Resolving PROFINET Communication Failures on SCALANCE W700 WLAN

Problem Overview

On a wireless PROFINET network that pairs a SCALANCE W734-1 client (6GK5734-1FX00-0AB0) with a SCALANCE W774-1 access point (6GK5774-1FX00-0AB0) configured in access point mode, communication works reliably with a SIMATIC KTP700 Basic HMI but fails with PROFINET IO devices such as the ET 200 distributed I/O, RF180C RFID reader, and SINAMICS S110 drive. The S7-1500 CPU acts as the PROFINET IO controller on the wired side of the W734-1 client. The same client module, the same AP firmware, the same antennas, and the same static IP scheme are used in every test case; only the device attached to the AP changes.

The symptom is not a link-down event. The radio link is up, the client is associated, and an HTTP/TCP ping (when performed) is responsive. What fails is the PROFINET application relationship (AR) between the controller and the IO device. The CPU reports that the IO device cannot be reached, the device's status LED on the AP side stops blinking PROFINET identification, and the IO device is removed from the configured topology within seconds of a CPU restart.

This is a classic PROFINET-over-WLAN timing failure. The HMI tolerates the wireless latency because the KTP700 Basic PN exchanges cyclic PROFINET traffic at relatively long update times (16-32 ms is typical for a basic panel and the connection is robust to jitter). The ET 200, RF180C, and SINAMICS S110 each demand tighter deterministic timing; when the watchdog expires before the IO device responds, the controller drops the AR and the device enters a fault state.

Affected Hardware and Topology

The reference configuration used to reproduce the failure consists of the following components:

The 734/774 pair operates in bridge mode at Layer 2; PROFINET frames traverse the radio link as if both stations were on a shared Ethernet segment. The IP addressing is static on each Ethernet port of the SCALANCE W pair, and the W734-1 client is configured to attach to the W774-1 AP SSID with a pre-shared WPA2/AES key.

Root Cause: PROFINET Real-Time Constraints Over Wireless

PROFINET IO relies on a strict timing contract between the IO controller and the IO device. The contract is enforced by two related timers:

• Send clock (update time): the period at which the controller dispatches a new cyclic frame. Defaults: 1 ms for IRT, 1-4 ms for RT (depending on the device class).
• Watchdog time: the maximum interval the controller waits for a response from the IO device before tearing down the AR. The default is 3x the update time, which is too tight for a WLAN link that adds 5-30 ms of one-way delay plus retransmission latency when a packet is lost.

When a PROFINET frame is lost in the air, the SCALANCE W pair re-transmits it transparently. The retransmission, plus 802.11 acknowledgement and possible 802.11 contention back-off, easily extends the round-trip time (RTT) past the 3x update time watchdog. As soon as the watchdog fires, the CPU marks the IO device as failed and reports a station failure (diagnostic interrupt 0x001E / 0x001F and station-not-found / station-fault events in the CPU's diagnostic buffer).

The HMI survives this because HMI communication on the basic panel is a non-real-time PROFINET service. The cyclic HMI traffic is scheduled at a generous 50-100 ms update time, and an HMI timeout is recoverable - the panel simply requests the next data set. PROFINET IO devices, by contrast, drop their I/O channels the moment the AR is torn down and require either a power cycle or a controller-side AR re-establishment, which itself takes a full watchdog window.

A second, related root cause is the missing or non-deterministic MAC behaviour. SCALANCE W clients use the MAC-mode parameter to decide which MAC address the AP side advertises; if the client is in Automatic mode and the WLAN link is not yet up, the AP will not have the correct MAC in its bridge table and the first few frames from the controller will be flooded. The SCALANCE W774/W734 operating manual states that the client waits for the MAC address due to the setting "Automatic" for the MAC mode parameter and is connected only after the WLAN link is up. This is documented in the SCALANCE W774-1 / W734-1 operating instructions (PDF).

Solution Strategy

Resolve the failure in three coordinated steps, in this order:

1. Raise the PROFINET watchdog time on every IO device that traverses the WLAN link. This is the single most common fix and is the first action Siemens Support recommends in FAQ 109475919 (TIA Portal) and FAQ 109474533 (STEP 7 V5.5).
2. Enable the iPCF (industrial Point Coordination Function) feature on both the AP and the client. iPCF, together with iPCF-MC, converts the otherwise best-effort 802.11 MAC into a deterministic, centrally scheduled MAC that bounds the worst-case latency to a few milliseconds - exactly what PROFINET IO requires.
3. Confirm the PROFINET device name of every IO device attached behind the AP matches the name configured in the controller's PROFINET topology. A mismatch is the most common reason iPCF and the extended watchdog still fail to bring the AR up.

Apply the three steps together; raising the watchdog without iPCF will mask the immediate failure but leave the link vulnerable to bursts of radio interference, and enabling iPCF without the proper PROFINET naming will not create an AR at all.

Step-by-Step: Increase the PROFINET Watchdog Time in TIA Portal

Open the TIA Portal project that contains the S7-1500 station. In the project tree, expand the PROFINET IO device whose watchdog you want to change. The procedure below follows Siemens FAQ 109475919.

1. Select the IO device in the Project tree (e.g., ET 200SP > Head module > PROFINET interface).
2. Open Properties > PROFINET interface > Real time settings in the inspector window.
3. In the Update time dropdown, pick a value consistent with the wireless latency. A safe starting point is 32 ms (or 64 ms for IRT-class devices such as SINAMICS S110). 1 ms and 2 ms will not survive a WLAN retransmission and must not be used.
4. In the Watchdog time field, set a value at least 6-10x the chosen update time. With 32 ms update, a watchdog of 192 ms (6x) to 320 ms (10x) is reasonable; with 64 ms update, 384-640 ms.
5. Repeat for every PROFINET IO device that is connected through the W774-1 AP, including the RF180C and the SINAMICS S110 CU305 PN.
6. Compile the project and download the configuration to the CPU. The new values take effect on the next AR establishment.

Step-by-Step: Increase the PROFINET Watchdog Time in STEP 7 V5.5

If the controller is configured with STEP 7 V5.5 instead of TIA Portal, follow the procedure documented in FAQ 109474533:

1. Open HW Config in SIMATIC Manager.
2. Double-click the PROFINET IO device whose watchdog must be raised.
3. Select the PROFINET IO tab in the properties dialog.
4. Click Update time and select a value of 32 ms or 64 ms. Avoid 1 ms and 2 ms - they will trigger the same failure over WLAN.
5. Click Watchdog time and enter a value of at least 6x the update time (e.g., 192 ms for a 32 ms update, 384 ms for a 64 ms update).
6. Save, compile, and download the HW Config to the controller.

Reference: Recommended Update / Watchdog Pairings for PROFINET over WLAN

Step-by-Step: Enable iPCF on the SCALANCE W Pair

iPCF (industrial Point Coordination Function) is the Siemens proprietary enhancement to IEEE 802.11 that turns the SCALANCE W pair into a deterministic, slot-based wireless bridge. iPCF-MC (the management channel) carries the WBM traffic on a non-interfering channel. Both features must be enabled for PROFINET IO to work reliably over WLAN. Configuration is performed through the Web Based Management (WBM) of the SCALANCE W pair.

1. Open the WBM of the SCALANCE W774-1 AP (default URL: https://<AP-IP>).
2. Navigate to Layer 2 > iFeatures.
3. Set iPCF mode to On. The AP becomes the master of the cell; only clients that have iPCF enabled can attach.
4. Set iPCF-MC to On. This reserves a separate management channel for WBM and other non-real-time traffic.
5. Click Set, then Save and reboot the AP.
6. Open the WBM of the SCALANCE W734-1 client.
7. Navigate to Layer 2 > iFeatures and enable iPCF on the client. The client now negotiates iPCF on association.
8. Reboot the client and wait for the WLAN link to come back up.

Step-by-Step: Verify and Re-Assign the PROFINET Device Name

A PROFINET IO device is identified on the wire by its PROFINET name, not its IP address. The controller stores the device's name in the PROFINET topology editor and will only accept an AR from a device that announces exactly that name. A name mismatch is the most frequent cause of AR-establishment failure after the watchdog and iPCF have been corrected.

1. In TIA Portal, open Devices & Networks and select the IO device (ET 200, RF180C, or SINAMICS S110) that is failing.
2. Note the value in Properties > PROFINET interface > PROFINET device name. This is the name the controller expects (e.g., et200sp-hf-1).
3. Connect a PC directly to the AP's Ethernet port (or to the client via the wired side, or use a WLAN-capable engineering station).
4. Open Online & Diagnostics for the target IO device, switch to PROFINET device name, and click Assign name.
5. Type the exact string from step 2 (case-sensitive), select the IO device by its MAC address, and click Assign.
6. Power-cycle the IO device to force a re-arbitration with the controller.

The PROFINET LED on the IO device should now turn steady green (no flashing), and the IO device should appear in the CPU's online diagnostics as OK within one watchdog period.

SCALANCE W Device Modes and MAC Mode

SCALANCE W700 modules support three device modes. The choice of mode and the value of the MAC mode parameter on the client have a direct impact on PROFINET behavior over WLAN. The Editing properties and parameters (TIA Portal cloud docs) reference lists them as follows:

On the client, the MAC mode parameter can be set to Automatic, Manual, or Layer 2 tunnel. With Automatic, the client uses its own MAC address on the wireless side; the AP learns the MAC when the WLAN link is established. The SCALANCE W774/W734 manual notes that the client waits for the MAC address when MAC mode is Automatic and is connected only after the WLAN link is up. For PROFINET IO, set the client to Layer 2 tunnel if you need the WLAN to forward the controller's MAC transparently; otherwise, leave Automatic in place and accept the short delay during WLAN establishment. Avoid Manual unless the deployment is single-purpose and you understand the bridge-table implications.

Verification Procedure

After applying the three corrections, verify in this order:

1. Confirm both WBMs report iPCF: On and iPCF-MC: On and that the client shows connected, iPCF active in its status page.
2. Open the CPU's online diagnostics (Online & Diagnostics > Diagnostics buffer). The most recent entries should show that each IO device attached behind the AP was found and that no further station-fault or station-not-found events have been logged.
3. For each IO device, open Online & Diagnostics > PROFINET diagnostics and confirm the device is in state OK with the configured update time and watchdog reflected in the device's own PROFINET interface properties.
4. Force a WLAN interruption by briefly disabling the radio on the AP (or by walking the antenna out of range). The IO devices should report a station-down event, then recover automatically within one watchdog period once the radio is restored. If they do not recover, the watchdog is still too short - raise it and retest.
5. For the SINAMICS S110, open STARTER or the TIA Portal drive commissioning, go online, and confirm that PROFINET communication to the CU305 PN is established (the PROFINET LED on the CU is steady green, and STARTER reports Online via PROFINET).

Troubleshooting Matrix

Additional Reference: Industrial Wireless LAN in a PROFINET IO Environment

Siemens maintains a dedicated application document, "Industrial Wireless LANs in a PROFINET IO Environment", entry ID 22681042. The document covers:

• The full parameter set of iPCF, iPCF-MC, and iREF (industrial roaming) on SCALANCE W700.
• How to plan the cycle time budget for PROFINET IO across a wireless cell, including the formula T_watchdog >= 6 x (T_update + T_radio_latency_max).
• Compatibility matrices between SCALANCE W firmware versions and PROFINET device firmware.
• Roaming behaviour with iREF and how to size the overlap region between AP cells.
• Sample configurations for ET 200, RF180C, and SINAMICS S drives over WLAN.

Use this document as the primary reference when commissioning any new PROFINET-over-WLAN cell or when expanding an existing cell with additional IO devices.

Safety and Operational Notes

• PROFINET IO over WLAN is not a candidate for safety-related communication (PROFIsafe). The SINAMICS S110 with PROFIsafe, ET 200SP with PROFIsafe modules, and RF180C with safety blocks must not be operated over the wireless link for safety functions.
• Keep the SCALANCE W pair on a dedicated SSID with WPA2/AES and a strong pre-shared key. Do not mix PROFINET IO traffic with general IT traffic on the same SSID.
• Document the chosen update time and watchdog time in the project's network plan. A wireless PROFINET cell behaves differently from a wired cell during commissioning, and the values that work on a clean bench may not be sufficient once the cell is installed in a noisy industrial environment.

FAQ