WinCC Unified I/O Field: Display 'No User' When Logged Out (V17)

Overview

WinCC Unified (TIA Portal V17) exposes the system tag @UserName, which returns the currently logged-in user as a WString. When no user is authenticated, the tag returns an empty WString of length zero; it never returns the literal text "null". This article documents how to display either the username or the placeholder text "No User" in an I/O field, and how to drive a Boolean status tag that mirrors the login state for use by the PLC.

Two complementary approaches are covered:

1. A JavaScript dynamization triggered on a 1-second cycle (T1s) that combines string formatting and status-bit updates in a single function.
2. An event-driven approach tied to system alarms raised on user login and logout, which removes polling latency.

Prerequisites

• TIA Portal V17 Update 4 or later (V18 and V19 are compatible with the same syntax; minor differences in system alarm numbering may apply).
• WinCC Unified Runtime V17 or higher installed on the target device (Comfort Panel, MTP, or Unified PC).
• Configured user administration on the HMI device: Security → Users and Roles with at least one user defined.
• A test screen containing an I/O field (or a Text field, if display-only).
• An internal HMI Bool tag, or a Bool tag mapped to a PLC address, used as the UserLoggedIn status bit.

Understanding the @UserName System Tag

The @UserName tag is part of the WinCC Unified system namespace. It is generated automatically when a project is compiled and cannot be edited or deleted. Its properties are fixed by the runtime:

Because the empty state is a zero-length WString and not the JavaScript value null or undefined, a simple .length === 0 test reliably distinguishes the two cases. This is the foundation of the detection logic in the script below.

I/O Field Configuration

Configure the I/O field in Output mode so it functions as a read-only display and cannot be modified by the operator. Open the I/O field properties in TIA Portal:

1. Select the I/O field on the screen canvas.
2. In the Properties pane, expand General.
3. Set Mode to Output.
4. Confirm the Format setting is String or WString; the I/O field in RT Unified supports String display modes only.

The I/O field RT Unified element supports Mode, Process value, and Format properties per the official documentation. Refer to the IO field RT Unified reference for the complete property matrix.

Property Reference

JavaScript Dynamization for Username Display

To display either the username or the literal placeholder "No User", dynamize the Text property of the I/O field with a JavaScript function and trigger it on a 1-second cycle.

Configuration Steps

1. Select the I/O field on your screen.
2. Open the Properties pane and navigate to Appearance → Text.
3. Click the dynamization icon (lightning bolt) next to the Text property.
4. Select Script as the dynamization type.
5. Set the trigger to Cyclic with an interval of T1s (1 second).
6. Paste the function below into the script editor.

function DisplayUserName() {
    let userName = Tags("@UserName").Read();
    if (userName.length === 0) {
        return "No User";
    }
    return userName;
}

The function returns the placeholder when the read value has zero length and returns the username otherwise. Because @UserName is updated automatically on login and logout, a 1-second cycle is sufficient for human-perceptible display updates.

Why the Original .length Call Failed

Two common mistakes cause the original approach to fail:

1. Calling .length outside a function. The WinCC Unified dynamization evaluator expects a function with an explicit return statement. A bare expression like Tags("@UserName").Read().length returns a number, but the dynamization framework looks for a string result when bound to the Text property.
2. Missing return statement. Without a return value, the dynamization evaluates to undefined, and the I/O field displays the last cached value or an empty string.

Wrapping the read in a named function and returning a resolved string fixes both issues.

Setting a Status Bit

To drive a Boolean tag that mirrors the login state, extend the script to write to an internal HMI tag or a tag mapped to a PLC address. The same script can handle both the display text and the status bit, ensuring they stay synchronized.

Internal HMI Tag Setup

1. In the project tree, open HMI Tags.
2. Create a new tag named UserLoggedIn.
3. Set the data type to Bool.
4. Set the connection to <Internal tag> if the bit stays on the HMI, or to a PLC connection with a target address if the PLC needs to read it.

Combined Script for Display and Status Bit

function UpdateUserState() {
    let userName = Tags("@UserName").Read();
    let statusTag = Tags("UserLoggedIn");
    if (userName.length === 0) {
        statusTag.Write(0);
        return "No User";
    }
    statusTag.Write(1);
    return userName;
}

Wire this single script to both:

• The Text dynamization of the I/O field.
• The Value dynamization of the UserLoggedIn tag (via a tag trigger or a separate script dynamization on a hidden element).

Reading the Bit in the PLC

If UserLoggedIn is mapped to a PLC address, the controller reads the bit through standard input instructions. Typical address formats for S7 controllers:

In the PLC program, evaluate the bit with a normal contact, for example:

// SCL example for S7-1500
IF "HMI_UserLoggedIn" THEN
    // any user is logged on
END_IF;

Event-Driven Approach via System Alarms

WinCC Unified raises system alarms on user login, logout, and authentication failure. Configuring an event on these alarms runs the script immediately, eliminating the 1-second polling delay and reducing CPU load.

Identifying the System Alarm Numbers

Open HMI Alarms → System Alarms and filter by category System with a substring search for "User". Typical alarm numbers in V17 fall in these ranges:

Configuring the Alarm Event

1. Open HMI Alarms → System Alarms and locate the "User logged out" alarm.
2. Select the alarm and open its Events tab in the Properties pane.
3. Add a Coming event (raised when the alarm becomes active).
4. Configure the event to call the UpdateUserState() function defined above.
5. Optionally, repeat the configuration on the "User logged in" alarm with a Going event or its own Coming event so the bit transitions are immediate in both directions.

With the event-driven approach, the display updates within one scan of the alarm event, typically under 100 ms, and no cyclic script is required.

Verification Procedure

After compiling and downloading the project to the HMI device, validate the implementation with the following checks:

1. Initial state: With no user logged in, the I/O field should display "No User" and the UserLoggedIn tag should equal 0.
2. Successful login: Log in as a test user via the user view. Within 1 second (cyclic) or immediately (event-driven), the field should update to the username and the status bit should transition to 1.
3. Logout: Log out from the user view. The field should return to "No User" and the bit should transition to 0.
4. Failed login: Attempt login with an incorrect password. The state should remain "No User" and 0, because no successful authentication occurred.
5. PLC visibility: If the status bit is mapped to a PLC address, use TIA Portal online watch or a trace to confirm the bit transitions match the HMI events with the expected timing.
6. Multi-screen consistency: If the username appears on multiple screens, verify all instances update simultaneously and that no screen retains a stale cached value.

Troubleshooting Matrix

Performance and Security Notes

The 1-second cyclic script adds negligible load on Comfort Panels, typically under 0.5% CPU on an MTP1500. For multi-screen projects where the username appears on several screens, consider placing the script on a global screen template or in a scheduled task to avoid redundant triggers per screen. The event-driven approach scales better on large projects because the script runs only when the login state actually changes.

Security-wise, displaying the username in clear text on the operator screen is acceptable for standard HMI applications but should not be used as an authentication mechanism. Use the WinCC Unified user administration API for access control decisions, and never rely on @UserName alone for security-critical logic in the PLC. The status bit derived from this method is informational; enforce access control at the user administration level, not by reading the displayed username from the panel.

FAQ