WinCC Motor Start Confirmation: OK/Cancel C Script Implementation

Problem Overview

When a pushbutton on a WinCC runtime screen is wired through a direct connection to a binary tag in the automation system (AS), a single accidental click can write a 1 to the motor-start bit. For safety-critical actuators (motors, valves, heaters, conveyors), this one-click behavior is unacceptable. The standard mitigation is a confirmation prompt that requires the operator to explicitly acknowledge the action before the tag is written.

Two implementation paths exist in WinCC:

1. A Win32 MessageBox() call from a C script bound to a mouse event (quick to deploy, but officially discouraged by Siemens).
2. A native WinCC picture-window dialog using two buttons bound to internal tags and a visibility animation (recommended by Siemens for production systems).

This article documents both paths, the exact C code, the API flags involved, the tag wiring, and the commissioning checks required for a motor-start confirmation in WinCC V7.x / TIA WinCC Comfort/Advanced.

Prerequisites

Option A — Win32 MessageBox C Script (Quick Deployment)

The original solution in the field report uses the Win32 API MessageBox() directly from a WinCC C action. It is compact, requires no extra screen objects, and is sufficient for non-safety HMI prototypes.

MessageBox API flags used

Return values: 1 = IDOK (OK pressed), 2 = IDCANCEL (Cancel pressed / dialog closed).

Reference C script

// Event-bound C action on a button "Mouse click" event
// Tags used:
//   YOUR_TAG_NAME  - BOOL, external WinCC tag connected to the motor-start bit

#pragma code("kernel32.dll")
#pragma code("user32.dll")

#include <windows.h>

if (MessageBox(NULL,
               L"Are you sure you want to START Motor 1 ?",
               L"Confirm Motor Start",
               MB_OKCANCEL | MB_ICONEXCLAMATION | MB_SETFOREGROUND | MB_SYSTEMMODAL) == 1)
{
    BOOL temp;
    temp = GetTagBit("YOUR_TAG_NAME");
    if (temp == TRUE)
    {
        SetTagBit("YOUR_TAG_NAME", 0);
    }
    else
    {
        SetTagBit("YOUR_TAG_NAME", 1);
    }
}

Step-by-step wiring (WinCC V7 Graphics Designer)

1. Open the screen containing the motor-start button.
2. Right-click the button → Properties → Events → Mouse → Click.
3. Right-click the action symbol → C action….
4. Paste the script above, replacing YOUR_TAG_NAME with the exact WinCC tag name (case-sensitive).
5. Compile with Ctrl+F7. The status bar must show 0 errors, 0 warnings.
6. Save the picture and trigger a full RT build (Start → WinCC Runtime).

Step-by-step wiring (TIA Portal WinCC)

1. Open the HMI screen, select the button.
2. In the Inspector → Events → Click, add a function list or call a script.
3. For a global C script, declare the function in the Scripts editor and call it from the event.
4. For an inline VBS alternative, use HMIRuntime.Tags("YOUR_TAG_NAME") instead of GetTagBit/SetTagBit.

Why Siemens Officially Discourages MessageBox in WinCC

Siemens KB entry ID 22906363 states that MessageBox calls from WinCC are not recommended for production runtime. The technical reasons are:

• Language dependency: The literal text is hard-coded in the script; it cannot be translated through WinCC's Text Library or Text Distributor.
• Modal blocking: MB_SYSTEMMODAL can freeze the runtime if the dialog is dismissed abnormally (e.g., remote desktop disconnect, focus loss).
• Audit trail gap: The WinCC Audit option cannot log who clicked OK versus Cancel inside a MessageBox because the click happens in a non-WinCC window.
• Redundancy / WebUX: Modal Win32 dialogs are unsupported in WinCC/WebUX and WinCC/Server redundant pairs.
• Siemens support position: Errors that originate from a Win32 MessageBox in a C action are classified as customer code, not as a WinCC bug.

Option B — Native WinCC Confirmation Picture (Recommended)

A native dialog is built from standard WinCC picture objects: a picture window containing two buttons (Yes / No), a text field, and an optional warning icon. Visibility is driven by an internal tag triggered on the first button click.

Required tags

Screen construction

1. Create a picture window named PW_Confirm, configurable size (e.g. 400×200 px), centered, with a "Close on losing focus" option disabled.
2. Inside the picture window, place a static text "Confirm Motor 1 start?", a Yes button, and a No button.
3. Bind the Visible property of the picture window to the tag Confirm_Req with a Dynamic dialog (range 0/1) or, in TIA, an Animation → Visibility linked to the same tag.
4. On the Yes button Click event, use a direct connection:
   Tag: Motor1_Start = 1
   Tag: Confirm_Req = 0
   This writes the start bit and closes the dialog in one action.
5. On the No button Click event, direct connection: Confirm_Req = 0.

On the original start button (C action or direct connection)

// On Click of "Start Motor 1" button — C action
SetTagBit("Confirm_Req", 1);   // open confirmation dialog
SetTagBit("Motor1_Start", 0);  // ensure bit is low until Yes is pressed

This sequence is critical: resetting Motor1_Start to 0 at the moment the dialog opens prevents a race condition where the PLC sees a stale 1 from a previous operation.

Why this is safer than Option A

• All text is in the WinCC Text Library → translatable.
• The click event is logged by the WinCC Audit option if licensed.
• Visibility, position, and styling are part of the picture; consistent across themes.
• Works on WinCC/WebUX, WinCC/Client, and redundant servers.
• No kernel32.dll / user32.dll dependency — fully supported runtime.

Tag Configuration Reference

Direct Connection vs. C Action — When to Use Which

Verification and Commissioning Steps

1. Compile check: Trigger a full project recompile in WinCC Explorer. Read the Diagnostics window; no syntax errors allowed.
2. Tag simulation: In WinCC Graphics Designer, switch to Runtime with simulation mode and use the tag simulator to force Motor1_Start to 0.
3. Click test — Cancel path: Click the start button. The dialog must appear. Click "Cancel". Verify in the tag simulator that Motor1_Start is still 0 and the PLC diagnostic buffer shows no write to DB10.DBX0.0.
4. Click test — OK path: Click the start button, then "Yes". Verify Motor1_Start = 1 for the configured pulse time, and that the AS logic starts the motor.
5. Re-click test: While the dialog is open, click anywhere on the runtime outside the dialog. The dialog must remain modal (Option A) or simply stay visible (Option B). Background clicks must not write to the tag.
6. Power-cycle test: Restart WinCC Runtime while the motor is running. Confirm that the start tag defaults to 0 and that the operator must re-confirm to restart.
7. Multi-client test (Option A only): Open the same screen on a second WebUX client. MessageBox on client 1 must not block client 2, and vice versa. If it does, the deployment is not scalable to a multi-client setup — switch to Option B.

Troubleshooting Matrix

Edge Cases and Field-Proven Caveats

• Edge case — operator double-clicks: A double-click on the start button can fire the C action twice. The MessageBox call is modal, so the second call is suppressed by Windows, but a non-modal native dialog will show twice. Mitigate with a 500 ms debounce tag.
• Edge case — language switch at runtime: Option A text is fixed; switching language does not translate it. Option B pulls text from the Text Library, so it follows the active runtime language.
• Edge case — operator authorization: The dialog asks for confirmation, not authorization. If only Level-4 users may start the motor, gate the action in the AS with the WinCC user authorization, or in the HMI by checking GetTagBit("@CurrentUserLevel").
• Edge case — touch screens: A "Mouse Click" event does not fire reliably on WinCC RT with a single tap. Bind to Mouse Down for touch, or add a Long touch trigger for safety-critical starts.
• Edge case — timeouts: Neither option times out by default. A native dialog can implement a 10-second timeout with a global C script on a 1 Hz timer that clears Confirm_Req.

Security and Authorization Layer

Confirmation is orthogonal to authorization. Recommended stack for a motor start:

1. Operator logs in with a valid WinCC user (User Administrator).
2. Operator presses Start.
3. Authorization check: UserLevel >= 4 — if not, the action is rejected with a status message.
4. Confirmation dialog appears (this article).
5. Operator presses Yes.
6. Tag is written to the PLC.
7. PLC performs the F-CPU safety check, then energizes the contactor.

Summary

For non-safety prototype HMI screens, the Win32 MessageBox C script is the shortest path to a motor-start confirmation dialog in WinCC, using MB_OKCANCEL | MB_ICONEXCLAMATION | MB_SETFOREGROUND | MB_SYSTEMMODAL and the GetTagBit/SetTagBit pair. For any production system, multi-client deployment, audited environment, or translated runtime, build a native WinCC picture-window dialog driven by an internal confirmation tag and use direct connections on the Yes / No buttons. Both methods are valid; the Siemens support knowledge base entry 22906363 documents why Option B is the long-term choice.