WinCC Flexible 2008 S7-300 Connection Failure: MPI/PROFIBUS Fix

1. Problem Overview

Engineers commissioning WinCC Flexible 2008 Runtime on a PC (or Panel) frequently report a persistent "Disconnection from PLC" status indicator even when the physical PROFIBUS or MPI cable, the PC adapter (CP 5611 / CP 5612 / CP 5512 / USB PC-Adapter), and the PLC appear healthy. The HMI tag service shows a red dot, the area pointer Coordination stays inactive, and Status_Force remains at zero. The PLC is in RUN or RUN-P, but no cyclic data is exchanged.

Most of these incidents are not PLC faults, cable breaks, or defective adapters. They are configuration mismatches between three layers:

1. Windows access point (S7ONLINE) on the PG/PC interface
2. WinCC Flexible ES connection definition (AS-OS engineering view)
3. PLC partner / MPI address / baud rate in the S7-300 hardware configuration (HW Config)

This article walks through the full diagnostic and remediation sequence for a PC-based WinCC Flexible 2008 Runtime talking to a SIMATIC S7-300 (CPU 312, 314, 315, 315-2 DP, 317, 319, or ET 200S with IM 151) over MPI or PROFIBUS.

2. Symptoms and Error Indicators

Before changing anything, capture the exact symptoms. The error class determines the layer that is broken.

Common hex error codes the engineer will encounter:

3. Architecture: What the Software Stack Looks Like

WinCC Flexible 2008 Runtime on a PC does not talk to the S7-300 directly. The communication path is layered:

If the access point S7ONLINE is not assigned to a working CP 5611/CP 5512/CP 5621 profile, the chain breaks at step 3 even if the rest is correct. This is the single most common cause of the "unable to connect" report in the field.

4. Prerequisites Before You Start

• WinCC Flexible 2008 ES + Runtime installed with valid license (USB hardlock or floating license server).
• SIMATIC S7-300 station with a known MPI address (default 2) or PROFIBUS DP address (default 2).
• Siemens PC adapter: CP 5611 (PCI), CP 5621 (PCIe), CP 5512 (PCMCIA), or USB PC-Adapter (6ES7972-0CB20-0XA0).
• STEP 7 (SIMATIC Manager) installed on the engineering PC, with the correct PC adapter driver installed. If SIMATIC Manager is not installed, install the standalone SIMATIC NET PC Software to expose the Set PG/PC Interface control panel applet.
• PROFIBUS cable with termination on both ends, 9.6 kbit/s to 12 Mbit/s matching the configured baud rate on the CPU.

5. Step-by-Step Diagnostic Procedure

1. Open Start → Control Panel → Set PG/PC Interface on the runtime PC. On modern Windows 7/10, navigate to Start → All Programs → Siemens Automation → SIMATIC → Set PG/PC Interface.
2. Confirm the Access Point of the Application list contains S7ONLINE (case sensitive, no spaces). It must not be missing.
3. Set S7ONLINE → PC Adapter (MPI) for MPI, or PC Adapter (PROFIBUS) for PROFIBUS. If a CP 5611 is installed, choose CP 5611 (MPI) or CP 5611 (PROFIBUS).
4. Click Properties and verify:
   • Transmission rate matches the bus: 187.5 kbit/s for MPI, 1.5 Mbit/s typical for PROFIBUS.
   • Highest station address (HSA) is at least equal to the largest address on the bus (e.g. 31 or 126).
   • PC is the only master on the bus is unchecked if other DP masters exist.
5. Open the WinCC Flexible ES project and go to Project → Communications → Connections. Double-click the connection to the S7-300.
6. Verify:
   • Communication driver: SIMATIC S7 300/400
   • Station address: the MPI/PROFIBUS address of the CPU (commonly 2)
   • Rack: 0
   • Slot: 2 (for CPU 31x in the central rack)
   • Connection resource: OP / S7 connection (1), depending on the S7-300 CPU type and configured PG/OP connections
7. Open the S7-300 project in STEP 7 / SIMATIC Manager. In HW Config, right-click the CPU → Object Properties → Interface. Confirm the MPI/PROFIBUS address and the Configured connections table allows at least one OP / S7 connection (the default of 1 is normally sufficient for a single HMI).
8. From the runtime PC, run Start → Programs → Siemens Automation → SIMATIC → SIMATIC Manager → Options → Set PG/PC Interface → Diagnostics. Click Read to test whether the CPU responds at the configured address. A successful test shows the module order (CPU 31x, slot 2, etc.).
9. If STEP 7 / SIMATIC Manager is not installed, run PG/PC Interface Diagnostics from SetPgPc.exe or install the SIMATIC NET Commissioning Tool.
10. Restart the WinCC Flexible Runtime after any change to S7ONLINE. The S7DOS service caches the access point assignment; a reload of the project is mandatory.

6. Solution A: Correct the S7ONLINE Access Point

The fastest fix for the typical "unable to connect" symptom reported in the field is to re-point the S7ONLINE access point at a working CP profile. Procedure:

1. Launch Set PG/PC Interface.
2. In the Access Point of the Application list, select S7ONLINE.
3. In the Used Interface Parameterization list, select the correct CP entry (for example PC Adapter (MPI) active or CP 5611 (PROFIBUS) active).
4. Click OK and confirm the warning about overwriting an existing assignment.
5. Reboot the PC. S7ONLINE is read at boot by the S7DOS subsystem; a warm reload is sometimes not sufficient.

7. Solution B: Correct the WinCC Flexible ES Connection

Open the connection editor and re-enter the partner parameters. A typical valid configuration for a CPU 315-2 DP at MPI address 2 looks like this:

Name           = S7_300_Connection
Driver         = SIMATIC S7 300/400
Station address = 2 (MPI) or 2 (PROFIBUS)
Rack           = 0
Slot           = 2
Connection     = OP / S7 connection (1)
Cycle / Update = 1 s (default)
Active         = yes

If the PLC is a CPU 31x-2 DP accessed through its PROFIBUS interface (not the MPI/DP combined port), set the Network in the connection dialog to PROFIBUS and enter the DP address in Station address. Use the same value for Highest station address (HSA) in the Set PG/PC Interface properties.

8. Solution C: Recover a Missing SIMATIC Manager

A frequent follow-up symptom is that after installing WinCC Flexible 2008, SIMATIC Manager no longer appears in the Start menu. This is normal: WinCC Flexible 2008 is delivered as a separate install and does not include STEP 7. The fix is one of the following:

• Install STEP 7 V5.5 (or V5.4) on the engineering PC if the project is configured with SIMATIC Manager.
• If only the runtime is required on a separate PC, install SIMATIC NET PC Software (S7-1613 or later) to obtain the Set PG/PC Interface applet and the CP 5611/CP 5621 driver.
• If the project was edited with WinCC Flexible 2008 ES + STEP 7 V5.4 integration (the integrated "Professional" install), verify that the integration component was not removed during a WinCC-only reinstall.

9. Reinstall / Re-load Procedure After a Driver Change

If S7ONLINE was pointing at a non-existent or corrupted CP, the CP driver must be reinstalled:

1. Open Device Manager, expand SIMATIC NET, confirm the CP is listed without a yellow exclamation mark.
2. If the CP is unknown, run the SIMATIC NET Commissioning Tool and run Install/Remove Interface for the correct CP type.
3. Re-assign S7ONLINE to the new interface.
4. Open WinCC Flexible Runtime, Project → Transfer → Runtime Files, then re-transfer the compiled .fwx project from ES to the runtime PC.
5. Start the runtime. The status indicator should turn green within 5–10 s.

10. Verification Checklist

11. Common Pitfalls and Field-Proven Caveats

• Two different PCs, two different S7ONLINE assignments. The ES PC may be configured for TCP/IP → RFC1006 while the runtime PC is configured for PC Adapter (MPI). Always check both machines.
• USB PC-Adapter driver version. The 6ES7972-0CB20-0XA0 USB PC-Adapter requires SIMATIC NET 6.4 or later. WinCC Flexible 2008 ships with an older PC-Adapter driver; an update may be required to use modern USB adapters.
• Terminating resistors. PROFIBUS requires terminators on both physical ends only. Setting termination on a mid-bus node breaks the bus.
• CPU in STOP. S7-300 stops responding on MPI/PROFIBUS during STOP in some firmware versions. Always verify the CPU mode first.
• Connection resource exhausted. If the CPU has more than 1 OP connection configured and the project uses multiple S7 connections, increase the Configured connections in HW Config.
• Firewall on the runtime PC. WinCC Flexible RT does not need an open port for MPI/PROFIBUS, but Windows Firewall can block the SIMATIC NET services. Disable the firewall rule for Siemens SIMATIC NET during commissioning.
• 32-bit vs. 64-bit Windows. WinCC Flexible 2008 is a 32-bit application. On a 64-bit Windows 7/10 install, the Set PG/PC Interface applet is in C:\Program Files (x86)\Siemens\Automation\SIMATIC — never in C:\Program Files\Siemens.

12. When the Project Is Configured with TIA Portal Instead

If the S7-300 was re-engineered in TIA Portal (V13 / V14 / V15), WinCC Flexible 2008 will not load TIA-compiled S7-300 projects directly. Use the HMI Tag Simulation interface or migrate to WinCC (TIA Portal) for new builds. For legacy WinCC Flexible 2008 installations, the SIMATIC Manager project file (*.s7p) must be retained and edited with STEP 7 V5.x to keep the WinCC Flexible connection synchronised.

13. Reference: Official Siemens Documentation

• Siemens FAQ: Configuring a connection between WinCC flexible Runtime and S7-300/S7-400 via PROFIBUS/MPI
• Siemens FAQ: S7ONLINE access point configuration for STEP 7 / WinCC
• SIMATIC NET PC Software installation and PC Adapter setup
• WinCC Flexible 2008 Communication manual (PDF)
• S7-300 CPU 31x communication / connection resources manual