WinCC Flexible 2008: Start DataLog from BOOL Tag in VBScript

Overview

In WinCC Flexible 2008 SP3 (and throughout the WinCC Flexible family, including WinCC Comfort/Advanced in TIA Portal), a DataLog is the object that records multiple process tags - temperature, current, pressure, etc. - to a file at scheduled intervals or on demand. The common field requirement is: start the log when a BOOL goes to 1, stop the log when it returns to 0. Operators press a Start pushbutton, a process interlock is released, or a machine mode switches to "Auto-Run"; the HMI should record the batch from that point forward and stop cleanly when the BOOL is cleared.

The built-in LogTag function only writes a single tag/value pair to a log on demand, so it is the wrong tool when the log already references several tags. The correct surface for multi-tag DataLogs is the pair of VBScript functions StartLogging and StopLogging, driven either by a cyclic scheduler that polls the BOOL, or by the BOOL change-event configured directly in the DataLog's trigger properties.

This reference covers both methods, the scheduler pattern, the HMI-restart safety concern that trips up most first implementations, the full function reference, and a field-tested troubleshooting matrix.

Prerequisites

1. Engineering system. SIMATIC WinCC Flexible 2008 SP3 installed (any edition: Standard, Advanced, or the ES2 bundle).
2. Runtime target. A SIMATIC Panel with VBScript support, or PC Runtime. Confirm VBScript is enabled under Device settings > Runtime > Scripts.
3. External BOOL tag. A BOOL tag connected to the PLC area that the operator or process toggles to control logging (e.g. HMI_Tag_StartLog mapped to DB10.DBX0.0).
4. Internal state tag. A BOOL tag internal to the HMI used to remember the last requested state (e.g. HMI_LoggingActive).
5. Configured DataLog. A DataLog (e.g. Data_log_1) that references the process tags to record. Configure the storage path, file format (CSV or RDB), and the cyclic logging interval under Data Log properties > Storage.

Method 1 - VBScript Called from a Cyclic Scheduler

The most flexible pattern is a short VBScript that checks the BOOL tag and calls StartLogging only on the 0-to-1 transition and StopLogging only on the 1-to-0 transition. The one-shot form prevents redundant state changes that can, on some panel firmware versions, cause log-file rollover glitches and loss of the last partial record.

Step 1 - Configure the Tags

1. In the project tree, expand Tags and add an external tag HMI_Tag_StartLog of type BOOL pointing to the PLC address (e.g. DB10.DBX0.0). Acquire cycle: 1 s. Connection: the configured HMI-PLC connection.
2. Add an internal BOOL tag HMI_LoggingActive for state tracking. This tag is read and written by the script and should be marked Persistent if you intend to use the restart-safety Pattern B described below.

Step 2 - Create the Scheduler

1. In the project tree, right-click Schedulers and select Add scheduler.
2. Name it LogScheduler. Set the trigger to Cyclic with a cycle of 1 s. The minimum cycle is 100 ms on MP 377 and 250 ms on TP 177 4".
3. On the Event tab of the scheduler, add a VBScript function and paste the script below.

Step 3 - The VBScript

' One-shot DataLog control from a BOOL tag.
'   HMI_Tag_StartLog : external BOOL, 0 = stop, 1 = start
'   HMI_LoggingActive: internal BOOL, tracks the last requested state
'   Data_log_1       : the name of the configured DataLog

If SmartTags("HMI_Tag_StartLog") = 1 Then
    If SmartTags("HMI_LoggingActive") = 0 Then
        StartLogging hmiDataLog, "Data_log_1"
        SmartTags("HMI_LoggingActive") = 1
    End If
Else
    If SmartTags("HMI_LoggingActive") = 1 Then
        StopLogging hmiDataLog, "Data_log_1"
        SmartTags("HMI_LoggingActive") = 0
    End If
End If

The first argument of StartLogging/StopLogging is the provider constant: hmiDataLog for data logs, hmiAlarmLog for alarm logs, or hmiRecipeLog for recipe logs. The second argument is the configured log name and is case-sensitive.

Step 4 - Verification

1. Download the project to the panel and start Runtime.
2. Set the BOOL in the PLC to 1. Within one scheduler cycle the log file Data_log_1.csv (or .rdb) should begin to grow in the configured directory.
3. Set the BOOL back to 0. New records should stop being appended; the existing file is not deleted - StopLogging pauses the log, it does not erase it.
4. Watch HMI_LoggingActive on a diagnostics screen. It should track the requested state exactly.

Method 2 - DataLog's Built-in Tag Trigger

For a configuration-only solution with no VBScript, the DataLog object itself can be triggered by a tag change.

1. Open the DataLog Data_log_1 and select the Trigger property in the inspector.
2. Add a new trigger of type Tag.
3. Select the BOOL tag HMI_Tag_StartLog. Set the trigger condition to On value change or, more typically, On value = 1 for the start event.
4. Under Start/Stop behavior select Start log on rising edge and Stop log on falling edge.

This pattern uses the BOOL directly to start and stop the log without any script. The trade-off is that it only supports simple rising-edge / falling-edge behaviour and cannot apply preprocessing, timeouts, hysteresis, or compound conditions.

Method Comparison

The HMI Restart Problem and Three Field-Proven Patterns

The naive script - "if BOOL = 1 then start, else exit" - has a well-known failure mode. If the panel power-cycles while the BOOL is 1, Runtime comes back up with the BOOL still 1 but no log running, because the script has not yet been called. Any code that fires only on the 0-to-1 transition will miss the event entirely. Three patterns address this.

Pattern A - Startup Script Reads the Initial Value

Add a second VBScript bound to the project's Runtime started event. It reads the BOOL once and, if it is 1, starts the log immediately. The cyclic scheduler takes over once Runtime is up.

' Startup script - run once when Runtime starts
If SmartTags("HMI_Tag_StartLog") = 1 Then
    StartLogging hmiDataLog, "Data_log_1"
    SmartTags("HMI_LoggingActive") = 1
End If

Bind this to the project event under Project tree > Runtime settings > Events > Runtime started.

Pattern B - Persistent State Tag

Mark the internal tag HMI_LoggingActive as Persistent. After a power cycle Runtime restores its last value, and the cyclic scheduler sees a consistent state. In the tag's properties, enable Persistent and assign a persistence area (default area 1, up to 32 kB on most panels). This area is battery-backed on panels that support it and is written to flash on graceful shutdown.

Pattern C - Heartbeat from the PLC

Have the PLC write a 1-Hz square wave into a separate BOOL HMI_Log_Heartbeat. Treat any heartbeat as "log active" and any missed heartbeat as "stop". This is the strongest pattern because it tolerates both HMI and PLC restarts, at the cost of some PLC code.

' Cyclic script with heartbeat supervision (assumes 1 s cycle)
Dim tNow
tNow = Now

If DateDiff("s", SmartTags("HMI_Heartbeat_Time"), tNow) > 3 Then
    ' No heartbeat received in 3 s - assume link or PLC is down
    If SmartTags("HMI_LoggingActive") = 1 Then
        StopLogging hmiDataLog, "Data_log_1"
        SmartTags("HMI_LoggingActive") = 0
    End If
End If

If SmartTags("HMI_Log_Heartbeat") = 1 And SmartTags("HMI_LoggingActive") = 0 Then
    StartLogging hmiDataLog, "Data_log_1"
    SmartTags("HMI_LoggingActive") = 1
    SmartTags("HMI_Heartbeat_Time") = tNow
End If

Tune the timeout to roughly three times the heartbeat period. Two seconds is the practical minimum on a 1 s scheduler cycle.

VBScript Function Reference

Scheduler Configuration in Detail

Schedulers are project-tree objects that fire a configured event at a fixed interval or on a calendar time. They are global - one scheduler runs regardless of which screen is open.

1. Right-click Schedulers and choose Add scheduler.
2. Set Trigger to Cyclic and Cycle to 1 s. The shortest cycle is 100 ms on MP 377 and 250 ms on TP 177 4".
3. Add an event of type VBScript. Either link to a function or paste a snippet directly.
4. Scripts in schedulers are global to the project. Avoid naming collisions with screen-level scripts.

For lower CPU load, lengthen the cycle to 2 s or 5 s for batch-oriented logging, but be aware that the response time of the start/stop transition will also grow.

DataLog Storage and File Format

The default storage path and file naming are set in the DataLog's Storage property.

On PC Runtime the default is C:\ProgramData\Siemens\Automation\Logs\ unless redirected in the project.

Architecture Diagram

Code Examples

Example 1 - Two-Line Minimum

If SmartTags("HMI_Tag_StartLog") = 1 Then StartLogging hmiDataLog, "Data_log_1"
If SmartTags("HMI_Tag_StartLog") = 0 Then StopLogging  hmiDataLog, "Data_log_1"

This is the form most often quoted in support tickets. It is functionally correct but calls StartLogging on every cycle while the BOOL is 1, which is acceptable on most panels but may produce extraneous log rollover on the MP 277 with firmware < 4.15.

Example 2 - One-Shot with Internal State (Recommended)

The pattern shown in Method 1 above. Recommended for production deployments.

Example 3 - Stop After N Records

' Stop logging automatically after 1000 records
If SmartTags("HMI_LoggingActive") = 1 And SmartTags("HMI_RecordCount") >= 1000 Then
    StopLogging hmiDataLog, "Data_log_1"
    SmartTags("HMI_LoggingActive") = 0
    SmartTags("HMI_Tag_StartLog") = 0  ' optionally clear the request back to the PLC
End If

HMI_RecordCount is logged into the data log on each cycle and can be read back via SmartTags. Add it as one of the tags on the DataLog's tag list if it is not already present.

Example 4 - Mux: Different Log per BOOL Value

Select Case SmartTags("HMI_Tag_StartLog")
    Case 0
        If SmartTags("HMI_LoggingActive") <> 0 Then
            StopLogging  hmiDataLog, "Data_log_idle"
            SmartTags("HMI_LoggingActive") = 0
        End If
    Case 1
        If SmartTags("HMI_LoggingActive") <> 1 Then
            StartLogging hmiDataLog, "Data_log_run"
            SmartTags("HMI_LoggingActive") = 1
        End If
    Case 2
        If SmartTags("HMI_LoggingActive") <> 2 Then
            StartLogging hmiDataLog, "Data_log_fault"
            SmartTags("HMI_LoggingActive") = 2
        End If
End Select

Example 5 - Audit Trail with Operator ID

' Append a single event row whenever logging starts or stops
If SmartTags("HMI_LoggingActive") = 1 And SmartTags("HMI_LastLoggedState") = 0 Then
    LogTag "Operator_ID", SmartTags("HMI_Operator")
    LogTag "Event",       "LogStart"
    SmartTags("HMI_LastLoggedState") = 1
End If
If SmartTags("HMI_LoggingActive") = 0 And SmartTags("HMI_LastLoggedState") = 1 Then
    LogTag "Operator_ID", SmartTags("HMI_Operator")
    LogTag "Event",       "LogStop"
    SmartTags("HMI_LastLoggedState") = 0
End If

Verification Checklist

Troubleshooting Matrix

Performance and Limits

DataLog performance is bounded by the panel's flash or SD-card write speed. On WinCE panels with internal flash, sustained logging above 200 records per second is not recommended. On panels with a removable SD card (MP 277, MP 377), 1000 records per second is typical. PC Runtime on SSD is limited only by the SQL or CSV write path and the OS file system.

The 1 s scheduler cycle in Method 1 introduces at most 1 s of latency between the BOOL transition and the StartLogging call. If lower latency is required, switch to Method 2 (event trigger) or to a 100 ms scheduler cycle on panels that support it.

Migration to TIA Portal (WinCC Comfort/Advanced)

The same scripting model carries over to TIA Portal V13 and later (WinCC Comfort, WinCC Advanced, WinCC Professional). The VBScript surface, the hmiDataLog provider, and the scheduler model are unchanged. Code that works in WinCC Flexible 2008 SP3 generally works in TIA Portal without modification once the project is migrated. Two notes for the migration:

• In TIA Portal, schedulers are now under Runtime settings > Events > Scheduled tasks.
• The migration tool reports script errors as warnings; review each one before downloading the migrated project.

For reference, the WinCC Flexible 2008 SP3 documentation set is available in the Siemens Industry Online Support portal; the WinCC product page at Siemens SIMATIC HMI links to the current TIA Portal documentation and migration guides.