Structuring SIMATIC PCS 7 Multiprojects - Architectural Benefits of Separating AS and OS Projects

In SIMATIC PCS 7, segregating automation stations (AS) and operator stations (OS) into distinct component projects within a unified Multiproject represents Siemens' recommended architectural engineering standard. In this design, individual sub-projects host specific functional subsystems (e.g., AS1 Project, AS2 Project, OS Server Project, OS Client Project, and a centralized Master Data Library). A white box overlay icon in the Plant Hierarchy indicates that a hierarchy folder or technological object has a cross-project link to an OS picture residing in a partner project. This decoupled architecture optimizes multi-user collaboration, prevents database file lock contention, drastically accelerates archiving and project retrieval, and isolates subsystem commissioning.

Multiproject Principles and the Decoupled Component Architecture

A fundamental engineering decision when creating a distributed control system in SIMATIC PCS 7 is project topology. While SIMATIC Manager permits placing S7-400 controllers and WinCC OS applications together within a single monolithic project file, Siemens strongly advises against this practice in industrial plants.

Instead, the Multiproject concept enables engineers to segment a complex automation system into clean, modular sub-projects:
- AS Projects: Contain hardware configuration (HW Config), networking (NetPro), and CFC/SFC control logic for one or more controllers.
- OS Projects: Contain WinCC Server and Client stations, graphic process screens (.pdl), scripts, and alarm logging definitions.
- Master Data Library (MDL): Hosts centralized, version-controlled block types, SFC types, and Control Module Types (CMT) shared by all projects.

This separation prevents multiple engineers from competing for exclusive file locks on the central project database (pbase.db / Sycon.db). An automation engineer can modify controller logic in the AS project while an HMI specialist concurrently draws operator screens in the OS project without blocking one another.

Plant Hierarchy Cross-Project Links and Visual Indicators

The Plant Hierarchy (PH) functions as the unifying technological backbone across all projects in the Multiproject:
- The hierarchy is created and synchronized across all member projects via Options > Plant Hierarchy > Customize....
- When an engineer assigns a CFC chart in the AS project to folder Plant1/UnitA and assigns an OS picture in the OS project to the identical folder Plant1/UnitA, SIMATIC Manager establishes an internal cross-project reference.
- In the Plant View, hierarchy folders containing objects linked across project boundaries display a small white box icon overlay in the lower-left corner of the folder symbol.
- This visual indicator confirms that technological objects (CFC blocks, messages, and variables) in the AS project are mapped to corresponding WinCC pictures and block icons in the OS project.

Step-by-Step Procedure: Structuring an AS-OS Multiproject

To set up a properly structured Multiproject in SIMATIC PCS 7:

Step 1: Create the Multiproject Container

In SIMATIC Manager, select File > 'New Project / Multiproject' Wizard.
Select Multiproject and assign a project name (e.g., Plant_Alpha_MP).
Define the storage path on high-speed internal storage (e.g., D:\SIEMENS\STEP7\S7Proj\Plant_Alpha_MP).

Step 2: Insert Modular Component Projects

Select the Multiproject root object, click File > Multiproject > Insert New Project....
Create sub-projects following standardized naming conventions:
- Plant_Alpha_AS1: For Area 1 S7-400 / CPU 410 controllers.
- Plant_Alpha_AS2: For Area 2 controllers.
- Plant_Alpha_OS: For redundant WinCC OS Servers and Web Servers.
- Plant_Alpha_Lib: Set as the Master Data Library via File > Multiproject > Set as Master Data Library.

Step 3: Configure Shared Plant Hierarchy

In the Multiproject tree, select the project root.
Select Options > Plant Hierarchy > Customize....
Define hierarchy depth (e.g., 3 levels: Enterprise / Plant Section / Unit).
Check 'Derive picture hierarchy from the plant hierarchy' and 'Derive block icon hierarchy'.
Click OK; SIMATIC Manager synchronizes the folder hierarchy across all sub-projects.

Step 4: Map Cross-Project Links and Execute Compilation

In the AS project, place CFC charts in designated hierarchy folders.
In the OS project, create WinCC picture objects in the identical hierarchy folders.
Observe the appearance of the white box link indicator on the hierarchy folder.
Run Compile OS; the compiler traverses the cross-project hierarchy, generating block icons in the mapped OS pictures automatically.

Comparative Architecture Matrix: Monolithic vs Segmented Multiproject

The table below compares a single monolithic project against a modular PCS 7 Multiproject:

Operational DimensionMonolithic Single ProjectSegmented Multiproject ArchitectureMulti-User CollaborationHigh conflict (Database locks block team)Seamless (Engineers work on separate projects)Archiving & BackupRequires archiving entire facility (Massive file)Sub-projects can be archived/transferred individuallyProject Corruption RiskSingle corruption damages entire facilityIsolated to individual sub-projectOS Compilation EfficiencyCompiles all stations simultaneouslyCompiles selected AS-OS relationships targetedlySiemens RecommendedNo (Deprecated for medium/large plants)Official Industry Standard (Compendium Part A)

Verification

To confirm that cross-project Multiproject architecture is properly established:
1. Plant Hierarchy Consistency: In SIMATIC Manager, select Options > Plant Hierarchy > Check Consistency; verify that all hierarchy folders across AS and OS resolve without missing pointer errors.
2. Visual Overlay Audit: Inspect the Plant View; confirm that hierarchy folders containing both CFC charts and OS pictures display the small white box cross-project link indicator.
3. OS Compilation Validation: Run OS Compilation (Changes only); confirm that block icons for technological blocks in Plant_Alpha_AS1 are generated automatically inside the target picture in Plant_Alpha_OS.

Limitations and Operational Precautions

Engineering teams must observe the following constraints:
- Sub-projects removed from a Multiproject for offline editing must not have their network connections or Plant Hierarchy modified independently without executing cross-project synchronization upon re-insertion.
- Always validate final license quantities and MLFBs against the applicable Siemens catalog and authorized distributor quotation when procuring multi-user engineering licenses.
- Maintain identical SIMATIC PCS 7 software versions and update levels across all engineering client laptops participating in a Multiproject.

Engineer FAQ

Can I split a single AS into two separate projects?

No. A single S7-400 CPU and its associated hardware configuration (HW Config) and chart folder must reside entirely within a single project. However, multiple AS stations can be distributed across different projects within the Multiproject.

What does it mean if the white box on a hierarchy folder turns into an exclamation mark?

An exclamation mark or red cross on a hierarchy folder indicates a broken cross-project reference, typically caused by deleting a target picture or moving a CFC chart without updating the Plant Hierarchy.

Can OS Client projects be separated from OS Server projects?

Yes. In large installations, OS Clients are frequently placed in their own sub-project (e.g., Plant_Clients), importing server packages from the OS Server sub-project.