Resolving PCS 7 AS-OS S7 Connection Failures with PLCSIM

1. Problem Overview

When commissioning a Siemens SIMATIC PCS 7 project with S7-PLCSIM as the target runtime, engineers frequently encounter a situation in which the Automation Station (AS) compiles successfully but the Operator Station (OS) compile wizard reports that no S7 connection is available. The OS compile dialog lists the available connection targets, and the field for the PLCSIM (MPI/TCP) variant is either blank or absent. In SIMATIC Manager, opening NetPro on the Engineering Station (ES) and attempting to insert a new S7 connection under the ES object produces no entries; right-clicking the partner (AS) and selecting Insert New Connection either returns no valid targets or throws an inconsistency warning.

This article isolates the root cause of the missing AS-OS connection, documents the diagnostics used to confirm the fault, and provides a step-by-step fix that has been verified on PCS 7 V9.0/V9.1 with PLCSIM V16/V17. The procedure also covers the equivalent configuration when the AS is replaced by a S7-400H station, an AS 410, or an ET 200SP CPU used in PCS 7.

2. Root Cause Analysis

The fundamental cause of the missing AS-OS connection is a subnet mismatch between the Automation Station and the PC Station (ES/OS). In a PCS 7 project the AS is typically networked on one bus (MPI, PROFIBUS, or PROFINET) while the PC Station is networked on a different bus (usually Industrial Ethernet). The WinCC OS compile wizard can only create an S7 connection when both endpoints are members of the same S7 subnet object inside NetPro.

Three concrete conditions trigger the symptom described above:

1. Different subnet types: AS connected to PROFIBUS or MPI, while the PC Station (OS single-station or OS server) is connected only to an Industrial Ethernet subnet. Without a shared subnet object, NetPro cannot resolve a partner for a new S7 connection.
2. Same subnet type, different subnet IDs: Both stations are connected to PROFINET but to different PROFINET subnet objects (for example, PN/IO_1 vs. PN/IO_2). NetPro treats these as separate broadcast domains and refuses to generate an S7 connection.
3. Missing PROFINET interface on the AS: The CPU has no PN port configured, or the PN port has been disabled. The OS wizard scans for valid PN endpoints; when none exist, the PLCSIM (TCP) entry is omitted.

A secondary cause is a multi-project split: when the AS project and the OS project are stored as separate sub-projects inside a PCS 7 multi-project and the multi-project has not been "adjusted" (merged) since the last hardware change, NetPro cannot resolve cross-project S7 connections.

3. Prerequisites and Environment

Before starting diagnostics, confirm the following project-side prerequisites:

• The PCS 7 project opens without "Project inconsistent" warnings.
• The AS hardware (HW Config) compiles cleanly: Station > Consistency Check shows no errors.
• The PC Station HW Config contains an IE General (or CP 1623) entry with index 1 and a PROFINET IO or Industrial Ethernet subnet assigned.
• The WinCC project inside the PC Station has the SIMATIC S7 PROTOCOL SUITE channel loaded.

4. Diagnostic Procedure

Follow these diagnostic steps in order. Each step produces a verifiable observation that confirms or eliminates a possible cause.

4.1 Inspect NetPro on the ES

1. Open SIMATIC Manager and select the ES project.
2. Double-click NetPro in the component view.
3. Verify the AS and the PC Station are both visible in the network view.
4. Note the subnet icon attached to each station's interface. The AS will show its PROFIBUS/MPI/PROFINET subnet; the PC Station will show its Industrial Ethernet subnet.

4.2 Confirm Subnet Membership

Right-click the AS PROFINET interface > Object Properties > tab Interfaces > field Subnet. The drop-down must show a PROFINET subnet that is also assigned to the PC Station's IE General. If the two are different subnets (even if the names look identical), the OS compile wizard cannot establish a logical S7 connection.

4.3 Attempt a Manual S7 Connection

1. In NetPro, right-click the AS and choose Insert New Connection.
2. In the partner dialog, select (Unspecified) or PC Station.
3. If no valid partner is listed, the OS side of the project is either missing or on a non-matching subnet.

4.4 Verify WinCC OS Project Network Entry

1. Open the OS project in WinCC Explorer.
2. Right-click Tag Management > SIMATIC S7 PROTOCOL SUITE > TCP/IP.
3. Confirm a connection named S7OSLNK or a user-defined connection exists, with a valid IP address and rack/slot of the AS CPU.
4. If the entry is missing or shows a red "X", the OS compile wizard has not yet generated it because no valid AS endpoint was found.

5. Solution: Merge Subnets in NetPro

The canonical fix for the symptoms above is to ensure that the AS PROFINET interface and the PC Station IE interface share the same subnet object inside NetPro. There are two proven methods; use the first unless a multi-project setup is in use.

5.1 Method A — Assign the Existing Subnet

1. In NetPro, right-click the AS PROFINET interface > Object Properties.
2. On the Interfaces tab, click New if no PROFINET subnet exists, or select the existing subnet that the PC Station already uses.
3. Click OK. NetPro now shows both stations linked by a single subnet line.
4. Save and compile NetPro (Network > Save and Compile). Confirm no errors in the output window.

5.2 Method B — Multi-Project Merge

When the AS and the PC Station reside in separate sub-projects of a PCS 7 multi-project, the merge operation unifies the network view.

1. In SIMATIC Manager, right-click the multi-project root.
2. Select Multi-Project > Adjust Projects (or Adjust to Multi-Project in older versions).
3. When prompted, choose Merge Subnets for PROFINET and PROFIBUS as applicable.
4. Confirm with OK and wait for the consistency check to complete.
5. Re-open NetPro and verify both stations are now connected to the same subnet.

6. Solution: Configure PROFINET IO on the AS

If the AS hardware has no PROFINET interface configured, the OS compile wizard cannot create an IE-based S7 connection even if the PC Station is on Industrial Ethernet. The fix is to enable the PN interface on the CPU.

1. Open HW Config for the AS.
2. Double-click the CPU entry (for example, CPU 416-3 PN/DP or CPU 410-5H).
3. In the Properties dialog, select the PROFINET Interface tab.
4. Click New to create a PROFINET subnet, or attach the CPU to the existing Ethernet subnet.
5. Assign an IP address consistent with the OS project (default: 192.168.0.1 for AS, 192.168.0.2 for OS).
6. Save and compile HW Config.
7. Return to NetPro; the AS now appears with a PN subnet. Repeat Method A from Section 5 to merge with the PC Station subnet.

The general principle of S7-connection establishment across subnets is documented in Siemens' official S7 communication reference, which describes the configuration of an S7 connection between two S7 CPUs across PROFINET. The same end-to-end logic applies when one of the endpoints is a WinCC OS application: both endpoints must share exactly one common subnet, and that subnet must be of a type supported by the OS channel driver (TCP/IP for PROFINET/Industrial Ethernet, Named Connections for PC internal, MPI for legacy).

7. AS-OS Connection Compilation in WinCC

Once the subnet topology is correct, regenerate the AS-OS connection through the OS compile wizard.

1. In SIMATIC Manager, select the OS object (for example, OS1).
2. Right-click and choose Compile OS (or use Options > OS > Compile OS).
3. The wizard scans NetPro for S7 connections between the AS and the OS. A list of available connections appears.
4. Select S7 Connection with type TCP/IP and check the box PLCSIM (TCP/IP) if targeting PLCSIM.
5. Click Compile. The wizard writes the connection entry to the WinCC project and generates the matching blocks in the AS S7 program.

8. Verification and Commissioning

After applying the fix, validate the AS-OS link end-to-end before declaring the project ready.

8.1 NetPro Consistency

1. Open NetPro and select Network > Consistency Check.
2. Confirm the output window reports "No errors" and "No warnings".

8.2 S7-PLCSIM Connection Test

1. Start S7-PLCSIM and download the AS program to the simulated CPU.
2. Start the WinCC Runtime on the OS.
3. In the WinCC channel diagnosis (Start > SIMATIC > WinCC > Channel Diagnosis), confirm the connection state shows a green check.
4. Force a tag value in PLCSIM and verify it updates in the WinCC screen within one acquisition cycle (typically 1 second).

8.3 Tag Visibility

Open WinCC Explorer > Tag Management > SIMATIC S7 PROTOCOL SUITE > TCP/IP > the OS-generated connection. The tag list should now show the AS process tags without red exclamation marks. A red exclamation indicates that the tag address could not be resolved against the AS symbol table — re-run Compile OS to refresh the tag database.

9. Common Configuration Errors

10. PLCSIM-Specific Limitations

PLCSIM emulates an S7 CPU but imposes constraints that do not exist on physical hardware. Engineers using PLCSIM as a virtual AS must observe the following boundaries:

• PLCSIM V16+ supports TCP/IP-based S7 connections to WinCC Runtime. Earlier versions (V13-V15) support only MPI/DP and cannot host an AS-OS connection over Industrial Ethernet.
• PLCSIM instance limit: Only one PLCSIM instance can run per Windows session on the same IP. For redundant OS servers, configure two distinct IP addresses on two virtual NICs.
• Time behavior: PLCSIM executes scan cycles at the configured OB1 cycle time; do not expect real-time I/O response when validating with simulated I/O.
• F-CPU support: PLCSIM does not support F-runtime groups; safety-related S7 connections (F-S7) cannot be tested with PLCSIM. Use a physical F-CPU for F-commissioning.

11. Diagnostic Decision Matrix

12. Frequently Asked Questions