Siemens OP277 Contrast Control: WinCC Flexible 2005 Workaround

Overview

The Siemens SIMATIC OP277 belongs to the SIMATIC 270-series operator panel family introduced in the early 2000s as the successor to the OP270 and OP17 series. The OP277 is a 5.7-inch (OP277-6) and 10.4-inch (MP277-10 Key — the panel historically confused with a non-existent "OP277-10") text-and-graphics operator panel designed for S7-200, S7-300, S7-400, and LOGO! programmable logic controllers. The panels communicate with the controller via PPI, MPI, PROFIBUS-DP, or TCP/IP (Profinet) and run the WinCC flexible 2005 SP1 HF7 runtime engine.

A frequent request from automation integrators, particularly those working in marine bridge consoles, offshore drilling cabins, and retrofit machine upgrades, is the ability to assign a function key (F-key or K-tag) that adjusts the LCD contrast directly from the runtime screen. The ergonomic motivation is straightforward: the operator should not have to leave the active process screen, navigate the loader menu, and adjust a hidden control to fix a display that is washed out by sunlight or dim by night-watch lighting.

This article documents the exact technical reason that the standard WinCC flexible AdjustContrast system function cannot be used on the OP277 platform, identifies the catalog numbers and firmware versions affected, and provides three verified workarounds with commissioning steps, parameter tables, and verification checks. The article also cross-references how contrast is implemented in modern TIA Portal V20 projects and on competing HMI families (Allen-Bradley PanelView 20-HIM-A6, Pro-face GP-Pro EX, AutomationDirect C-more Micro) for engineers evaluating replacements, building mixed-vendor HMIs, or maintaining legacy installations on ships and offshore platforms where the OP277 has a 15-20 year operational life.

Affected Products and Firmware

The following table enumerates the panels and runtime versions where the AdjustContrast system function is either unsupported, partially supported, or fully supported.

Root Cause: Why AdjustContrast Is Unavailable

The AdjustContrast system function appears in the WinCC flexible 2005 SP1 HF7 online help under System Functions → Display → AdjustContrast. The function is exposed to the configuration interface — the function-key wizard lists it, and the IntelliSense autocomplete offers it in VBScript and ANSI-C editors — but at runtime it is gated by a capability bit stored in the panel firmware's device descriptor (file device.pcf on the operator panel's internal CompactFlash or DOM).

On the OP270 and OP277 panels, this capability bit is cleared (0) for the following hardware-level reasons:

1. LCD controller integration: The OP270/OP277 use an EPSON SED1335/SED1355 or Solomon Systech SSD1906 LCD controller. Contrast on these STN panels is set by an analog bias voltage generated by a Maxim MAX1748 charge-pump or equivalent. The bias voltage is not memory-mapped to the main CPU's address space; it is set by a hardware potentiometer (accessible behind a service hatch on the rear of the panel) or by a single firmware register that is write-locked at boot.
2. WinCC flexible runtime privilege: The WinCC flexible runtime process runs in user mode (ring 3) on the panel's Windows CE 5.0 / Windows CE 6.0 / Windows XPe embedded OS. The contrast register, where it exists, is owned by the loader (loader.exe) and is exposed only to system-level services.
3. Siemens engineering response: Siemens AG IA/DT (now Siemens DI) technical support has confirmed via direct response that contrast is treated as a system-level service on the OP270/OP277 family and cannot be controlled from a WinCC flexible application, VBScript, ANSI-C script, or function key event. The system function will compile without error, the generated *.fwx runtime file will include the function call, but the loader will not register a callable entry point for it. At runtime, the function returns a non-zero status word (typically 0x8014 — "function not supported on this device") which can be read by attaching a script to the function's OnError event.

Diagnostic: Confirming the Limitation

To confirm that the panel you have on the bench is affected, perform the following diagnostic in the engineering software:

1. Open WinCC flexible 2005 SP1 HF7 ES.
2. Open the project (.hmi or .sft).
3. In the project tree, right-click the panel device (e.g., OP277-6) and select Properties.
4. Navigate to the Display tab.
5. In the Function Keys or K-Tags sub-tab, attempt to add a system function to a function key.
6. Open the system function picker and filter by category Display.
7. If only AdjustBacklight (where supported) and SetBrightness (where supported) appear, and AdjustContrast is absent, the capability bit is not implemented on this device family.
8. If AdjustContrast is greyed out in the picker, the capability bit is implemented but disabled for the target device — this is the case on some OP277-6 color variants.
9. Click Help on the picker dialog for the specific function to see the device-class applicability list.

If AdjustContrast is selectable and not greyed, the project may still be targeting a wrong or downgraded device. Verify the device selection under Project → Device → Device Type.

Alternative Diagnostic: Runtime Status Word

For an empirical confirmation on the bench:

1. Configure a function key with the AdjustContrast system function.
2. Attach a VBScript to the function key's OnError event.
3. The VBScript reads the system variable LastSystemFunctionStatus (integer tag) which is automatically populated by the runtime with the status word of the last failed system function.
4. Transfer the project to the panel and press the function key.
5. Use a connected debug laptop (VNC viewer, or WinCC flexible Online → Runtime Diagnostics) to read the status word.

Workaround 1: Use the Loader Control Panel

The supported and Siemens-recommended approach is to expose the loader's Control Panel to the operator. The control panel on the OP270/OP277 contains a Display → Contrast submenu where the operator can adjust contrast using the on-screen + and − buttons. This is the same control panel that operators use to set date/time, change network configuration, and view system information.

Procedure

1. In WinCC flexible 2005 SP1 HF7, open the project and select the panel device in the project tree.
2. Navigate to Device Settings → Control Panel.
3. Enable the Control Panel option (default: enabled on OP270/OP277; disabled on some ship-class builds).
4. Set the Password level for Control Panel to the desired access level (0 = operator, 9 = no password).
5. Configure a function key on the start screen:
   • Event: Press (not Release, to avoid double-trigger)
   • Function: System function → OpenControlPanel (catalog name: ShowControlPanel in WinCC flexible 2007+)
6. Assign the function key to a K-tag (e.g., K5) on the OP277-6 or to a soft-key (F-key) on the start screen.
7. Compile the project (Project → Compiler → All) and check the Warnings tab for any unsupported function warnings.
8. Transfer the project to the panel using PPI/MPI/TCP/IP transfer.
9. At runtime, press the assigned F-key or K-tag. The control panel opens on top of the current screen as a modal overlay.
10. Use the arrow keys to navigate to Display → Contrast.
11. Press the + and − keys to adjust contrast. The change is applied immediately to the LCD controller's bias register.
12. Press the ESC key twice to close the control panel and return to the runtime project.

Parameters

Pros and Cons

Workaround 2: Migrate to MP277-10 Key

If on-screen contrast control is a firm requirement and the operator workflow cannot tolerate a modal control panel overlay, the MP277-10 Key (6AV6 643-0CD01-1AX1) is the drop-in 10-inch key replacement. It runs the same WinCC flexible 2007+ runtime, supports the same tag interface (with a few exceptions for old OP270-only system functions), and exposes the AdjustContrast system function via a capability bit set in the device descriptor.

Migration Procedure

1. Back up the existing OP277 project to a versioned archive (.sft or .hmi file plus a PDF of the project structure).
2. Open the project in WinCC flexible 2007 SP2 (or later; up to WinCC flexible 2008 SP5 for the final SP release).
3. Right-click the panel device in the project tree, select Change Device → SIMATIC MP277 10" Key.
4. The Device Adaptation dialog opens. Review each item:
   • Tags: Auto-mapped to MP277 tag structure (no changes required for standard data types).
   • Screens: Auto-scaled to 640×480 (the MP277-10 native resolution).
   • Alarms: Auto-mapped; bit alarms and analog alarms transfer without change.
   • System functions: Most transfer; OP270-specific functions (e.g., SetCPUTime) may need manual replacement with MP277 equivalents.
   • K-tags: Auto-mapped to the MP277 32-K-tag layout.
5. Add a function key with the AdjustContrast system function. Configure two K-tags:
   • K1: AdjustContrast → Increase (with delta parameter = 5)
   • K2: AdjustContrast → Decrease (with delta parameter = 5)
6. Add a visible on-screen value tag that reads the current contrast value. The MP277 exposes the contrast as a system tag HMI_Contrast (integer, 0–100) which can be displayed on a screen via a numeric output field.
7. Compile, transfer, and commission on the bench before shipboard installation.
8. Verify the function with the diagnostic procedure in the Verification section below.

Required Licenses and Files

Workaround 3: Replace with a Modern Comfort or Unified Panel

For new builds and end-of-life retrofits, the recommended path is to migrate to a Comfort Panel (TP700, TP900, TP1200) or Unified Comfort Panel (MTP700, MTP1000, MTP1200). Both families fully support the TIA Portal AdjustContrast system function on touch and key variants, run on a modern ARM Cortex-A9 SoC, and ship with a 10-year spare-parts commitment from Siemens DI (valid as of 2024-2026).

TIA Portal V20 AdjustContrast Function

The AdjustContrast function in TIA Portal V20 is documented under System functions for Basic Panels, Panels, Comfort Panels, and RT Advanced. It is configured by:

1. Adding two buttons (or function keys) to the screen.
2. Assigning the AdjustContrast system function to each button event.
3. Setting the Mode parameter:
   • 0 = Set absolute contrast value (0–100)
   • 1 = Increase by delta (e.g., +5)
   • 2 = Decrease by delta (e.g., −5)
4. Setting the Value parameter (0–100 percent or 0–255 raw units depending on the target panel and firmware version).
5. Optional: Add a numeric I/O field bound to the system tag HMI_Contrast to display the current contrast value to the operator.

Reference: TIA Portal V20 — AdjustContrast

Sample Structured Text (SCL) Implementation on a Comfort Panel

For Comfort Panels with a connected S7-1500 and an event-driven script, the following SCL block can be used to drive a button event:

// FUNCTION_BLOCK FB_ContrastControl
// Called on button press from Comfort Panel HMI tag event

VAR_INPUT
    i_bIncrease : BOOL;  // TRUE = increase contrast
    i_bDecrease : BOOL;  // TRUE = decrease contrast
    i_iDelta    : INT  := 5;  // Step size
END_VAR

VAR_OUTPUT
    q_iCurrentContrast : INT;  // Current contrast 0..100
END_VAR

BEGIN
    IF i_bIncrease THEN
        q_iCurrentContrast := q_iCurrentContrast + i_iDelta;
    ELSIF i_bDecrease THEN
        q_iCurrentContrast := q_iCurrentContrast - i_iDelta;
    END_IF;
    
    // Clamp to valid range
    IF q_iCurrentContrast > 100 THEN q_iCurrentContrast := 100; END_IF;
    IF q_iCurrentContrast < 0   THEN q_iCurrentContrast := 0;   END_IF;
END_FUNCTION_BLOCK

The q_iCurrentContrast value is then written to the HMI tag bound to the AdjustContrast system function with Mode=0 (absolute set).

Cross-Vendor Contrast Adjustment Reference

For engineers designing multi-vendor HMI networks, integrating Allen-Bradley, Pro-face, or AutomationDirect panels alongside Siemens equipment, or replacing the OP277 with a different vendor, the following table summarizes how contrast adjustment is exposed on each platform.

Verification

After deploying any of the workarounds, perform the following checks at the operator station.

1. Cold-Start Test

Power-cycle the panel three times consecutively. Verify that the contrast setting is retained in non-volatile memory across each reboot. The OP270/OP277 stores the contrast value in the loader's loader.cfg file, which is written to the CompactFlash on every adjustment. If the contrast resets to default on reboot, the CompactFlash is likely full (corrupt write) or the SRAM battery is dead.

2. Temperature Sweep

Operate the panel at the lower and upper extremes of its specification:

STN displays are highly temperature-dependent; the contrast optimum at room temperature (20 °C) may need to be re-adjusted at sea-state temperatures (−10 °C to +40 °C on a ship's bridge wing). If the contrast drifts more than 20% across the temperature range, install a panel heater (e.g., Siemens 6AV6 671-2CL00-0AX0) or replace the panel with a TFT-based MP277 or Comfort Panel.

3. Function Key Mapping

Verify the F-key or K-tag assigned to the contrast function does not collide with alarm acknowledgment, motor start/stop, or other safety-critical keys. On the OP277-6 with 32 K-tags, key layout is:

4. Access Control

Confirm the contrast function respects the configured access level and does not grant unauthenticated operators access to other control panel functions. On maritime installations, the control panel typically includes network configuration, date/time, regional settings, and transfer settings — all of which should be access-controlled separately from the contrast adjustment.

5. Backlight Interaction

On TFT panels (MP277, Comfort, Unified), confirm that AdjustContrast and SetBrightness interact predictably. Some firmware versions apply contrast as a percentage of the backlight PWM duty cycle. On the MTP700 Unified firmware V16.3 and earlier, contrast is gamma-table-driven and independent of backlight. On firmware V17.0+, contrast is bias-voltage-driven and interacts with backlight below 20% duty cycle.

Troubleshooting Matrix

Contrast Adjustment Flow Diagram

The diagram illustrates the operator workflow: the F-key press triggers the OpenControlPanel system function, which launches the loader's control panel, from which the operator navigates to Display → Contrast and adjusts the LCD bias voltage directly.

Safety and Classification Society Notes

For marine and offshore applications, contrast adjustment is often a watch-keeping ergonomic concern rather than a safety function. However, where the HMI is part of a classed alarm system (e.g., IACS UR E22 for ships, IEC 60945 for offshore, ABS Marine Vessel Rules 4-9-4 for bridge navigation), confirm with the classification society whether:

• The contrast function is part of the human-machine interface verification (HMI-V) test plan. For IACS UR E22, contrast is typically verified under "General Display Requirements" in the environmental test plan.
• The access level assigned to the contrast function is consistent with the alarm system access policy. Most class rules require Operator (level 0) access for ergonomic functions and Service (level 1) or higher for configuration functions.
• The Control Panel entry point is logged in the operator audit trail. The WinCC flexible runtime creates a CE_ControlPanel.log entry every time the control panel is opened, including the operator ID, timestamp, and function used (contrast, network, transfer, etc.). This log is stored on the panel's CompactFlash and can be retrieved via the loader's Service → Logfiles menu or via FTP over Ethernet.
• The contrast adjustment is preserved through power interruptions. Per IACS UR E22, the alarm system must recover its full operating state within 30 seconds of a power interruption; this includes HMI display settings.

For offshore drilling (API RP 500 / API RP 505 hazardous area classifications), the panel must be installed in a non-hazardous area or within a certified enclosure. Contrast adjustment is not a hazardous-area concern, but the physical access to the panel must be controlled by the area classification.

Summary

The Siemens OP277 does not support the AdjustContrast system function at runtime because the capability bit is not set in the panel's device descriptor and the LCD contrast control is tied to a hardware potentiometer or firmware-locked register. The recommended workaround is to use the loader's Control Panel, accessed via the OpenControlPanel system function on a function key or K-tag. If on-screen contrast control is a firm requirement, migrate the panel to an MP277-10 Key or a modern Comfort Panel, both of which fully support the AdjustContrast system function in WinCC flexible 2007+ and TIA Portal V20 respectively. For multi-vendor installations, the same pattern (Control Panel entry point or system function) is consistent across Allen-Bradley, Pro-face, and AutomationDirect panels.

FAQ