Overview
The typical PCS7 visualization tier is built around an OS Server that owns the WinCC project and one or more OS Clients that subscribe to the server's tag database. The same project can additionally host an OS Single Station (single-user WinCC runtime) that communicates directly with the Automation Station (AS) and is intended for engineering, commissioning, or as a low-cost fallback HMI. A frequently asked engineering question is whether a PCS7 multi-project built for 1 AS + 1 OS Server + 3 OS Clients can also contain a fourth OS station configured as a single-user station while still pointing to the same AS — without the cost of a fully redundant OS Server pair.
The answer is yes, but not as a member of the same multi-project. The OS Single Station must live in a separate WinCC project (or a separate PCS7 sub-project) with its own ES compile, its own OS project, and its own license key. This article documents the exact project topology that works, the licensing boundary, the WinCC compilation/download steps, the WinCC Application Reference alternative, and the field-proven limitations of running a single-user station in parallel with a server/client pair.
PCS7 OS Architecture Constraints
SIMATIC PCS7 V9.x and V10 organize the visualization layer as Windows-based SIMATIC WinCC stations that connect to one or more S7-400/S7-1500 AS stations over Industrial Ethernet (ISO-on-TCP, S7 Communication, or, since V9.0 SP2, OPC UA). Per the SIMATIC PCS7 Manual Collection (entry ID 109751785), only the following WinCC station roles are first-class citizens inside one PCS7 multi-project:
| Role | Process Connection | Owns Project | Redundancy Model |
|---|---|---|---|
| OS Server | Direct to AS via S7 | Yes (master) | Can be paired with a redundant OS Server |
| OS Client | Server-only (TCP/IP to OS Server) | No | N/A |
| OS Single Station | Direct to AS via S7 | Yes (standalone) | Standalone; no automatic failover |
| ES (Engineering Station) | Both S7 and OS Server for download | Yes (multi-project master) | Often mirrored as backup ES |
An OS Single Station is functionally a WinCC Runtime that talks directly to the AS and is unable to subscribe to an OS Server's process image. The PCS7 plant hierarchy (PH) and the WinCC tag database that the OS Server publishes to its clients are not exposed to a single station unless you explicitly build a parallel project.
Why a Single Station Cannot Sit in the Same Multi-Project
Inside a single PCS7 multi-project, every OS object must declare a single station type. A multi-project can host any combination of OS Server, OS Client, and ES, but the OS Single Station is a project-level type that is mutually exclusive with the OS Server role on the same WinCC project node. Concretely:
- When you insert a new OS in SIMATIC Manager (or the TIA-equivalent) into the WinCC project, the wizard asks for either Single-user station or Server + Client. Once set, that single object holds all process pictures, archives, and alarms.
- Changing the role of an existing OS in a built project (server → single, or single → server) is not a supported operation. You must create a new OS object, copy/import the picture tree, rebuild the server packages, and re-license.
- You can register multiple OS Servers in the same PCS7 project pointing to the same AS (e.g. for hot-standby redundant servers, or to scale a large plant across two visualization computers). These are still OS Servers, not single stations. The three OS Clients subscribe to whichever server is currently the master.
OS Single Station as a Budget Backup
If the actual requirement is "if the OS Server PC dies, we still want a PC that can talk to the AS and let operators see the process," then the budget-constrained architecture the original question describes is technically valid as long as you accept manual failover. The expected behaviour is:
- Normal operation: the OS Server + 3 OS Clients carry all operator activity. The OS Single Station is online but the operators do not use it; it functions as a hot-standby view-only HMI.
- Server failure: the three OS Clients go dark because the server is gone. The OS Single Station keeps running independently on the AS connection and operators switch their monitors / input devices to it.
- Recovery: the OS Server is repaired or replaced, the project is re-downloaded, and the clients reconnect. The single station is left in place as permanent backup.
There is no automatic failover, no shared archive, and no tag cross-load between the single station and the server/client pair. Each side has its own archive database, its own message list, and its own user administration. Long-term trending continuity is therefore not preserved across the failover; only live operation is preserved. The archive gap must be explained to operations if regulatory reporting depends on PCS7 archive continuity.
WinCC Application Reference as a Better Alternative
One of the cleanest Siemens-supported ways to mirror data from a primary WinCC project into a secondary one is the WinCC Application Reference (also called WinCC Reference Application). It is documented in the WinCC V7.5 / V7.6 documentation portal (entry 109748625) and the PCS7 add-ons section. With a reference application:
- The backup WinCC project reads selected tags from the primary project's tag database at runtime using the WinCC API, not by polling the AS again.
- No additional AS connection or load is generated on the PLC.
- Only the relevant tag subset is mirrored; the project footprint is small.
- You do not need a full PCS7 OS Single Station project; you can run a plain WinCC Runtime project with a reference license.
For a budget backup that only needs to keep the most important monitoring tags alive when the server is down, this is usually a more efficient route than maintaining a full parallel OS Single Station. The trade-off is that Application Reference requires the primary WinCC project to still be running and online; it is not a true PLC-direct backup. If the primary OS Server is lost, the reference project loses its data source. The PCS7 OS Single Station, by contrast, talks to the AS directly and survives an OS Server outage.
Project Topology Options Compared
| Option | Resilience to OS Server Failure | License Cost | Engineering Effort | Recommended Use |
|---|---|---|---|---|
| OS Single Station in a separate PCS7 sub-project | High (direct AS link) | 1 × OS Single Station license | Medium: full picture copy, separate OS compile, separate download | Budget backup PC next to the server room |
| Redundant OS Server pair + 3 OS Clients | Very high (automatic failover) | 2 × OS Server + redundancy license + 3 × OS Client | High: redundant server configuration, archive mirroring, user administration replication | Production-critical plants with regulatory archive continuity |
| WinCC Application Reference project | Low (depends on primary) | 1 × Reference license | Low: API configuration, no AS connection | Reporting / secondary dashboards that only need a subset of tags |
| OS Simulation on the ES | None (offline only) | None beyond ES | Lowest | Engineering, factory acceptance test, training |
Step-by-Step: Adding the OS Single Station
The following procedure covers PCS7 V9.0 / V9.1 with WinCC V7.5. Adapt folder names and DLL references to V10 if you are on the newer generation.
- Insert a new PCS7 sub-project. In SIMATIC Manager, right-click the multi-project, choose Insert New Object > SIMATIC PC Station. Add a WinCC Application of type Single-user station. This creates an independent project root inside the multi-project tree (alongside the existing OS Server project), not under it.
- Copy the picture tree. In WinCC Explorer of the OS Server project, use Project Duplicator to copy graphics, scripts, and tag prefixes into the new single-station project. Manually verify the S7 channel connection: the single station must point to the same AS IP address and use the same S7 connection name and rack/slot parameters, otherwise alarms will not map correctly.
- Re-import the AS symbol table. Run OS Compile with the Entire OS scope on the single station. Open Options > OS Compile / Export, select Compile OS, target Single-user station, and check the Settings > Process tags tab to confirm the S7 connection S7OSServer_1 maps to S7OSE_1 on the single station.
- Re-assign the Plant Hierarchy (PH). The single station is registered to the same PH only if you import the PH export. In SIMATIC Manager, on the PH object, choose Export Plant Hierarchy from the OS Server, then Import Plant Hierarchy on the single station. Without this step, operator pictures will display as empty because the picture-@ prefix mapping will not resolve.
-
Compile and download. From the ES, right-click the new single-station PC station, choose Target System > Compile and Download Objects. On the target PC, WinCC Runtime starts automatically; verify with the
CCStart/CCStopshortcut in the Start menu that the project loads without 100% project load warnings. - License the single station. Install the OS Single Station license (e.g. 6ES7658-2XX00-0YB5 family; the exact catalog number depends on the PCS7 version and the number of process tags). Activate via the Automation License Manager. Without a valid license, WinCC Runtime enters demo mode and resets every two hours.
- Repeat the OS compile cycle. Whenever the AS program changes, run OS Compile on the OS Server and on the single station, in that order, so the picture tags and the S7 connection are regenerated consistently.
Licensing and Runtime Considerations
The PCS7 OS license model is per-role and per-tag-count. According to the SIMATIC PCS7 licensing notes (entry 109760604) and the PCS7 price list, the relevant license classes are:
| Role | License Class | Behavior Without License |
|---|---|---|
| OS Single Station | Single-user, point-count band (e.g. 250 / 1k / 5k / 10k / 30k process tags) | Runtime starts in demo mode; exits after ~2 h |
| OS Server | Server with redundant partner option | Runtime starts in demo mode |
| OS Client | Client license (read/write) | No picture load, error on connect |
| Engineering ES | ES is licensed implicitly with PCS7 ES | Compile-only allowed; runtime disabled |
An OS Simulation on the ES, useful for offline testing of pictures, runs inside the ES license and is intended for short-term use. It is documented in PCS7 OS simulation FAQ (entry 109770085) as being time-limited per session; it is not a substitute for a backup HMI in production. For a permanent fallback HMI you must own a real OS Single Station license.
Plant Hierarchy and Multi-OS Assignment
The PCS7 Plant Hierarchy (PH) is a logical tree that maps operator areas to process tags and pictures. An OS can be assigned to a PH, but each OS can have only one active PH mapping. Two scenarios are valid:
-
One PH per OS Server, identical structure on the single station. The PH export from the OS Server is imported into the single station; the resulting tree looks identical and the picture prefix
@resolves on both. - Sub-area assignment. The single station is given a different PH (e.g. only Area 1 and Area 2 of a five-area plant). This is useful when the backup HMI is sized for a subset of the process.
Assigning more than one OS to the same PH is permitted, but the assignment is exclusive: each PH node maps to one OS object within the same multi-project. Two OS Servers pointing at the same PH are typically the redundant pair (master + standby). The OS Single Station is a different OS object and is therefore a peer to the OS Server, not a child. The PH export/import is the mechanism that keeps the picture names aligned between the two.
Verification and Commissioning Checks
After installation, run the following verification sequence on the single station:
- Process connection. Open WinCC Explorer > Tag Management > S7 Channel and confirm the channel is in Connected state, with the partner IP matching the AS CPU. A red state indicates a routing or rack/slot mismatch.
- Picture load. Open the standard picture @main.pdl directly from the file system on the target PC and confirm the alarm line, area buttons, and trend controls render without red X markers.
- Alarm acknowledgement. Trigger a test alarm on the AS (e.g. a level high-limit in AS program simulation). Confirm the alarm appears in the single station's message list and that acknowledgement toggles the bit in the AS.
-
Archive writing. Confirm
C:\Program Files (x86)\Siemens\Automation\WinCC\ArchiveManagercontains a configured TagLoggingSlow and TagLoggingFast archive. Manually disconnect the OS Server and observe that the single station continues to write its own archive files; this proves independent operation. - Failover exercise. Power off the OS Server, observe the three OS Clients going dark, and confirm the single station remains operational. Re-power the OS Server, re-run OS Compile, and confirm clients reconnect automatically.
- Time sync. Verify the single station is synchronised to the same NTP source as the AS; mismatched timestamps will corrupt alarm ordering between the server archive and the single station archive.
Troubleshooting Matrix
| Symptom | Likely Cause | Action |
|---|---|---|
| Single station starts in demo mode | OS Single Station license missing or wrong tag-count class | Install correct license in Automation License Manager; verify HW ID matches the USB stick or software-license container |
| All values are ### on the single station | S7 connection name mismatch after OS Compile | Re-compile OS on the single station; verify S7OSE_1 matches the connection name in Tag Management |
| Operator pictures do not open | PH not imported, so @ picture prefix cannot resolve |
Export PH from OS Server, import on single station, re-run OS Compile |
| Archives are empty | Archive path not local; mapped network drive disconnected | Move archive path to a local SSD; configure backup via the OS Server's archive backup service |
| Alarms duplicated after server recovery | Both server and single station have acknowledged locally | Document operational rule: only the active HMI acknowledges; the backup is read-only during normal operation |
| Runtime stops every 2 hours | License mismatch or demo mode is active | Re-check license; if evaluation mode was installed, replace with full license |
| Picture tree copy is missing user-defined function blocks | Project Duplicator only copies pictures and tags, not custom FBs/VBScripts if path differs | Manually re-import @Script folder and re-register standard functions |
Operational Caveats and Field Notes
- Archive duplication is a regulatory risk. Two PCs writing parallel process archives for the same plant may violate 21 CFR Part 11 / GxP audit-trail rules if the operator can manipulate one archive independently of the other. Document the operational rule that the single station is a fallback display only and not the audit-relevant archive of record.
- Process tag count. If the OS Single Station license is for a smaller tag count than the OS Server (e.g. 5k vs 30k), the OS Compile will silently drop tags beyond the licensed count. Make sure the single station license covers the union of all process tags actually used in the picture tree.
- AS connection load. Two HMI stations connected to the same AS add S7 communication load. For a 416 CPU and a moderate HMI tag count, this is usually negligible; for a 414 with 800 tags, watch the S7 connection resources counter in the CPU diagnostic buffer.
- Anti-virus and Windows patching. The single station must be on the same patching and anti-virus policy as the OS Server. A Windows Update reboot of the single station during a server outage is a common production incident root cause.
- User administration. Both OS Server and OS Single Station maintain their own user lists unless you centralise on a Windows domain. Plan for this duplication if the plant follows SIMATIC Logon.
Can I add an OS Single Station to the same PCS7 multi-project that already has 1 OS Server and 3 OS Clients?
Not in the same WinCC project. The OS Single Station must be inserted as a separate SIMATIC PC Station / sub-project within the PCS7 multi-project, with its own WinCC project, its own OS Compile, and its own OS Single Station license. The single station will then talk to the AS independently of the OS Server / Client pair.
Can I just copy the OS Server project, change its type to single-user, recompile, and download it?
In theory yes; in practice it is cleaner to use the WinCC Project Duplicator and then re-run OS Compile with the Single-user station target role. Renaming the type of an existing server project is not supported, and the picture tree, S7 connection name, and PH export must be re-imported so the operator pictures resolve correctly.
What is the difference between an OS Single Station and WinCC Application Reference?
An OS Single Station connects to the AS directly via S7 Communication and runs a full WinCC Runtime project, including graphics, archives, and alarms. A WinCC Application Reference reads selected tags from another WinCC project's tag database over the WinCC API, without touching the AS, and cannot run a full picture tree. The single station is the right choice for true server-outage resilience; the reference is the right choice for reporting and dashboards.
Can two OS stations be assigned to the same Plant Hierarchy (PH)?
Within a single PCS7 multi-project, each PH node maps to one OS object. You can have the OS Server and the OS Single Station both reference the same exported PH file, but each is a separate OS object. The redundant OS Server pair also uses the same PH on both members; that is the supported pattern for a single PH across two OS nodes.
Is OS Simulation on the ES a viable backup option for a budget project?
No. OS Simulation runs only inside the ES license, is time-limited per session, and is intended for engineering tests and FAT, not for production fallback. For a permanent backup HMI you need a real OS Single Station license on a separate PC.