Resolving Profinet Communication Loss on S7-1500 with Scalance

Problem Overview

A Profinet (PN) installation built on a Siemens S7-1500 CPU 1517-3 PN/DP (6ES7 517-3UP00-0AB0, firmware V2.9.7) and a SCALANCE X208 (6GK5 208-0BA10-2AA3) exhibits an intermittent loss of the Profinet connection from a C# Windows service on a WinCC HMI panel (6AV7 260-1xxx-xxxx with Windows 7 Ultimate) to the PLC. The HMI's native WinCC channels continue to read tags without interruption, but the external S7netplus-based polling process loses its S7 connection after several hours of stable operation. A power-cycle of either the CPU or the SCALANCE X208 restores connectivity for a further period of several hours. The behaviour was first observed after a network migration: an ET200M station on PROFIBUS with a Siwarex FTA weighing module was replaced by an ET200SP head module with a WP321 weighing module on Profinet. PRONETA diagnostics and TIA Portal online diagnostics both report the network as healthy at the moment of inspection.

Affected Hardware and Software Configuration

The exact configuration in the TIA Portal V17 project is summarised below.

Reference for the CPU: Siemens Product Support – 6ES7517-3UP00-0AB0. Reference for the switch: Siemens Product Support – 6GK5208-0BA10-2AA3.

The two logical networks on the panel PC are:

• VLAN1 – corporate / SQL server traffic (default route).
• VLAN2 – Industrial Ethernet / Profinet, on the SCALANCE X208 port facing the CPU.

The CPU's Profinet interface is configured for an HMI connection on PN/IE_1. The C# service opens a parallel S7 connection through the same physical interface to read process data; it explicitly disables optimized block access on the data blocks it targets.

Symptom Pattern and Timing

The failure exhibits a characteristic pattern that is important for triage:

1. The system runs without errors for a number of hours (typically 4 to 12 hours in the affected installation).
2. The C# service's S7 connection terminates (TCP RST or timeout). WinCC HMI tags continue updating, proving the Profinet link is not physically down.
3. A restart of the S7-1500 CPU or a restart of the SCALANCE X208 restores the connection for a similar interval.
4. PRONETA and TIA Portal online diagnostics show no active alarms on the Profinet network or on the CPU's PN interface at the time of the failure.
5. Diagnostic buffer entries on the CPU do not show a Profinet station failure; there is no AR (Application Relationship) abort for the ET200SP or the HMI.

The cycle load due to communication in TIA Portal is reported at 30 %, and the utilisation on the switch port facing the CPU peaks at 35 %. These numbers are below saturation thresholds and do not, on their own, explain a hard drop of the S7 channel after several hours.

Root Cause: Firmware Mismatch Between Project and Device

The decisive diagnostic step in this case is to compare the firmware version configured in the TIA Portal hardware catalog against the firmware version actually running on the installed SCALANCE X208. The investigation revealed:

• Historical hardware configuration in TIA: SCALANCE X208 with firmware V4.5.
• Physical device on site: SCALANCE X208 with firmware V5.2.8 (updated during the PROFIBUS-to-Profinet migration but not propagated into the TIA project).

When a SCALANCE device is updated in the field using the Web Based Management (WBM) or a separate Service Pack but the TIA Portal project retains the older device version, the engineering tool and the live device diverge in their interpretation of Profinet records, LLDP TLVs, and PN-DCP device roles. In particular, the following behaviours are known to be sensitive to firmware skew on SCALANCE X-200 family switches:

This is consistent with the observed "works for several hours, then drops, recovers on switch restart" pattern: the switch's internal control plane walks into a state that is incompatible with the controller's view of the network, the S7 connection that the C# service opened is torn down, and a power-cycle is required to clear the switch's state. WinCC on the same HMI is unaffected because WinCC uses the S7-1500's HMI connection which, in TIA V17, can be configured with a different keep-alive behaviour and a more aggressive reconnect policy on the HMI side.

For the firmware release history of the SCALANCE X208, see the Siemens Industry Online Support entry 6GK5208-0BA10-2AA3 under "Firmware" and the corresponding release notes. For the S7-1500 CPU firmware history, see the entry 6ES7517-3UP00-0AB0 under "Firmware updates".

Diagnostic Workflow

Before assuming a firmware mismatch, run the following ordered checks. Each step adds information that the next step relies on.

1. Read the live firmware on every Profinet device. In TIA Portal: Online → Accessible nodes, select the SCALANCE X208, and read its diagnostics. Compare the firmware version printed on the WBM Information → Version page with the version pinned in the device configuration (right-click device → Properties → General → Device information → Firmware). Any deviation is a candidate root cause.
2. Read the CPU's diagnostic buffer. In TIA Portal: Online → Online & diagnostics → Diagnostics buffer. Look for entries of type Communication error, IO device failure, PN IO AR abort, and any OB82 / OB83 / OB86 / OB122 activations. Time-stamp them against the last successful read of the C# service.
3. Read the SCALANCE X208 system log via WBM. Connect a service laptop to the switch, browse to https://<switch-ip>, log in, and open Information → System → Event log. Scroll to the timestamp of the last dropout. The X208 with V5.x firmware retains a ring buffer that records LLDP, PN-DCP, STP, and IGMP/ARP anomalies.
4. Run PRONETA in passive scan for 24 h. Start PRONETA in Network analysis mode and let it run over one full failure cycle. Verify that no Profinet station briefly drops from the network during the run.
5. Capture traffic on the CPU-facing port. Use Wireshark with the "PNIO" dissector on a mirror port of the SCALANCE. Filter on pn_dcp and look for unprovoked DCP identify requests or responses; these indicate a Profinet supervisor mismatch.
6. Check the OB1 / OB82 time stamps. Compare the OB82/OB86/OB122 execution times with the service dropout time. If OB82 fires only on the C# service's connection and not on the HMI's connection, the issue is on the external S7 channel, not on Profinet IO.
7. Validate the C# service. Temporarily disable the S7netplus-based service and let the system run for 24 h with only WinCC. If the failure no longer occurs, the issue is on the S7 connection policy of the external service. If the failure still occurs, the issue is on the switch or the controller.

Resolution Procedure

Once the firmware mismatch is confirmed, perform the following actions in the order given. All actions require a coordinated maintenance window because they trigger Profinet AR aborts.

1. Update the TIA Portal device configuration to the actual firmware. Open the Devices & Networks view, select the SCALANCE X208, open Properties → General → Catalog, and reassign the device to the firmware that is currently running on the physical device (V5.2.8 in this case). Compile the project.
2. Update the CPU firmware if required. The CPU is on V2.9.7. Siemens recommends keeping the S7-1500 CPU firmware at the same major release as the TIA Portal project. TIA V17 supports S7-1500 CPU firmware up to V3.1.x. Updating to V3.1.5 is a supported step that brings the controller into the same firmware family as recent SCALANCE V5.x line devices. Use the Siemens Support file S7-1500 CPU firmware update V3.1.5 from the 6ES7517-3UP00-0AB0 support page. Perform the update in RUN via the SIMATIC Automation Tool or in STOP via the online & diagnostics menu.
3. Update the SCALANCE X208 firmware via TIA Portal. In the device configuration, right-click the SCALANCE → Update firmware. TIA Portal will transfer the V5.2.8 firmware image that is already present in the project to the live device, ensuring the engineering and runtime firmware views are now consistent. The switch reboots during this operation.
4. Verify that the ET200SP with WP321 has the latest firmware. The WP321 weighing module firmware is part of the same Profinet migration. Bring it to the revision specified in the TIA V17 hardware catalog to remove any residual cross-device firmware skew.
5. Recompile and re-download the PLC program. After the firmware harmonisation, recompile the TIA V17 project and download the hardware configuration and software to the CPU. This forces the controller to re-establish ARs with all Profinet devices, including the SCALANCE.
6. Restart the C# service and the WinCC Runtime. Bring the HMI panel back up. Verify that the WinCC connection establishes within 30 s and that the S7netplus service can read at least one tag without error.

Verification

After the harmonised firmware download, validate the system against the original symptom:

1. 24-hour no-fault run. Leave the line running with the C# service active. Confirm that no S7 connection is lost over a continuous 24 h period. The original symptom was a dropout every 4 to 12 h, so 24 h is the minimum for a clean verdict.
2. Stress test the network. Increase the C# service poll rate by a factor of five for 1 h. Confirm that the cycle load due to communication remains below the configured ceiling (TIA Portal default ceiling is 50 %). This validates that the system has headroom for traffic bursts.
3. Verify PN/IE_1 HMI connection. In TIA Portal: Online & diagnostics → Communication, check the status of the HMI connection. The "HMI connection" should show "Connected" and stay "Connected" throughout the test.
4. Validate the diagnostic buffer. After the 24 h test, read the CPU's diagnostic buffer. No entries of type Communication error or AR abort should be present.
5. Validate the SCALANCE log. Read the SCALANCE event log. No PN-DCP or LLDP errors should be present. Any warnings about the ring buffer should be reviewed against the configured log retention.
6. Validate the C# service logs. The service should log a continuous series of successful S7 reads, with no TCP resets or timeouts. If timeouts do occur, the S7netplus keep-alive and reconnect parameters must be tuned, not the network.

Preventive Measures and Network Hardening

Several measures reduce the probability of a recurrence and accelerate the next investigation if one happens.

Maintain a firmware registry

For every Profinet device, keep a single source of truth in a spreadsheet or in the TIA Portal project header. The columns should at minimum include: order number, firmware version (project), firmware version (live), date of last update, and the engineer responsible. This single document catches drift between engineering and physical worlds long before it causes a fault.

Use PRONETA for periodic audits

Run PRONETA at least once per quarter in network analysis mode. Export the inventory and diff it against the firmware registry. PRONETA reports each device's firmware and its configuration mismatches.

Separate Profinet from office traffic

The HMI panel currently carries both VLAN1 (office/SQL) and VLAN2 (Profinet) on the same physical machine. This is a legitimate design, but the SCALANCE must be configured to keep Profinet frames in their VLAN with the PVID set to the Profinet VLAN on the CPU-facing port. Verify under Layer 2 → VLAN → Port-based VLAN in the SCALANCE WBM.

Set a sane PN/IE cycle and watch the cycle load

The cycle load due to communication was reported at 30 % on this CPU. S7-1500 CPUs can be configured for a higher share of the OB1 cycle dedicated to communication, but the default of 20 % is conservative. If Profinet traffic is going to grow, raise the share in Properties → PROFINET interface → Real-time settings → Communication load in 5 % steps and re-measure with PRONETA.

Replace the X208 with a higher-tier switch when Profinet IO grows

The SCALANCE X208 is an 8-port unmanaged/lightly managed switch suitable for small Profinet cells. Once Profinet IO is expanded to multiple devices and the port utilisation on the CPU-facing port is consistently above 30 %, an upgrade to a SCALANCE XC-200, XB-200, XR-300, or XC-300 is the recommended path. See the comparison on the SCALANCE Product Overview in the Siemens Industry Online Support.

Related Hardware Migration Considerations

The PROFIBUS-to-Profinet migration is a likely catalyst for the fault. When ET200M on PROFIBUS with Siwarex FTA was replaced by ET200SP with WP321 on Profinet, three things changed simultaneously:

1. The Profinet AR topology gained a new device (WP321) that uses the full PROFINET IO specification including the IRT options for high-resolution time synchronisation.
2. The SCALANCE X208 was updated from V4.5 to V5.2.8 in the field, presumably to gain PN-DCP filter and improved ring detection features.
3. The cycle load and traffic profile on the CPU-facing port changed from a single dominant PROFIBUS gateway to a Profinet IO controller with multiple ARs and a parallel S7 connection from the C# service.

All three changes are individually safe. Combined, they create a state in which any firmware skew between the engineering view and the physical view is exposed. The mitigation is a coordinated firmware and configuration update after every Profinet topology change. Document each migration with a "before/after" firmware matrix and require both the project and the live devices to be in the same firmware train before sign-off.

When to escalate to SINEC NMS

For installations with more than 20 Profinet devices, manual WBM access to each SCALANCE becomes impractical. Siemens SINEC NMS is the central network management system that polls all SCALANCE devices, harvests their firmware versions, raises alarms on configuration drift, and provides topology visualisation. Deploying SINEC NMS at the start of a Profinet migration is the most cost-effective preventive measure available.

Frequently Asked Questions