Troubleshooting S7-1500 PROFINET Port Partner Mismatch Error

1. Problem Overview

A recovered TIA Portal V14 project for a SIMATIC S7-1516F-3 PN/DP (and equivalent S7-1500 CPUs such as 6ES7516-3AN01-0AB0 / 6ES7516-3AN00-0AB0) starts up with the CPU ERROR LED flashing red instead of going to RUN. The online diagnostics describe the alarm as:

"An interconnection has been configured but another partner has been recognized on the port."

This is a PROFINET port partner (neighbor) mismatch raised by the port X1 P1 of the CPU. In the topology editor the port is assigned to a SINAMICS S120 drive object that is currently deactivated (grey icon), while the physical port is actually patched to the SIMIT Simulation Unit's port P1, which presents one or more virtual PROFINET devices.

The diagnostic alarm is raised at power-up, after the CPU performs its PROFINET startup phase, because the device that physically answers the LLDP/PROFINET identification exchange on X1 P1 does not match the configured neighbor partner stored in the topology view.

2. Root Cause Analysis

The root cause is not a defect in the S7-1500 hardware, the SIMIT Simulation Unit, or the NX10 coupling. It is a PROFINET port monitoring mismatch created by the interaction of three settings:

1. Topology editor assignment in TIA Portal – A port-to-port interconnection is configured between the CPU X1 P1 and a specific S120 drive port. This is mandatory when IRT (Isochronous Real-Time) is used.
2. Port partner monitoring – Enabled either by default or explicitly, this function compares the LLDP neighbor detected at runtime with the neighbor configured in the topology. A mismatch raises a diagnostic interrupt and contributes to the error LED state.
3. IRT requirement for topology – Because the project uses IRT (a deterministic PROFINET scheduling mode), the port interconnection cannot simply be removed from the topology editor; IRT depends on the topology for path and bandwidth planning.

The combination produces the specific diagnostic text shown above. The S7-1500 system manual documents this status for the ERROR LED as "An error has occurred" when the LED flashes red, with the RUN/STOP/MRES LED providing the secondary state.

3. LED Status Reference for the CPU 1516-3 PN/DP

The following table summarizes the documented CPU LED meanings from the SIMATIC S7-1500 manual collection. Use this as the first sanity check when the ERROR LED flashes red.

Source: CPU 1516-3 PN/DP Manual (6ES7516-3AN01-0AB0) and CPU 1516-3 PN/DP Manual (6ES7516-3AN00-0AB0). Cross-reference: TIA Portal Manual Collection – Status and Error Display of the CPU.

4. Reading the Diagnostic Buffer

Before changing the topology, capture the exact diagnostic entry. This identifies whether the alarm is purely a port partner mismatch or whether additional IRT/sync errors are present.

1. In TIA Portal, open the project and connect online to the CPU.
2. Right-click the CPU in the project tree and choose Online & Diagnostics → Diagnostics buffer.
3. Filter for PROFINET IO and Port entries.
4. Locate the entry timestamped at the last CPU restart.

Expected diagnostic text for this fault:

Diagnostic interrupt from PROFINET IO
Port: X1 P1 (interface X1)
Status: Interconnection configured, but another partner recognized
Diagnostic: Port partner mismatch

If the buffer instead shows "Sync master missing," "Sync slave not found," or "IRT topology inconsistent," the fix path differs – see Section 8 below.

5. Why IRT Prevents a Simple Topology Delete

PROFINET with IRT (Isochronous Real-Time) requires the controller to know the topology of the network before runtime. The IRT scheduler uses port-to-port interconnections to:

• Reserve bandwidth along each cable segment.
• Compute phase-aligned transmission windows.
• Validate that the configured sync domain is physically contiguous.

Deleting the topology entry for X1 P1 in TIA Portal removes the IRT bandwidth reservation on that port and therefore prevents the IRT cycle from starting. This is why the error persists even though the user is logically satisfied with the SIMIT/NX10 coupling – the configuration is incomplete from the PROFINET IRT perspective.

6. Resolution Path A – Disable Port Partner Monitoring for SIMIT

This is the recommended approach when IRT must remain active. Port partner monitoring is a per-port, per-device property – it does not affect IRT scheduling, only the diagnostic interrupt.

1. In TIA Portal, open Devices & Networks and select the CPU.
2. Switch to the Topology view (not the Network view).
3. Click on port X1 P1 of the CPU. The properties dialog opens in the inspector window.
4. Navigate to PROFINET interface → Port interconnection / Port options.
5. Locate the option "Monitoring of the port partner / LLDP neighbor" (German: "Überwachung Port-Partner").
6. Set the option to "Do not monitor" or uncheck the corresponding checkbox, depending on TIA Portal version. In V14 the wording is: "Port-Partner-Überwachung: deaktiviert".
7. Repeat the setting on the SIMIT Simulation Unit port P1 side to suppress the alarm there as well.
8. Compile the project (Hardware and Software, full rebuild).
9. Download the hardware configuration to the CPU.
10. Restart the CPU.

The CPU will now skip the LLDP partner comparison on X1 P1 and accept the SIMIT-presented neighbor. The ERROR LED should extinguish after the next diagnostic refresh cycle.

7. Resolution Path B – Remove Topology (RT-Only Projects)

If the project does not actually require IRT (only RT class 1 or class 2), the topology entry is optional:

1. Open Topology view.
2. Right-click the interconnection line between CPU X1 P1 and the (deactivated) S120 drive port.
3. Choose Delete interconnection.
4. Recompile and download the hardware configuration.

PROFINET RT no longer validates the LLDP neighbor against a configured partner; the diagnostic alarm disappears. Verify that no other part of the project depends on the topology (e.g., port statistics, cable diagnostics, iDevice neighbor relations).

8. Resolution Path C – Reactivate the S120 Device Object

If the original intent is to keep the S120 drive as the configured partner and only suppress the SIMIT path temporarily, reactivate the drive object instead of editing topology:

1. In the project tree, locate the SINAMICS S120 device under Ungrouped devices or its assigned IO controller.
2. Right-click the device and select Activate device (or clear the "Deactivated" checkbox in Properties → General).
3. Confirm that the device's PROFINET name and IP address match the actual S120 hardware (or the SIMIT-presented virtual S120).
4. Verify the PROFINET device name on the SIMIT Simulation Unit: open SIMIT → Project Editor → PROFINET device list and confirm the S120 PN name matches the TIA Portal configuration byte-for-byte (case-sensitive).
5. Recompile and download.

This is the only path that lets topology remain intact and the ERROR LED clear without disabling port monitoring.

9. SIMIT-Specific Configuration Checklist

SIMIT presents PROFINET devices to the controller through the Simulation Unit's own PN interface. Misalignment between the TIA Portal device names and the SIMIT-presented names is a frequent cause of partner mismatch alarms.

10. Verification

After applying Path A, B, or C, validate the fix with the following sequence:

1. Perform a CPU restart (MRES or power cycle).
2. Confirm that the ERROR LED is off and RUN LED is solid green after ramp-up.
3. Open Online & Diagnostics → Diagnostics buffer and confirm no new port partner mismatch entries appear.
4. In the topology view, right-click and choose Go online → Compare topology. The offline topology and online topology must match (offline = online).
5. Confirm that the SIMIT virtual drives respond to the PLC program (e.g., motion setpoints are accepted, status word bits toggle in the NX10 coupling).
6. Force a PROFINET device replacement test: disconnect and reconnect the SIMIT Simulation Unit's port P1 cable and verify the alarm does not reappear.

11. Troubleshooting Matrix

12. Edge Cases and Field-Proven Caveats

• Memory card with stale topology: The S7-1500 stores the topology alongside the hardware configuration on the SIMATIC memory card. If the card is reused from a prior project, the old topology persists even after a new project is downloaded. Always re-format or use a fresh card during commissioning.
• IRT with multiple sync masters: If more than one CPU is configured as sync master in the same IRT domain, the IRT startup will fail and the ERROR LED will latch red. Reduce the configuration to a single sync master per domain.
• SIMIT port LED behavior: The Simulation Unit's port P1 LED indicates link status only, not LLDP validity. A green link LED on P1 does not guarantee that the LLDP neighbor is the same as the one configured in TIA Portal.
• NX10 coupling independence: The NX10 Motion Controller coupling remains functional even when the S7-1500 reports a PROFINET alarm. The machine can continue to move under SIMIT simulation while the CPU displays the alarm, because NX10 and SIMIT exchange data through a separate channel.
• Firmware version sensitivity: The exact wording of the port partner monitoring option varies between TIA Portal V14, V15, V15.1, V16, V17, and V18. In V14 it appears under "Port interconnection / Port options"; in V17/V18 it is grouped under "Port statistics & diagnostics." Always verify against the specific TIA Portal version in use.
• Multiple deactivated drives: When more than one drive is greyed out in the device view, each contributes its own port partner entry. Fix all of them or apply Path A globally for the X1 interface.

13. Related PROFINET Diagnostics Worth Knowing

The port partner mismatch alarm belongs to the broader family of PROFINET port diagnostics defined in IEC 61784-2 and the PROFINET specification. Other alarms in the same family that may surface in similar situations include:

• Port link down – Physical layer loss.
• Neighbor mismatch – Wrong LLDP neighbor detected (this article's subject).
• MAU type error – Media attachment unit mismatch (e.g., forced speed/duplex).
• Port speed/duplex mismatch – Negotiated speed differs from configured speed.

Each of these alarms feeds into the same diagnostic interrupt path on the CPU and produces the same visual symptom (ERROR LED flashing red). Always distinguish them by reading the diagnostic buffer before applying a fix.

14. Summary

The S7-1516F-3 PN/DP ERROR LED flashing red with the diagnostic text "interconnection configured, but another partner recognized" is the symptom of a PROFINET port partner monitoring mismatch. It is not a hardware defect. The fix depends on whether the project uses IRT:

• IRT active: Keep the topology; disable port partner monitoring on CPU X1 P1 (Path A) or reactivate the drive (Path C).
• RT only: Delete the topology entry for X1 P1 (Path B).

After the fix, verify with a CPU restart, a diagnostic buffer check, and a topology online/offline comparison. The PROFINET device names in SIMIT must match the TIA Portal configuration exactly.