or a rectangular region of touch input is ignored, but the screen image is otherwise correct.
• Tapping the screen with a fingertip produces no tactile confirmation; using a USB mouse still navigates the entire screen correctly.
• Values in IO fields update, but tapping a soft button does not invoke its event — this is the canonical symptom in the source report.

4. Root Cause Analysis

For the MP 377 12" Touch, the most common root causes of partial button unresponsiveness, in descending order of frequency observed in field service:

1. Touch calibration drift. The 4-wire resistive overlay calibration stored in non-volatile memory has shifted, usually because of mechanical impact, vibration, thermal cycling, or a recent firmware / project update. The reported finger coordinate falls outside the acceptance window of a soft button, so the runtime rejects the press.
2. Touch controller hang. The internal touch controller firmware on the panel's mainboard has stopped servicing interrupts. Project values continue to update because the display pipeline is independent of the input pipeline.
3. Project-level lock or animation block. A configured button has the "Operating enable" tag cleared by PLC logic, or an invisible / overlapping object on a higher layer is swallowing the press. Verify by inspecting the active screen in the engineering station and confirming all "Events" of the failed button are correctly wired.
4. Front-overlay damage. Physical cracks or contamination on the touch foil locally kill the resistive contact. This is hardware — not recoverable by calibration.

For the MP 377 12" Key, the root cause in the vast majority of cases is membrane key failure: the conductive dome beneath the affected key has lost its snap or the trace is open. Siemens service treats this as a non-repairable field fault and replaces the front overlay or the entire panel.

5. MLFB Identification Procedure

Before opening the cabinet, capture the panel's full identification. This is required for the spare-parts RMA and for matching the correct device manual.

1. On the panel itself, press Start → Settings → Control Panel.
2. Double-tap OP (Operator Panel) or, on later firmware, System Properties.
3. Read the fields: Device, Image version, Bootloader version, and Device version.
4. Cross-check the MLFB with the rating plate on the rear.

If the touch is fully dead and the panel cannot be navigated with a finger, plug a USB mouse into the USB-A port on the rear or side of the panel before the procedure. The MP 377 enumerates a standard USB HID mouse on Windows CE 5.0 without any driver load.

6. Touch Screen Recalibration Procedure

Use the on-device calibration tool. This is the same routine that the panel runs on first commissioning, and it persists the new matrix into the registry of Windows CE.

1. Press the physical Calibrate key on the panel, or navigate Start → Settings → Control Panel → OP.
2. Switch to the Touch tab.
3. Tap Calibrate Touch Screen. A 5-point target sequence appears (center, then four corners).
4. Use a fine-tip stylus (a PDA stylus or the back of a mechanical pencil works) and tap each target accurately. Avoid using a fingertip for the calibration step — the calibration point is read at the moment of contact, and a finger on a resistive overlay has a larger contact patch than the visual crosshair.
5. Confirm with OK; the new calibration is stored immediately. A power cycle is not required.

If the on-device Start → Settings → Control Panel path is inaccessible because the touch is too far off, use the USB mouse workaround in Section 7 to drive the panel, then run the calibration routine.

Reference: Siemens Support entry 18796013 — Calibrating the touch screen on a panel.

7. USB Mouse Workaround

The MP 377 supports a USB HID mouse on the rear Type-A port without installing additional drivers. This is the canonical field workaround for an unresponsive touch and lets the operator keep the line running until a planned touch replacement.

1. Power down the panel. Plug a standard USB mouse (wired, no driver required) into the rear USB-A port.
2. Power up. Windows CE enumerates the mouse within 10–15 seconds; the cursor appears on screen.
3. Use the mouse to navigate to the control panel, run the touch calibration as in Section 6, then test the soft buttons.
4. If the calibration succeeds, remove the mouse and confirm finger touch works. If finger touch still fails, the touch overlay is electrically damaged and the front overlay must be replaced.

8. Recovery with ProSave and WinCC flexible

If calibration does not restore touch, the next step is to re-flash the operating system image and re-install the runtime project from a known-good backup. Use ProSave (Siemens service tool) over Ethernet.

1. On the engineering station, open ProSave. Set the device type to MP 377 Touch 12".
2. Select OS Update → Reset to Factory Settings. The panel reboots into the loader.
3. Use OS Update → Update OS to push the firmware matching the original MLFB. The image must match the panel variant (a Touch image on a Key panel will brick the input layer).
4. After the OS update completes, use Backup/Restore → Restore to push the last known-good runtime project.
5. Reboot. Re-test the soft buttons. If still dead, the touch overlay is the failure.

For TIA Portal, the equivalent service tool is TIA Portal → Online → HMI Device Maintenance → Reset / Update OS. See TIA Portal HMI Device Maintenance (entry 109746530).

9. Hardware Failure Indicators

Escalate to hardware replacement when any of the following are observed:

• Visible crack, dent, or burn mark on the front overlay.
• Touch behaves erratically: phantom presses, presses registered at the wrong location, or only a partial region responsive — symptoms of a fractured ITO layer.
• USB mouse works perfectly across the entire screen, confirming the display and runtime are healthy and the touch overlay is isolated as the fault.
• Calibration routine accepts taps but the new matrix is ignored on the next reboot — non-volatile storage of the calibration is damaged.
• For the Key variant: any function key on the front membrane does not register a press.

10. Replacement and Spare Parts

Spare part ordering uses the same MLFB with the addition of a spare suffix or a separate spare-part number. For the MP 377 12" Touch, the typical replacement is the complete front overlay assembly (touch + display bonded). Siemens parts logistics can usually ship a 12" Touch spare overlay within 24–48 hours inside the EU and the US.

If the panel is out of service and a CPU slot cannot be freed, consider a functional equivalent migration to a current SIMATIC HMI Comfort Panel — for example the TP1200 Comfort (6AV2 124-1MC01-0AX0) — using the TIA Portal project migrator. The mechanical cutout and depth differ; verify against the Comfort Panels device manual (entry 63481215) before scheduling the swap.

11. Verification Checklist

After any of the procedures above, verify with this sequence:

1. Boot: Panel reaches the configured start screen within 60 s of power-on, no system error dialog.
2. Communication: Tag values in IO fields update at the configured polling rate; no PLC timeout alarms in the alarm view.
3. Touch test: Tap a soft button that is known to be wired to ActivateScreen; confirm the screen change is instantaneous and the previous screen's tag subscriptions terminate.
4. Calibration regression: Reboot the panel three times. On each boot, re-tap a corner of the screen; the cursor (or calibration crosshair) should follow the finger accurately within ±2 mm.
5. Alarm acknowledgement: Trigger a configured alarm; confirm the Acknowledge button clears the bit in the PLC.
6. USB fallback: If a USB mouse is provisioned as a permanent fallback, leave it plugged in for 24 hours of production and confirm zero phantom-input events.

12. Frequently Asked Questions