Configuring Weekly CSV Data Logging on SIMATIC HMI Panels

Problem Statement: Weekly INT-to-CSV Logging on SIMATIC HMI

Logging a small set of INT counters from an S7-1200/1500 Data Block (DB) to a CSV file once per week is a common shop-floor requirement. Operators want a scheduled snapshot of standstill counters, hours-run tags, or batch quantities that survives a runtime restart, can be opened in Excel, and lives on a USB stick that is swapped once a week. SIMATIC Comfort Panels and Unified Comfort Panels support this scenario natively, but three recurring defects make the configuration look broken on first attempt:

1. Multiple lines per value - one LogTag call appears to write ten rows.
2. Logs stop after USB removal - removing the storage medium pauses the data log silently.
3. Logs only appear after a runtime restart - the file is not refreshed during running operation.

All three defects share a single root cause: the data log is not configured for On demand acquisition with a single, edge-triggered call to the LogTag system function. This article walks through the correct configuration on a TIA Portal project, the PLC scheduler that fires the trigger once per week, the path conventions for the USB stick, and a verification procedure that proves the snapshot is one row, written on Monday, and retrievable without restarting the panel.

Prerequisites and Compatible Engineering Environment

Before opening the project, confirm the HMI panel is on the same PROFINET subnet as the PLC and that the HMI connection in the TIA project is in Online state. A panel that cannot see the PLC will compile the data log without error but will not write a single row.

Data Log Architecture in TIA Portal

SIMATIC HMI distinguishes three log types. Each has its own storage path, retention rule, and export format.

For weekly INT snapshots, only the Data log is used. The two relevant configuration blocks are:

• Log properties - name, path, max entries, file format, file name pattern.
• Per-tag acquisition mode - Cyclic continuous, On change, or On demand.

For a weekly counter snapshot, the correct mode is On demand. Selecting Cyclic continuous with a 1-week cycle writes only one row per week but cannot be triggered manually; selecting On change writes a row every time the counter increments, which is what produces the "ten rows per snapshot" symptom in the field.

Configuring the Data Log and Logged Tags

Open the HMI device in the TIA Portal project tree, expand Historical data, and add a new data log. The following parameters are mandatory for the weekly snapshot to work correctly.

1. Name: WeeklyCounters - the name is referenced by the LogTag call and must be unique within the HMI.
2. Storage location: select the USB stick. In TIA Portal V17+ the symbolic path is \USB\Logs\; the runtime maps this to /media/usb/Logs/ on a Comfort Panel and to /home/industrial/Logs/ on a Unified Panel.
3. Entries per log: set to a value greater than 52 (one year of weekly snapshots) plus 10% headroom. For a TP700 with 8 MB of internal flash, 200 is safe; for a TP2200 with SD card, 2000 is acceptable.
4. File format: CSV for direct Excel import.
5. File name: WeeklyCounters_<DateTime>.csv - the runtime substitutes the creation timestamp automatically.

After creating the data log, add the INT tags from the PLC DB. The two critical settings per tag are:

Triggering Logs with the LogTag System Function

LogTag is a WinCC system function in the Logs group. In a Comfort Panel project it appears in the event editor under System functions > Logs > LogTag. The signature takes two string parameters: the tag name and the data log name.

The recommended configuration is to attach the LogTag call to a value-change event on the trigger tag. This makes the event fire exactly once per rising edge of the trigger bit and avoids duplicate writes.

1. In the HMI project tree, open Screens > <Start screen> (or any always-on background screen).
2. From the toolbox, drop a rectangle or invisible button anywhere on the screen - it acts only as an event carrier.
3. In the Properties pane, switch to the Events tab.
4. Add an event of type Tag > Value change. Bind it to HMI_Trigger.
5. From the function list, add System functions > Logs > LogTag.
6. Configure the function with two parameters: HMI_Counter1 and WeeklyCounters. Repeat for HMI_Counter2 and HMI_Counter3 so the single edge writes all three counters in one row.

Calling LogTag three times in the same event is correct: the runtime writes one row containing all three tag values, not three rows. The function is synchronous; a subsequent LogTag call within the same event uses the snapshot from the same scan cycle.

VBScript Equivalent (Optional)

Unified Comfort Panels and WinCC Professional projects can call the same function from VBScript for more complex orchestration:

' On a value-change event of HMI_Trigger
Sub OnChange(ByVal Item)
    If SmartTags("HMI_Trigger") = True Then
        HMIRuntime.LogTag "HMI_Counter1", "WeeklyCounters"
        HMIRuntime.LogTag "HMI_Counter2", "WeeklyCounters"
        HMIRuntime.LogTag "HMI_Counter3", "WeeklyCounters"
    End If
End Sub

PLC-Side Weekly Scheduler in SCL

The trigger must be generated by the PLC, not the HMI, because the HMI runtime clock can drift and is not synchronized with the operator's shift schedule. S7-1500 firmware V2.0 and later expose RD_SYS_T, which returns a DTL (Date and Time Long) structure with weekday information.

The following SCL block produces a one-second positive edge on HMI_Trigger every Monday between 06:00:00 and 06:00:59, and latches the trigger to prevent multiple writes within the same minute.

// FB_WeeklyLogTrigger
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
VAR_INPUT
    i_Enable   : BOOL;   // Master enable from operator screen
END_VAR
VAR_OUTPUT
    o_Trigger  : BOOL;   // Connected to HMI_Trigger tag
END_VAR
VAR
    s_Time       : DTL;
    s_Latch      : BOOL;
    s_PrevSec    : INT;
END_VAR
BEGIN
    s_Time := RD_SYS_T();

    // Monday = 2 in DTL (Sun=1, Mon=2 ... Sat=7)
    // Window: 06:00:00 to 06:00:59 on Monday only
    IF i_Enable
       AND s_Time.WEEKDAY = 2
       AND s_Time.HOUR = 6
       AND s_Time.MINUTE = 0
       AND s_Time.SECOND < 2
       AND NOT s_Latch
    THEN
        o_Trigger := TRUE;
        s_Latch   := TRUE;
    END_IF;

    // Reset latch and trigger at 06:01:00
    IF s_Time.HOUR = 6
       AND s_Time.MINUTE = 1
    THEN
        s_Latch    := FALSE;
        o_Trigger := FALSE;
    END_IF;

    // Safety: force reset if trigger has been high more than 5 s
    IF o_Trigger AND s_Time.SECOND - s_PrevSec > 5 THEN
        o_Trigger := FALSE;
    END_IF;
    s_PrevSec := s_Time.SECOND;
END_FUNCTION_BLOCK

Compile the block, download it to the S7-1500, and connect o_Trigger to "DB20".HMI_Trigger. Set the master enable i_Enable from a button on the HMI so operators can pause the weekly snapshot during maintenance.

Alternative: HMI Scheduler

If the PLC does not need to be involved, the same trigger can be generated entirely in the HMI using a scheduled task. In TIA Portal open Schedules > Add new and configure a weekly event with the following properties:

Add a second scheduled task at 06:01:00 that sets the same tag to FALSE. This recreates the edge without PLC code, but loses the protection of a master enable tied to the operator screen.

Storage Paths, USB Behavior, and File Retrieval Methods

The HMI runtime differentiates between three storage media. The data log path in the project must match the medium that is physically present at runtime, otherwise the log is created on a different partition than expected.

File Retrieval Options

1. MiniWeb (HTTP file browser). Enable the web server on the HMI (Project tree > HMI > Web server > Activate). On a Comfort Panel, navigate to http://<panel-ip>/Files/; the data log appears in the folder tree. On a Unified Panel, the same path is https://<panel-ip>/ with a login prompt if HTTPS is enabled.
2. FTP/SFTP. Comfort Panels do not include an FTP server. Use the web server or remove the storage medium directly.
3. Direct removal. For shift-based workflows, the operator removes the USB stick after the Monday snapshot, copies the CSV, and reinserts the stick. The runtime reopens the data log automatically within 5 to 10 seconds.
4. Recipe synchronization. For automated retrieval, configure a recipe on the HMI that exports the data log file to a network share via the ExportDataRecords system function (WinCC Professional only).

Defect Matrix and Field-Proven Diagnostics

Enabling the WinCC Runtime Trace

For deeper diagnostics, enable the system trace on the panel. On a Comfort Panel, connect via ProSave and open Logs > System. Filter for the keyword LogTag to see every call, including the tag name and data log name. A failed call produces an entry of the form HmiRtLog - LogTag failed: log not started or LogTag failed: tag not found. The corresponding error codes are listed in the TIA Portal Information System under Diagnostics > System events > Logs.

Commissioning and Verification Procedure

1. Compile the HMI project, download to the panel, and start runtime.
2. Insert the USB stick and wait until the runtime logs a Storage medium connected system event. On a Comfort Panel this is visible in the diagnostics view; on a Unified Panel it appears in the notification area.
3. Open an online watch table on the PLC and force HMI_Trigger to TRUE for 2 seconds, then back to FALSE.
4. Within 3 seconds, navigate to \USB\Logs\ via MiniWeb and download WeeklyCounters_*.csv.
5. Open the CSV in Excel. The expected content is one header row and one data row, with the three INT counters aligned to columns 1, 2, and 3.
6. Confirm the runtime time and the timestamp in the CSV match within 1 second. A mismatch of hours indicates a UTC vs local-time misalignment on the panel - correct via Control Panel > Date/Time.
7. Set the PLC clock forward to the next Monday 05:59:55 and observe the trigger pulse. Verify the new row appears in the CSV without restarting the panel.
8. Remove the USB stick for 30 seconds, reinsert, and confirm the data log resumes within 10 seconds by inspecting the next Log resumed system event.

Performance Considerations

Each LogTag call writes a fixed header plus the tag values to the CSV. On a TP700 with a USB 2.0 stick, the typical write latency is 80 to 150 ms per call. A snapshot of three INT tags therefore completes in under 500 ms, which is below the 1-second window used in the SCL scheduler. Avoid calling LogTag for more than 20 tags in a single event; if more are required, batch the calls across two events 1 second apart to keep the runtime responsive.

Field-Engineering Notes

On a TP900 Comfort Panel paired with an S7-1500 CPU 1511-1 PN, the configuration described above writes a 200-byte CSV per week. With 52 weekly entries per year plus header, the file size remains under 20 KB. The default 2,000-entry limit per data log supports nearly 38 years of weekly snapshots on a single file, but most plants rotate the USB stick quarterly and archive the CSVs to a network share.

For multi-shift plants where the operator needs to read the counters on a different day, expose a manual trigger button on the HMI that calls LogTag directly. The button event should not be tied to a tag change, and the button should be access-protected through the user administration. This provides a fallback path when the PLC clock is wrong or the scheduler is disabled during commissioning.

For plants in regulated industries (pharma, food, water), the Audit Trail is the correct log type, not the Data log. Audit Trail entries are append-only, signed, and cannot be deleted from the runtime. The trigger mechanism is the same LogTag function, but the destination path is fixed and the format is GMP-compliant CSV with operator ID and reason code.

References to Official Documentation

Authoritative sources for the procedures in this article:

• Siemens Industry Online Support - main portal for firmware updates, manuals, and FAQ entries on WinCC data logging.
• SIMATIC HMI systems product page for panel specifications and supported firmware versions.
• TIA Portal Information System, accessible from the Help menu of TIA Portal, for the canonical reference of the LogTag system function, the data log editor, and the scheduled tasks editor. Always verify the parameters against the installed TIA Portal version's help, because the function signature has evolved between V15 and V19.

Frequently Asked Questions