S7-1200 and TP1200 Comfort Data Logging for Continuous RTD Trends

Overview

Continuous data logging on a Siemens SIMATIC S7-1200 PLC combined with a TP1200 Comfort HMI is a routine requirement for test stands, environmental chambers, and process validation runs where 10 or more RTD inputs must be captured without gaps. The engineering question is not whether the system can log, but where to log: inside the PLC's load memory and loadable file system, inside the HMI's archive store on SD/USB/network, or in parallel on both with a synchronization strategy.

This reference covers both options end-to-end: the DataLog instruction set in the S7-1200 firmware (DataLogCreate, DataLogOpen, DataLogWrite, DataLogClose, DataLogNewFile, DataLogDelete), and the WinCC Comfort/Advanced data log and tag logging configuration on the TP1200 Comfort Panel. File formats (CSV ANSI, RDB, TXT), storage media limits, web-server-based retrieval, and verification procedures are documented with concrete parameters.

Prerequisites

Before configuring continuous logging, confirm the following hardware and software baseline.

System Topology and Data Flow

The following diagram shows the physical and logical path for logged data. The PLC periodically samples RTDs via SM 1231 modules, then writes rows either to its own internal CSV files (DataLog) or to the HMI's archive over Ethernet. The HMI can additionally hold its own logs independent of the PLC.

PLC Data Logging Architecture (S7-1200)

The S7-1200 CPU maintains a file system on its load memory (internal flash, optionally SIMATIC Memory Card). Data logging is implemented with the following IEC instructions, all from the "Data log" group in STEP 7:

Files are stored under /DataLogs/ in the CPU's user file system. Maximum concurrent open logs is limited by RAM; on CPU 1214C this is typically 8 open logs at any time, with each file bounded by available load memory (internal flash size: 1 MB on CPU 1211C, 2 MB on 1214C, 4 MB on 1215C).

Configuring DataLog on S7-1200 — Step-by-Step

1. Create a global data block. Add DB "RTD_Logger" with the following structure:
   DataLogName : STRING[20]; → e.g. 'RTD_2025_01_Test42'
   LogID : DWORD; → handle returned by DataLogCreate
   LogStatus : INT; → 0 = closed, 1 = open, negative = error
   Columns : ARRAY[1..10] OF REAL; → RTD scaled values
2. Define a data block for one logging row. DB "RTD_LogRow" with one timestamp REAL (or DTL) plus 10 REAL columns matching physical RTDs.
3. Initialize the log in OB100 (startup). Call DataLogCreate with RECORD = DB RTD_LogRow, NAME = 'RTD_', then DataLogOpen to obtain the handle into LogID.
4. Trigger sampling. In a cyclic OB (OB1) or a time-of-day interrupt OB (OB10, configured to 1 s), copy the SM 1231 RTD process image words into DB RTD_LogRow.Columns[1..10], then call DataLogWrite with ID = LogID.
5. Close on test end. Triggered by an HMI tag or end-of-test boolean: call DataLogClose, then optionally DataLogNewFile to sequence the next test.
6. Activate the Web Server (CPU properties → Web server → Activate; add user "anonymous" with read-only data log permission) to retrieve files via browser without TIA Portal.

ST Code Examples

The following SCL (Structured Control Language) snippet implements the create / write / close sequence for a 10-RTD test. Drop this into a function block FB_RTD_Logger and call it from OB1.

// FB_RTD_Logger — DB instance background
// Input:  iStart (BOOL), iStop (BOOL), iSample (BOOL, 1 Hz from OB10)
// Output: qStatus (INT), qFileSize (DWORD), qFileName (STRING)
// InOut:  ioRTD : ARRAY[1..10] OF REAL

IF iStart AND (qStatus = 0) THEN
    "RTD_Logger".DataLogName := 'RTD_';
    DataLogCreate(
        RECORD  :=  "RTD_LogRow",
        NAME    :=  "RTD_Logger".DataLogName,
        ID      :=  "RTD_Logger".LogID,
        HEADER  :=  't;RTD1;RTD2;RTD3;RTD4;RTD5;RTD6;RTD7;RTD8;RTD9;RTD10',
        FORMAT  :=  0,                 // 0 = CSV ANSI comma
        TIMECOL :=  TRUE               // auto-insert timestamp column
    );
    IF "RTD_Logger".LogID <> 0 THEN
        qStatus := 1;
    END_IF;
END_IF;

IF iSample AND (qStatus = 1) THEN
    // copy live RTDs into row buffer
    FOR i := 1 TO 10 DO
        "RTD_LogRow".Column[i] := ioRTD[i];
    END_FOR;
    DataLogWrite(
        ID     := "RTD_Logger".LogID,
        RECORD := "RTD_LogRow"
    );
END_IF;

IF iStop AND (qStatus = 1) THEN
    DataLogClose(ID := "RTD_Logger".LogID);
    qStatus := 2;                       // 2 = closed & archived
END_IF;

HMI Data Logging on TP1200 Comfort

The TP1200 Comfort Panel hosts its own log engine, fully independent of any PLC DataLog. Configuration is done in TIA Portal under HMIs → TP1200 → Historical data → Data logs. The Comfort family supports the following capacities:

To configure a 10-RTD data log on the TP1200:

1. In the project tree, expand the TP1200 device → Historical data → add a new Data log named "RTD_Test_Log".
2. Add 10 log tags, each pointing to a PLC DB word address (e.g. DB_RTD_Buffer.Column[1..10]) via the configured HMI connection.
3. Set Acquisition cycle = 1000 ms (1 s). For sub-second acquisition, enable Trigger = tag change for a heartbeat bit on the PLC; this allows acquisition down to 100 ms without flooding the connection.
4. Define Storage location = \Storage Card SD\Logs\ and select CSV (ANSI) format.
5. Enable Segmentation and set file size limit to 4 MB or daily rollover — this prevents single-file bloat on long-duration tests.
6. Add an f(t) trend view on the same screen, bind all 10 tags, and enable "Save trend data" so the on-screen trend corresponds to the historical log.

PLC vs HMI Data Logging — Comparison

Recommendation: For a 10-RTD validation test where the operator wants both live trending on the TP1200 and recoverable historical CSV files that survive HMI replacement or SD card failure, configure both paths. Use the HMI log for operator visualization (f(t) trend) and the PLC DataLog for archival retrieval via web server. The PLC DataLog becomes the authoritative record.

File Formats and Storage Paths

Storage paths:

• PLC DataLog: internal flash, accessible via https://<CPU_IP>/DataLogs/<filename>.csv when Web Server is activated and "Allow data log access" is enabled.
• TP1200 Comfort: \Storage Card SD\Logs\<logname>\ on the SD card, or \USB\Storage USB\Logs\<logname>\, or \<SMB_share>\Logs\<logname>\ for network archive.

Memory and Performance Sizing

For 10 RTDs at 1 s acquisition: each row consumes roughly 50 bytes (timestamp + 10 × REAL + separators). Calculate duration before memory exhaustion:

Duration (hours) = Memory (bytes) / [50 × 3600 / SamplePeriod(s)]

For tests longer than the table indicates, implement DataLogNewFile to roll over, then move files off via web server to a PC. Alternatively, use the TP1200 Comfort with a 32 GB SD card, where 32 GB / (50 B × 3600 / 1) = ~178 h of continuous 1 s logging is feasible.

Retrieving Data — Web Server (PLC) and Export (HMI)

From the PLC (DataLog CSV)

1. Open a browser at http://<CPU_IP>.
2. Log in if a user/password is configured.
3. Navigate to Data Logs → click the file → browser download dialog → save .csv.

For scripted retrieval, use a Python or PowerShell snippet with HTTP basic auth; the CPU responds with standard text/csv MIME. The download URL is http://<IP>/DataLogs/<filename>.csv.

From the TP1200 Comfort (HMI log)

1. Insert USB stick in TP1200.
2. Open the panel's Control Panel → Backup/Restore → Export logs.
3. Select log, choose format, copy to USB.

Alternative: configure the log's storage path as \\NAS\share\ via SMB, so each rollover writes a new file directly to the network share. The TP1200 supports SMBv1/v2 with username/password; v3 requires TIA V17+ panel image.

Verification Procedures

1. Confirm DataLog creation: in TIA Portal online view, watch "RTD_Logger".LogID after iStart; expect non-zero DWORD.
2. Confirm row writes: read qFileSize; expect monotonic growth matching sample period.
3. Pull the file via web server within 30 s of writing 10 rows; open in Excel; confirm 1 header line + 10 data lines and timestamp monotonically increasing.
4. Confirm TP1200 trend shows the same numeric values as the PLC DB within ±0.1 °C.
5. Stress test: run for 1 h at production sample rate; verify no DataLog error code returned (codes 0001–001A in qStatus; see matrix below).

Troubleshooting Matrix

Field-Proven Caveats

• Power-loss resilience: PLC DataLog files are committed only on DataLogClose or every 100 rows internally buffered; mid-test power loss may lose up to ~100 rows. The HMI RDB format is more transactional but still not ACID-grade — for regulatory 21 CFR Part 11 compliance, add a UPS and a verification hash on close.
• Clock drift: Use the PLC's DTL (DATE_AND_TIME) for row timestamps, not the HMI's internal clock, to maintain a single time authority across PLC and HMI logs.
• RTD scaling: SM 1231 RTD returns values already in °C when configured for Pt100/Pt1000. For raw resistance logging (Ω), scale by 0.1 in the AI config — useful when the test is sensor-characterization rather than temperature validation.
• Segmentation naming: TP1200 segmented logs append a six-digit sequence (_000001). Include the test ID in the log name to avoid confusion across re-runs.