WinCC Tag Logging Hysteresis: Configure Dual Archive Tags

Problem Statement: Combining Hysteresis and Time-Based Archiving in WinCC

In a PCS7 power plant application, the control room operator requires that an analog process value (for example, a slowly drifting temperature or a noisy flow signal) be archived under two distinct conditions simultaneously:

1. Change-driven archiving – record the value into the Tag Logging archive database only when it deviates from the last archived value by more than a defined percentage, e.g. ±2 %.
2. Time-driven archiving – record a sample at a fixed interval (e.g. once every minute, or once every two minutes) regardless of whether the change threshold has been crossed.

The intent is to suppress the storage of redundant samples on a high-noise signal while still guaranteeing a minimum chronological trace for diagnostics, trend reconstruction, and regulatory evidence logging. The same requirement exists in legacy Teleperm XP installations, where the archiving subsystem natively supports both an absolute change threshold and a guaranteed logging interval on the same point.

The constraint in WinCC Tag Logging is structural: each archive tag accepts exactly one archiving method – either a hysteresis-controlled event, a fixed acquisition cycle, or a non-cyclic on-demand event. There is no single archive tag that natively supports both "archive on change with hysteresis" and "archive at fixed interval" at the same time.

The standard, field-proven workaround is to define two archive tags that point to the same process tag and configure each archive tag with a different archiving strategy. The two resulting archive rows are then interpreted by the trend, alarm, or external historian consumer as a single logical point.

WinCC Tag Logging Model: Acquisition, Archiving, and Events

Before configuring the dual-tag solution it is necessary to understand the three orthogonal parameters that WinCC Tag Logging exposes for every archive tag. The terminology differs between WinCC V7 (classic) and WinCC Professional (TIA Portal), but the underlying model is identical.

How the Hysteresis Mechanism Works

When the archiving method is set to Event-driven with hysteresis, the WinCC runtime compares every newly acquired value (sampled at the acquisition cycle) with the last value that was actually written to the archive. The comparison rule is:

archive_now  =  ABS( current_value - last_archived_value )  >=  hysteresis_value

If the inequality is true, the current value is appended to the archive; if false, the value is discarded. The hysteresis value can be expressed as:

• Absolute – a fixed engineering-unit span (e.g. 1.5 °C, 0.2 bar).
• Relative – a percentage of the last archived value or the configured measurement range, e.g. ±2 %.

The hysteresis check is evaluated at the acquisition cycle rate, not at the archiving cycle rate, so the fastest possible detection latency is one acquisition cycle. If the acquisition cycle is 500 ms and the hysteresis is ±2 %, a 2 % step in the process is detected within 500 ms; a 1.9 % drift is not archived at all until a subsequent cycle pushes the deviation past the threshold.

The Single-Archive-Tag Constraint

For every archive tag, the WinCC Tag Logging editor allows exactly one selection under Archiving → Method:

• Cyclic, with a configurable archiving cycle (1 s, 2 s, 5 s, 10 s, 1 min, … user-defined).
• Event-driven, with a Boolean trigger tag or a script.
• Event-driven with hysteresis (the subject of this article).
• On change (without explicit hysteresis – effectively zero hysteresis).

The two archive tags coexist in the same Tag Logging database. They share the same source process tag, but they produce independent time-stamped records. From a runtime CPU and storage standpoint the overhead is approximately 2× the storage of a single tag and a negligible CPU cost (one extra comparison per acquisition cycle), because the underlying PLC connection is reused.

Dual-Archive-Tag Solution: Step-by-Step Configuration

Prerequisites

• PCS7 V9.x or higher, or any WinCC V7.x / WinCC Professional installation that uses Tag Logging as the process-value archive.
• A WinCC process tag that is already configured in the Tag Management, e.g. Plant_H1_FT_0231_PV, with a known acquisition cycle (typical: 500 ms, 1 s, 2 s).
• Editor access to the Tag Logging configuration (WinCC Explorer → Tag Logging, or TIA Portal → HMI tags → Archive tags).
• If the data will be consumed externally, the WinCC Archive Connector must be installed and licensed as part of the WinCC DataMonitor option.

Step 1 – Create the First Archive Tag (Change-Driven, ±2 % Hysteresis)

1. Open Tag Logging in the WinCC Explorer (or the equivalent editor under TIA Portal).
2. Right-click the destination archive (typically PROCESS_VALUE_ARCHIVE) and select Add Archive Tag.
3. Assign a descriptive name, e.g. PV_FT_0231_Hyst, and bind it to the WinCC process tag Plant_H1_FT_0231_PV via the Tag drop-down.
4. Set the Acquisition cycle to match the underlying process tag's cycle (e.g. 1 s).
5. Under Archiving → Method, select Event-driven with hysteresis.
6. Set the Hysteresis field to 2 and the unit to % (relative to the last archived value). For absolute mode, set the value to the engineering-unit span, e.g. 0.4 bar.
7. Confirm the entry and recompile / activate the runtime.

Step 2 – Create the Second Archive Tag (Time-Driven, 1-Minute Cycle)

1. In the same archive, add a second archive tag, e.g. PV_FT_0231_Time, and bind it to the same process tag Plant_H1_FT_0231_PV.
2. Set the Acquisition cycle to the same value as in Step 1 (it is mandatory that both archive tags read identical raw values).
3. Under Archiving → Method, select Cyclic and set the Archiving cycle to 1 minute (or 2 minutes if the operator requirement is the longer interval mentioned in the field report).
4. Confirm the entry. Do not enable hysteresis on this tag – the cyclic method already guarantees the time-based sample.

Step 3 – Verify Tag Logging Compilation

1. Open the Tag Logging diagnostics view in the WinCC Explorer.
2. Right-click the archive and select Check Compilation. Both archive tags must compile without warnings.
3. Activate the runtime. The Tag Logging runtime log (TagLogRT.txt) will show one line per archive tag, e.g. Archive tag PV_FT_0231_Hyst registered and Archive tag PV_FT_0231_Time registered.

Step 4 – Confirm the Two Streams at Runtime

Open the Tag Logging – Online Trend viewer and add both archive tags to a single trend window. The expected behaviour is:

• The _Hyst trace shows step-like samples, each separated from the previous by at least ±2 % of the last archived value.
• The _Time trace shows one sample per minute on a regular grid, even when the _Hyst trace is quiet.
• When the process actually changes by more than 2 %, the _Hyst trace writes an extra record while the _Time trace continues to write on its 1-minute grid – the two records will be within a few hundred milliseconds of each other at the same time.

PCS7-Specific Configuration Notes

Within a PCS7 project, archive tags are typically generated automatically by the CFC / SFC compile step using the AS process tag list. The PCS7 Add-on OS Project Editor creates the WinCC archive structure based on the CFC chart's signal category. For the dual-tag approach to work cleanly inside PCS7:

• Configure the signal in the CFC as a standard analog measurement (e.g. MEAS_MON or CTRL block output).
• After the OS compile, manually add the second archive tag in the WinCC Explorer. PCS7's auto-generation only creates one archive tag per process tag, so the time-driven companion must be added by the engineer.
• Mark both archive tags as belonging to the same archive segment in the Tag Logging editor to ensure consistent partitioning when the archive is rotated to a long-term storage (e.g. the standard PCS7 1-year process-value archive).
• For redundant OS pairs (OS Server A / OS Server B in a PCS7 plant bus), the Archive Connector on the standby server will mirror the active server's archive, including both records.

Exposing the Combined Archive via the WinCC Archive Connector

The WinCC Archive Connector is part of the WinCC DataMonitor option and is the recommended method for granting external applications read access to the Tag Logging database through the standard WinCC OLE DB provider. The connector abstracts the underlying database (Microsoft SQL Server, in the case of PCS7 V9 and WinCC V7.5+) and presents a stable schema to consumers.

A typical consumer query written against the OLE DB provider to merge the two streams looks like the following SQL pattern (executed by an external historian or Excel pivot on the archive database):

SELECT [Timestamp], [Value], 'Hyst' AS Source
FROM dbo.PV_FT_0231_Hyst
WHERE [Timestamp] BETWEEN @Start AND @End
UNION ALL
SELECT [Timestamp], [Value], 'Time' AS Source
FROM dbo.PV_FT_0231_Time
WHERE [Timestamp] BETWEEN @Start AND @End
ORDER BY [Timestamp];

Downstream consumers that require a single value stream (e.g. an OPC UA Historical Access server) can apply a deduplication window of one second: the latest record in the window wins. This eliminates the visual duplication of the two streams in trend views and reports.

Comparison with Teleperm XP Archiving

For migration projects from Teleperm XP to PCS7 / WinCC, the engineering effort of the dual-tag workaround is small (approximately 30 minutes of work per affected point) and is a one-off cost that does not affect the runtime behaviour visible to operators.

Verification, Commissioning, and Operational Diagnostics

Functional Verification Checklist

1. Compile – both archive tags must compile without warnings.
2. Runtime registration – TagLogRT.txt must contain one registration line per archive tag.
3. Step response – apply a simulated step of ±3 % on the process tag; the _Hyst archive must contain exactly one record at the time of the step, and the _Time archive must contain one record within the next 1-minute boundary.
4. Steady state – leave the process constant for 10 minutes; the _Time archive must contain 10 records, the _Hyst archive must remain empty.
5. Sub-threshold noise – apply a noise signal with peak-to-peak amplitude of 1.5 %; the _Hyst archive must remain empty, the _Time archive must contain one record per minute.

Diagnostic Tools and Files

• TagLogRT.txt – runtime log of the Tag Logging subsystem. Located in the WinCC project directory under <ProjectName>/<ComputerName>/Logs/.
• CCAlgRt.log – alarm logging runtime log; useful if the archive tag is also exposed as an alarm source.
• WinCC Tag Logging – Online Trend – native viewer for verifying the two streams in the WinCC Explorer.
• DataMonitor Web – browser-based view of the same archive, useful for remote commissioning from the engineering station.
• SQL Server Management Studio – direct query of the underlying Tag Logging tables when the Archive Connector is not available.

Common Field Issues and Resolutions

Parameter Reference Summary

Best Practices for Production Deployments

1. Name both archive tags with a common prefix (e.g. PV_FT_0231_Hyst and PV_FT_0231_Time) so the Archive Connector filter pattern and SQL queries are simple wildcards.
2. Apply identical acquisition cycles to both archive tags to ensure that the time and the change events see the same source values.
3. Document the deduplication rule in the engineering HMI documentation so future modifications know that the two streams are intentional.
4. Re-evaluate the hysteresis value after one month of operation. The source value of ±2 % is a rule of thumb – adjust to the actual process noise level so that 95 % of the values are within the dead band and the archive does not become a noise record.
5. For regulatory logs, keep the time-driven archive at 1 minute and never lower the hysteresis below the meter's repeatability specification.
6. For performance logs, a 2-minute time-driven archive with a ±1 % hysteresis is usually sufficient and halves the storage cost.