Siemens OP177b PN/DP Dark Spot: LCD Failure Diagnosis & Migration

Siemens OP177b PN/DP Dark Spot: LCD Failure Diagnosis, Field Mitigation & Migration

1. Product Identification and Hardware Profile

The OP 177B is a third-generation SIMATIC Operator Panel that replaced the OP 170B and predates the Comfort line. The unit uses a 5.7" STN LCD module with a separate CCFL inverter. The article-number suffix -1AX1 identifies a specific configuration; for variant-specific manuals always cross-check the MLFB against the Siemens Industry Online Support product tree.

Reference: Siemens Industry Online Support – OP 177B PN/DP (6AV6 642-0DA01-1AX1).

2. Symptom Description

A single, well-defined dark patch appears near a corner of the active display area. The patch:

• Has a slightly irregular circular/oval boundary.
• Is darker than the surrounding pixel field (true black, not simply unlit).
• Grows measurably across days/weeks (millimetres per day at worst).
• Is not a software artifact (it persists through power-cycle, project change, and screen re-load).
• Does not respond to touch calibration routines.
• Is not associated with dead rows/columns (the dark area is localized, not striped).

Touch continues to work through the affected region in the early phase, suggesting that the top resistive layer is still intact. Backlight remains at constant brightness. Communication (PROFINET/PROFIBUS) is unaffected. In short: the electronics and the IGU's top layers appear intact; the failure is in the optical stack of the LCD.

3. Root Cause Analysis – Why STN LCDs Develop Growing Dark Spots

The OP 177B uses a STN (Super-Twisted Nematic) LCD with a CCFL backlight and a polarizer/retardation film stack. Several failure mechanisms can produce a growing dark patch on a panel of this age and type.

3.1 Polarizer Delamination / IGU Seal Failure

The polarizer is a multi-layer plastic film bonded to the front glass of the LCD. Edge-seal adhesive degrades under:

• Sustained heat (junction temperatures above +60 °C inside the panel cavity).
• Repeated thermal cycling (cabinet cooled at night, warmed by day).
• Humidity ingress when the front gasket is no longer sealing (porous foam age-hardening, missing gasket).
• UV exposure from direct sunlight (rare in indoor cabinets, common if the door is left open).

Once the seal fails, atmospheric moisture reacts with the polarizer's iodine/dye complex. The reaction produces iodine migration and polymer breakdown that absorb light and appear as a dark spot. The spot grows because the chemical reaction propagates along the polarizer film.

3.2 LC Fluid / Liquid Crystal Localized Leak

Loss of LC material from a single cell, typically caused by:

• Mechanical stress at a panel mounting point (over-tightened clamps, cabinet distortion, door slamming).
• Thermal shock (cold morning + sudden heater-on in the cabinet).
• Cell-gap non-uniformity in a worn-out display module.

An empty LC cell scatters/absorbs light and appears as a dark, often slightly textured patch. Growth comes from continued flow-out and from corrosion of the ITO electrodes exposed to air.

3.3 TAB / COG Bond or Driver-IC Anomaly

The driver ICs are bonded to the glass by Tape-Automated Bonding (TAB) or Chip-On-Glass (COG). Bond-pad corrosion or a single damaged line can also darken one region. However, these faults typically present as a horizontal/vertical line, half-screen blanking, or all-or-nothing pixel block, not a single growing blob. If you see a true geometric blob, rule TAB/COG out first.

3.4 CCFL Heat Damage / Reflector Burn

The CCFL inverter sits at one edge of the LCD. If the backlight has aged (CCFL lifetime ≈ 25,000–50,000 h depending on dim level) and the reflector film has yellowed or peeled, local discoloration can occur. This is usually yellow/brown rather than black and is rarely growing day-to-day.

3.5 EMI from a Nearby VFD (Hoist Converter)

The OP 177B is mounted on a hoist control cabinet; the hoist drive (typically a Sinamics V20/G120 or older Masterdrives/6SE70 converter) is in the next room. While the OP 177B itself is CE-marked for industrial EMI, the following can produce a visible dark artifact:

• Common-mode currents injected into the panel's 24 V supply via shared PE/grounding paths, creating localized field stress on the LCD.
• Conducted interference rectified by the LCD's ITO layers and slowly damaging the IGU seal.
• Magnetic-field-induced shadowing of the CCFL (only if the CCFL is poorly shielded – uncommon).

In practice, EMI from a hoist drive accelerates a pre-existing failure rather than being the primary cause. Treat VFD proximity as an aggravating factor and verify proper EMC installation.

3.6 Supply-Voltage Stress

Hoist installations commonly share the same 24 V supply with contactors, brakes, and relays. Inductive switching produces voltage transients that the OP 177B's internal SMPS must clamp. Repeated transients age the SMPS caps and the LCD bonding. Use a dedicated 24 VDC branch for the HMI and an upstream UPS/buffer as a minimum.

3.7 Mounting and Mechanical Pressure

Over-torqued mounting clips flex the front bezel and put sustained pressure on the LCD edge. On a 5.7" STN module the cell gap is ≈5 µm. A long-term pressure of even 0.1 N/mm² can displace LC fluid and initiate a leak at the corner. This is the most common physical explanation when the spot sits exactly in one corner of the display.

4. On-Site Diagnostics (without Spare Parts)

Run the following checks in order. Each takes <10 minutes with a basic multimeter and IR thermometer.

4.1 Visual & Mechanical

1. Power down. Remove the panel from the cutout. Inspect the front gasket: foam is it still elastic, compressed, crumbly?
2. Loosen each mounting clip ½ turn, then re-seat. Look for twist in the bezel.
3. Inspect the LCD perimeter with a 10x loupe – look for a thin "halo" or moisture trail at the edge of the dark spot.
4. Run a finger nail lightly along the front film at the spot's edge. If the film feels soft, raised, or peelable, the polarizer is delaminating.

4.2 Environmental

1. Measure cabinet internal temperature at the HMI rear face with an IR thermometer after 1 h of operation. The OP 177B derates above +50 °C ambient; the panel surface should not exceed +55 °C.
2. Measure cabinet humidity at the HMI location. If > 60 % RH sustained, fit a cabinet heater (anti-condensation) or relocate the HMI.
3. Verify the cabinet ventilation/filter cleanliness. Blocked filters raise internal temperature 10–15 K above ambient.

4.3 Electrical

1. Measure the 24 V supply at the HMI's power terminals under hoist operation (peak demand). It must remain within +19.2 to +28.8 V DC.
2. Oscilloscope (if available) the 24 V rail at the HMI: look for transients > +28.8 V or negative spikes below 0 V. Hoist contactor drop-outs often produce -60 V spikes of 1–10 ms.
3. Verify PE bonding. From the HMI PE terminal to the cabinet PE bus bar: resistance < 0.1 Ω. From cabinet PE to plant ground: < 1 Ω.
4. Verify the shield of the PROFINET/PROFIBUS cable is bonded at both ends via the connector backshells and at the cable entry into the cabinet.

4.4 EMC / VFD Proximity

1. Confirm physical distance between OP 177B and the hoist converter: ≥ 200 mm is the conservative rule for panel-to-VFD separation; ≥ 500 mm if cables run parallel for > 1 m.
2. Check whether the hoist 24 V supply and the HMI 24 V supply share the same PSU or breaker. If yes, separate them and add a line filter (e.g., Siemens 6EP1 filter module or Schaffner FN2070).
3. Inspect the converter's input filter – is it still bonded to the cabinet back-panel with the contact-paint removed under the filter?

4.5 Display Health via HMI Software

1. Open the OP 177B's Start Center and check the diagnostics page: Start > Settings > OP > Display (WinCC flexible) for diagnostic counters.
2. Run a screen test pattern (full white, full black, color bars). Map the dark patch – if it stays put across all patterns, the failure is in the IGU, not in the project.
3. Check backlight hours if the project exposes them. CCFL dim level should still be > 80 % of nominal.

5. Field Mitigation – Slowing or Stopping the Growth

Once the polarizer/LC seal is compromised, the failure is irreversible. You can only slow it. Realistic mitigation extends operational life by weeks to months.

6. Replacement Strategy – When the Panel Cannot Wait

Because the OP 177B has been blocked for delivery since 2015, three replacement paths exist, in order of preference.

6.1 Identical Spare (Best, when available)

Source a verified identical 6AV6 642-0DA01-1AX1 from a Siemens distributor. Note the FW version printed on the rear label and on the Start Center. Do not assume cross-version compatibility of the commissioning file; older FW versions reject newer backups.

Procedure:

1. Power down and remove the failed unit. Record the MLFB and FW version.
2. Install the replacement mechanically. Restore the wiring (PROFINET, PROFIBUS, 24 V, PE).
3. Power up. The Start Center should display "Transfer" mode if no project is loaded.
4. Restore the commissioning backup via ProSave (serial) or Ethernet. See Section 7.
5. Verify the project starts and the tag connections come up. Cycle hoist functions.

6.2 Successor Migration

Siemens-recommended successors for the OP 177B 5.7" QVGA are:

Migration requires:

1. The original WinCC flexible project (the *.hmi / *.fwx file) – if unavailable, see Section 7.3.
2. A TIA Portal V16 or later installation with the migration toolchain.
3. Migration of the project: TIA Portal > Project > Migrate project. The OP 177B project migrates to a TP700 with no functional loss for typical hoist visualizations.
4. Re-validate tag connections, alarms, and recipes. The new touch panel uses projected capacitive touch (Comfort) or resistive (Basic) – calibrate accordingly.

Reference: Siemens – Migration from OP 177B / TP 177B to Comfort Panels.

6.3 Used / Refurbished (Last Resort)

Surplus dealers can supply OP 177B units pulled from decommissioned machines. Validate before commissioning:

1. MLFB exactly matches 6AV6 642-0DA01-1AX1.
2. FW version compatible with the project file.
3. No dark spot on the display – power on for 30 min and inspect with a flashlight at low angle.
4. Touch calibration runs cleanly across the full screen.
5. Backlight brightness is uniform (no yellow corners = no CCFL reflector degradation).

7. Backup, Restore and Commissioning Without the Original Project

7.1 Backup Locations

On the OP 177B the commissioning project can reside in three places:

1. Internal flash – primary; backed up automatically to internal memory on transfer.
2. External storage – the rear USB port accepts a FAT-formatted USB stick. The panel writes a *.psb or *.fwx backup automatically on every change if configured.
3. External engineering station – ProSave stores the last transferred project under %USERPROFILE%\Siemens\Automation\WinCC flexible\<project>\<HMI>\Transfer.

7.2 Restoring via ProSave (Ethernet)

1. Connect the engineering PC to the OP 177B's PROFINET port. Configure the PC IP in the same subnet (e.g., 192.168.0.1 / 255.255.255.0) and the panel IP per the original project.
2. Start ProSave (bundled with WinCC flexible or TIA Portal).
3. Select Device Type: OP 177B PN/DP; Connection: Ethernet; enter panel IP.
4. Choose Restore > Transfer. ProSave pushes the project to internal flash.
5. Reboot the panel. The Start Center should now show the project name and the configured runtime.

7.3 When No Backup Exists – Reverse Engineering from the Failed Panel

If the project was never backed up and the source code is lost, two last-ditch paths exist:

1. Upload from a working but ailing unit. As long as the panel can still start WinCC flexible and the flash is intact, perform ProSave > Backup over Ethernet. This uploads the compiled runtime project (*.psb) to your PC. You cannot edit it in this form, but you can re-deploy it to a replacement.
2. Read the internal flash via service tools. Siemens does not publish a public flash-dump procedure for the OP 177B. Authorized service centers can do it. If borders are closed, contact the nearest Siemens regional repair center by e-mail and request a remote data-extract service.

8. EMC and Earthing Best Practice for Hoist Panels

The classic layout – HMI on the hoist cabinet, VFD/converter in the adjacent room – is the most common source of EMI-related LCD ageing. Apply the following on every hoist retrofit.

8.1 Power Supply

• Use a dedicated 24 VDC power supply (Siemens SITOP 6EP1334-3BA10 or equivalent) for the HMI; do not share with the contactor/brake circuit.
• Add a SITOP 6EP1 961-3BA10 buffer module (≥ 100 ms hold-up) to ride through contactor switching.
• For full isolation, add a SITOP 6EP1 971-1AA01 UPS module with battery.

8.2 Grounding / Bonding

• HMI PE terminal to cabinet PE bus: dedicated flat copper braid, ≤ 100 mm length, ≥ 6 mm² cross-section.
• Cabinet PE bus to plant ground: ≤ 1 Ω total, measured with a four-wire ohmmeter.
• Back-panel paint removed under every EMC filter, gland plate, and converter mounting.

8.3 Cable Routing

• PROFINET/PROFIBUS cable: ≥ 200 mm separation from any VFD input/output cable run for ≥ 1 m of parallel length.
• Cross power and signal cables at 90° if they must meet.
• Shield bonded to connector backshells both ends; cabinet gland plates provide 360° shield contact.

8.4 Cabinet Thermal Management

• Filter fan sized to limit internal temperature rise to < 10 K above ambient.
• Anti-condensation heater (e.g., 50 W cabinet heater) for humid environments; thermostat set just above dew point.
• OP 177B mounted in the lower 2/3 of the cabinet to use the natural convection cold-pool.

9. Verification Checklist After Repair / Migration

1. Visual: the dark patch is absent on the new panel; background is uniform white across all color tests.
2. Touch: run the calibration screen; all four corner targets register correctly. Drag a stylus in a continuous line across the screen – the line should not skip or jump.
3. Communication: PROFINET/PROFIBUS partner establishes the connection; tag values update in the visualization.
4. Function: perform one full hoist cycle (up, down, slow, fast, stop, E-stop). Verify alarms clear and re-arm correctly.
5. Environmental: re-measure cabinet internal temperature and humidity at full load. Document the values.
6. Electrical: re-measure 24 VDC at the HMI terminals during a hoist start. Confirm ≤ +28.8 V peak.
7. EMC: PE loop resistance ≤ 0.1 Ω (HMI to cabinet), shield continuity verified on all signal cables.
8. Backup: write a fresh commissioning backup to USB stick and store on the engineering station. Schedule quarterly backups.

10. Troubleshooting Matrix

11. Documenting the Failure for the Manufacturer

When borders reopen and a service request can be filed, the following documentation accelerates the case:

1. Front and rear photographs of the panel (article-number label visible).
2. A timestamped time-lapse of the dark patch (a weekly photo with a ruler in frame is sufficient).
3. Cabinet environmental log: temperature, humidity, supply voltage, transient count.
4. EMC survey: PE resistance, cable separation, VFD proximity.
5. Project file or last commissioning backup (transfer via ProSave before the panel becomes unreadable).

Submit the package through your nearest Siemens Industry Online Support request page. Reference: Siemens Industry Online Support.

12. Long-Term Recommendations

• Replace OP 177B installations older than 10 years proactively with a TP700 Comfort or KTP700 Basic.
• Maintain an offline backup of every WinCC flexible project on the engineering station; verify the backup by test-restore at least once a year.
• Standardise cabinet layout: dedicated HMI 24 V supply, EMI separation rules, thermal management checklist.
• Train operators to recognise early LCD failure (single growing dark spot, slight yellowing, backlight flicker) so the panel can be replaced on a planned maintenance window rather than during a breakdown.

FAQ