PCS7 Tag Logging vs Process Historian Sizing & Configuration

Overview of PCS7 Data Logging Architectures

SIMATIC PCS7 offers two distinct long-term data retention paths inside its WinCC runtime: the built-in Tag Logging subsystem of the OS Server, and the optional SIMATIC Process Historian (PH) with the Information Server (IS) sitting on top of it. Both subsystems use the same underlying tag model, but they differ in retention horizons, query models, redundancy, and licensing. Selecting the wrong path for a project with ~100 analog measurements and a handful of PID loops either inflates cost or silently truncates the audit trail.

For PCS7 V9.0 and later (including V9.1 and the V10 RT Professional path), the OS Server comes pre-licensed for a base of 512 archive tags. If the number of archive tags, the retention horizon, or the reporting requirements exceed what Tag Logging alone can deliver, the project is migrated to a Process Historian. Process Historian and Information Server are separate licensed products ordered via the Siemens catalog (for example 6ES7652-... PH bundles and 6ES7652-... IS bundles) and are installed as additional Windows services on dedicated servers or VMs.

Use the following quick decision rule, then verify with the sizing sections in this reference:

• Less than 512 archive tags and less than ~2 years of online retention with the full OS Server query feature set -> OS Server Tag Logging only.
• More than 512 archive tags, multi-year retention, cross-server queries, web-based reports, or regulator-driven long-term archive -> Process Historian + Information Server.

WinCC Tag Logging on the OS Server

Tag Logging is the integrated archive engine of the WinCC OS Server. It writes measured values and messages to circular segment files on local or network-attached storage. Each segment is a self-contained time window with a header block, a data block, and an index that the OS Server uses for trend and table queries.

Tag Types and Acquisition

Two acquisition types are available per archive tag:

• Acyclic – stores a value only on process change. Sufficient for slow PID loops and valve feedbacks.
• Cyclic – stores a value on a fixed time grid (typical: 1 s, 5 s, 10 s, 1 min, 5 min, 15 min, 1 h, 1 day, 1 week, 1 month, 1 year).

For analog measurements feeding control loops, a 1 s acquisition with 1 min compression is a common starting point. For 100 analog measurements, that yields:

• 1 s acquisition: 100 tags × 86,400 samples/day = 8,640,000 rows/day.
• 1 min compression: 100 tags × 1,440 samples/day = 144,000 rows/day retained long term.

Tag Logging Licenses

The PCS7 OS Server archive is licensed per archive tag, with a base bundle of 512 tags included (see Siemens entry ID 108893968 – "PCS7 OS Archive without separate Process Historian"). Beyond 512 tags, the project must move to Process Historian, since additional WinCC archive tags are not sold as a stand-alone upgrade on the OS Server. The exact licensed tag count is visible in the WinCC Explorer under Tag Logging > License.

Process Historian (PH) Architecture

SIMATIC Process Historian is a SQL Server-based archive that aggregates compressed measurements and messages from one or more OS Servers / WinCC Servers. The typical topology is:

• Data Source: PCS7 OS Server (single or redundant pair).
• Process Historian Server: Windows Server with Microsoft SQL Server (Standard or Enterprise) plus the PH service.
• Information Server: IIS-based web portal for reporting and ad-hoc queries.
• Clients: WinCC RT clients, PCS7 Maintenance Station, IS web clients, and 3rd-party OPC UA clients via the PH OPC UA interface.

Tags are configured for long-term storage in the WinCC Explorer on the OS Server and then mirrored to the PH. The PH performs its own compression, swapping, and backup. For RT Professional projects, the equivalent configuration is done in TIA Portal under RUNTIME settings > Process Historian, where the server name of the PH is entered (see Configuring runtime settings for Process Historian (RT Professional)).

System Requirements Baseline

Siemens publishes the system requirements for PH in entry ID 109748863. Excerpts relevant for sizing:

• Minimum 8 GB RAM for a single-server PH; 16 GB recommended for > 10,000 archive tags.
• Disk I/O: a dedicated SSD array for the SQL data files; data files separated from transaction log files; transaction log on a separate spindle or NVMe.
• Network: redundant 1 Gbit/s minimum, separate VLAN for the PH if data rates are high.

Information Server (IS) – the Reporting Layer

The Information Server is the reporting and web client for PH. It is always licensed separately from PH. IS provides:

• Excel add-in that queries PH directly with parameterized time ranges.
• Web-based dashboards accessible from any standard browser.
• Scheduled report jobs (PDF/XLSX) delivered by e-mail or file share.
• OPC UA server interface for downstream historians and MES systems.

For a small installation of ~100 analog tags, IS is often skipped at first release and added later when management-level reports are required.

Storage Sizing Formulas

Use the following formulas to size disks before commissioning. The source formula referenced in PCS7 help is the "tag time-period calculation" and the segment sizing formula, both of which WinCC exposes in the editor.

Single-Channel Raw Storage (bytes per day per tag)

For a cyclic acquisition on one analog channel:

bytes_per_tag_per_day = (86400 / T_acq) * (S_timestamp + S_value + S_status)

where:
  T_acq        = acquisition period in seconds
  S_timestamp  = 8 bytes (WinCC FILETIME)
  S_value      = 8 bytes (IEEE 754 double)
  S_status     = 4 bytes (WinCC quality code + flags)

For T_acq = 1 s this evaluates to 1,728,000 bytes/tag/day (~1.65 MB). For 100 tags the raw rate is ~165 MB/day, or ~60 GB/year before any compression or message archive.

Compressed Storage Estimate

After WinCC compression (typical 1 min and 15 min averages) the steady-state storage is closer to:

bytes_per_tag_per_day_compressed = (1440 / T_comp1) * 20 + (96 / T_comp2) * 20

where T_comp1 = primary compression period, T_comp2 = secondary.

For 100 analog tags compressed to 1 min and 15 min averages the typical footprint is 4–8 MB/day, i.e. 1.5–3 GB/year for the analog archive alone.

Segment Sizing (OS Server Tag Logging)

Each Tag Logging segment has a header, an index, and a data block. The maximum number of segments loaded at runtime is constrained by the PCS7 runtime, not by the OS – for PCS7 V9.0 and later, the practical limit is ~200 active segments and ~2 GB online before query performance degrades. Configure the segment period so that 200 segments cover the desired online window:

segment_period = online_window / 200

Examples:
  online_window = 30 days   -> segment_period = 3.6 hours   (use 4 h)
  online_window = 90 days   -> segment_period = 10.8 hours  (use 12 h)
  online_window = 365 days  -> segment_period = 1.83 days   (use 2 days)
  online_window = 4 years   -> segment_period = 1 week     (1 segment = 1 week)

Active segments are loaded into the OS Server memory; older segments are exported to a backup path (local, network share, or NAS). Backup segments must be manually re-imported before they can be queried, which is the architectural reason Process Historian exists.

Storage Capacity for 1 TB of Disk

Assuming 100 analog measurements on a 1 s acquisition cycle with 1 min and 15 min compression, the rough breakdown against 1 TB (1,024 GB) of usable disk is:

Subsystem	Footprint	Notes
OS Server Tag Logging, 1 year online	~10–20 GB	200 active segments, 2-day period
OS Server exported segments, 4 years	~50–80 GB	Standby, manually reloadable
Process Historian, 5 years online	~150–300 GB	SQL data + indexes
SQL transaction log (PH)	~50–150 GB	Backed up daily, can be on a separate volume
Information Server report cache	~10–50 GB	Excel/PDF cache, scheduled jobs
Headroom for messages and alarms	~100–300 GB	Message archive is much larger than value archive

Conclusion: 1 TB holds 4–8 years of PH online data for ~100 analog measurements, depending on message traffic. The same 1 TB holds ~50 years of OS Tag Logging compressed data on a stand-alone OS Server. The bottleneck on the OS Server is not disk space but the 200-segment online limit.

Licensing Comparison Table

Feature	OS Server Tag Logging	Process Historian + Information Server
Archive tag license	512 included; no expansion option on the OS	10,000+ via PH bundles (6ES7652-…)
Online retention on the server	~2 GB / 200 segments, manual reload beyond that	Multi-year, SQL-managed, no segment reload
Cross-server aggregation	No – one OS Server has its own archive	Yes – one PH aggregates multiple OS Servers
Redundancy	OS Server redundancy only	Optional PH redundancy (paired SQL Server instances)
Web reports / Excel	Limited (OS Web Navigator trends only)	Full IS Excel add-in and web dashboards
OPC UA historical access	No (read-only via OS Web)	Yes – PH exposes a historical OPC UA server
Approx. one-time cost (Siemens catalog)	Included in PCS7 OS bundle	Substantial – PH + IS + SQL CALs

Configuration Procedure – OS Server Tag Logging

1. Open the WinCC Explorer on the OS Server engineering station.
2. Right-click Tag Logging and open the editor. Add each AS tag you want to archive. Typical selection: every analog used in a faceplate plus the PV/SP/OUT of each PID block.
3. Set the acquisition type (cyclic or acyclic) and the acquisition cycle. Use 1 s for fast control, 5 s for slow measurements, 1 min for non-critical KPIs.
4. Configure compression: define at least two levels (1 min average, 15 min average). Optionally add 1 h and 1 day averages for trend displays.
5. Open Tag Logging > Timed Compressed and confirm the segment size. Use the built-in segment calculator (Help menu) to enter 100 tags, the cycle, and the desired online window. The tool returns the recommended segment period.
6. Configure the backup path under Tag Logging > Archive Backup. Point it to a NAS volume with NTFS permissions for the WinCC service account.
7. Activate the project. Verify that segments are created in the configured path and that the trend control on the OS client can read the archive.

Configuration Procedure – Process Historian (PH)

1. Install PH on a dedicated server. Prerequisite: a supported Windows Server edition, Microsoft SQL Server (Standard or Enterprise) installed first, and the PH setup launched as administrator.
2. Install Information Server on the same host (or a separate host, depending on sizing). The IS web role requires IIS.
3. On the OS Server engineering station, open the WinCC Explorer, select Tag Logging, and in the tag list tick Archive to Process Historian for each tag that must be long-term stored. Configure the same acquisition and compression as for the local archive.
4. For RT Professional projects, configure the PH server name under Runtime settings > Process Historian in TIA Portal. See the TIA Portal cloud help topic Configuring runtime settings for Process Historian (RT Professional).
5. Activate the project. The OS Server's PH connector (WinCC Archive Connector) opens a session to the PH and starts streaming compressed values.
6. Open the PH Management Console and verify that the OS Server appears under Data Sources and that the tag count matches the engineering count.
7. Schedule a daily SQL backup of the PH database. Retain at least 30 days of transaction log backups to support point-in-time restore.

Segment Management and Backup Strategy

For OS Server Tag Logging, the recommended practice is:

• One segment = one week for medium-sized projects (~100 tags).
• Keep ~200 segments online (~4 years) to satisfy typical auditor requirements.
• Use a NAS volume for backup segments. Configure scheduled copy jobs that mirror the segment directory to a second physical disk or tape.
• Reload backup segments on demand: WinCC Explorer > Tag Logging > Archive Backup > Link to a backup path. Queries against linked segments are slower than against online segments.

For Process Historian the backup is a SQL Server concern, not a WinCC concern. Use SQL Server Maintenance Plans with full backups nightly and transaction log backups every 15 min. Test a restore at least once per year – PH data that has not been restored is data that does not exist for audit purposes.

Limitations of OS Server Tag Logging

For the 100-tag scenario, the practical limitations of skipping Process Historian are:

1. License ceiling of 512 archive tags. Projects that grow past this must migrate to PH anyway, so the migration cost is paid late and tends to be more expensive.
2. No cross-server aggregation. Multiple OS Servers (e.g. unit 1 + unit 2) each have their own archive. A plant-wide trend requires manual export + join in Excel.
3. Manual reload of segments older than the online window. This is operationally acceptable if the user base understands it, but it blocks ad-hoc long-range queries.
4. No standard web reporting. OS Web Navigator exposes trends only via the runtime, not as historical reports.
5. Query latency grows with segment count. Practical response time stays acceptable up to about 2 GB online, then trend dialogs slow noticeably.

Selection Guide for 100-Analog-Tag Projects

Scenario	Recommended Path
Greenfield, < 512 tags, < 2 years online, no cross-server queries, no reports	OS Server Tag Logging only
Greenfield, < 512 tags, 2–4 years online, one OS Server	OS Server Tag Logging with weekly segments and NAS backup
Brownfield, multiple OS Servers, plant-wide KPIs, > 1 year retention	Process Historian + Information Server
Regulated industry (FDA 21 CFR 11, ISA 88 batch reporting, GxP)	Process Historian + Information Server (audit-trail and e-signature support)
Plant-wide OPC UA historical access for MES / LIMS	Process Historian (PH exposes historical OPC UA)

Verification and Commissioning Checks

After the archive is active, perform the following checks before sign-off:

1. Tag count parity: WinCC Explorer > Tag Logging > Statistics should equal the engineering count. On PH: PH Management Console > Tag Statistics should match.
2. Acquisition timing: Generate a step on a test AI channel and verify the timestamp resolution in the trend (1 s cyclic must show sub-second jumps).
3. Compression math: Trend the same tag on 1 s and 1 min – the 1 min average should match the manual average of the 1 s samples within tolerance.
4. Segment rollover: Force a small segment period (e.g. 10 min) and watch a rollover. Verify the file count, file size, and the absence of gaps in the trend.
5. Backup link: Move a closed segment to the backup path, then re-link it. Query the time range – the values must appear.
6. PH stream health: PH Management Console > Connections shows green; lag counters stay under 5 s during a load test.
7. IS report job: Schedule a test report; verify the PDF is delivered and the data matches the trend.

Frequently Asked Questions