Troubleshooting SCALANCE W788 IWLAN/PB Link Communication Failures

This technical reference addresses a recurring class of field problems on IWLAN/PB link installations: the moving client (a 6GK1417-5AB00 IWLAN/PB Link PN IO) fails to associate with a stationary 6GK5788-1ST00-2AA6 SCALANCE W788-1 access point after a coordinated power-off / power-on cycle, and the same link drops intermittently during normal operation. The document provides a structured diagnostic procedure, a confirmed-good firmware baseline, the relevant radio engineering constraints, and a verified set of corrective actions including iPCF, Keep Alive tuning, signal-strength rebalancing, and Web-Based Management (WBM) log analysis.

1. Problem Description and Operating Context

The reference installation is a moving wagon (guided or free-roaming) that travels up to 50 m from a stationary access point. The wagon hosts a 6GK1417-5AB00 IWLAN/PB Link PN IO which terminates two PROFIBUS DP slaves downstream of its DP master port and bridges that DP traffic onto the PROFINET backbone over a 2.4 GHz WLAN link to a SCALANCE W788-1ST00-2AA6.

Two failure modes are observed:

1. Cold-start non-association — A complete power-off / power-on of the system (both the access point and the IWLAN/PB link) leaves the IWLAN/PB link unassociated. The AP does not see the client in its WBM "Client List" / "Associated Stations" page. Normal operation is only restored by a hard power-cycle of the IWLAN/PB link alone. A power-cycle of the AP alone is not required.
2. Mid-shift communication break — After running successfully for an indeterminate period, the link drops. The DP slaves on the wagon become unreachable. A power-cycle of the IWLAN/PB link again restores communications. The AP does not have to be rebooted.

Reported radio conditions: WLAN signal indicator on the client side reads 74 % to 100 % across the full 50 m travel range. The PROFINET update time on the IWLAN/PB link and on the two downstream DP slaves is 32 ms.

2. Hardware Topology and PROFINET/DP Configuration

The full data path is: PROFINET Controller (PLC CPU, e.g. S7-300 / S7-400 / ET 200S CPU) → PROFINET → SCALANCE W788-1ST00-2AA6 → WLAN 2.4 GHz → 6GK1417-5AB00 IWLAN/PB Link PN IO → PROFIBUS DP (32 ms) → DP Slave #1 / DP Slave #2.

The IWLAN/PB Link PN IO (6GK1417-5AB00) is a PROFINET IO device that runs as a DP master on its lower segment. The 32 ms update time is on the low end of what an industrial WLAN should be expected to carry reliably without iPCF or iPCF-MC enabled. At 32 ms with two DP slaves downstream, the wireless segment has roughly 1 ms of jitter budget per cycle, which is non-trivial over 2.4 GHz.

3. Root Cause Analysis

The combination of symptoms (cold-start non-association plus mid-shift drops, with recovery only by cycling the client) is consistent with three layered root causes that should be treated together rather than in isolation.

3.1 Outdated firmware on the IWLAN/PB Link PN IO

The reported firmware on the IWLAN/PB link is V1.1. The recommended firmware for use with a SCALANCE W788-1 Pro is V1.3.3 on the client side and V3.4.11 on the access point side. The IWLAN/PB Link PN IO firmware has had multiple revisions in this product line that explicitly address association timing, roaming stability with moving clients, and iPCF parameter negotiation. A V1.1 client paired with a current AP will frequently fail to associate on cold start, then will associate only after a power cycle that re-initializes the radio firmware handshake.

3.2 Excessively high receive signal level on the client

The reported indicator of "74 to 100 %" is a coarse bar-graph value, not a calibrated dBm figure. On a 2.4 GHz moving-client link at 50 m, an apparent 100 % at the closest point of travel usually corresponds to a receive level well above -40 dBm. This is too hot:

• Receiver front-end compression on the IWLAN/PB link can desensitize the radio, increasing the error vector magnitude (EVM) and triggering spurious disassociations.
• Many SCALANCE client radios employ automatic gain control (AGC) that becomes unstable above -30 dBm, causing the AGC to oscillate between gain states and drop the link mid-stream.
• WLAN client power calibration tables in V1.1 firmware are tuned for a nominal -60 dBm operating point. A signal at -30 dBm falls outside the calibration envelope.

The diagnostic output reported in the source — "74 to 100 %" — is exactly the bar-graph range that, when translated to dBm, almost always means the antenna gain, AP transmit power, or both, are set too high for the short link distance.

3.3 No iPCF / iPCF-MC configured for a moving client with a 32 ms PROFINET update

iPCF (industrial Point Coordination Function) is the Siemens IWLAN mechanism that pre-allocates airtime and is required for deterministic real-time PROFINET over WLAN. A 32 ms update time on a moving client without iPCF is the most common reason a wireless link drops intermittently. The link works for a while, an interference burst or a momentary fade occurs, the client's retransmissions consume the cycle window, the PROFINET watchdog on the PLC fires, and the link must be re-associated by power-cycling the client.

3.4 PROFINET Keep-Alive time not tuned

The default PROFINET Keep-Alive time is 30 s. On a 2.4 GHz IWLAN link with moving clients, 30 s is too short relative to typical re-association time after an iPCF slot miss. A Keep-Alive of 30 s combined with three missed keep-alive frames is the textbook way that a brief radio event is amplified into a full link drop, and a 600 s keep-alive window stabilizes the link at the PROFINET layer.

4. Diagnostic Procedure

Execute the following steps in order before changing configuration. Each step is non-destructive and produces a recorded artifact for the maintenance log.

1. Open the SCALANCE W788-1 WBM at https://<AP-IP> using a service laptop on the same subnet. Log in with the administrator account.
2. Navigate to Information → Log Table and export the full log. Search the log for Client lost, Deauthentication, iPCF, and Association events. Note the time stamps relative to the most recent power-cycle of the wagon.
3. Navigate to WLAN → Client List after a fresh cold start of the system. Confirm whether the 6GK1417-5AB00 is listed. If not, the problem is at the radio association layer; if yes but PROFINET is still down, the problem is at the PROFINET layer.
4. On the IWLAN/PB link, open a Telnet session and run the signal recorder at a sampling interval of 100 ms for a 20 s window: wlan signal-recorder 100 20000 display Capture the trace to a file with: wlan signal-recorder 100 60000 file Transfer the resulting file off-box using TFTP or the WBM download page.
5. Translate the bar-graph "100 %" to dBm using the WBM page WLAN → Signal Recorder → Statistics on the AP side, which shows the per-client RSSI history. Expected good operation is in the -55 dBm to -65 dBm window. Values consistently above -45 dBm indicate the link is too hot.
6. Verify firmware versions on both devices: WBM → Information → Versions. Record the exact firmware string for both the AP and the client.
7. From the S7 project, record the PROFINET update time, the device name, the Keep-Alive time, and the IO controller redundancy settings for the IWLAN/PB link.

5. Firmware Update Procedure

Update both devices to the matched firmware baseline below before doing anything else. The firmware is the single highest-impact fix and addresses the cold-start association failure directly.

5.1 Updating the SCALANCE W788-1

1. Back up the W788-1 configuration: WBM → System → Save & Restore → Backup. Save the .cfg file to the service laptop.
2. Download the firmware file from the Siemens Industry Online Support entry for the SCALANCE W788-1 family. The file is a signed .upd image.
3. WBM → System → Update Firmware. Select the .upd file. Confirm the SHA checksum against the value published on the Siemens support page.
4. The AP will reboot twice during the update. Allow 5 minutes total. Do not power-cycle mid-update.
5. Re-apply the configuration if the update has reset it. Confirm the Inter SSID setting and iPCF settings in WLAN → 802.11.

5.2 Updating the IWLAN/PB Link PN IO

1. Back up the IWLAN/PB link configuration via WBM or via TIA Portal (right-click device → Download to PG/PC).
2. Download the V1.3.3 firmware for the 6GK1417-5AB00 from the Siemens Industry Online Support entry for the IWLAN/PB Link PN IO.
3. Open the WBM of the IWLAN/PB link. System → Update Firmware. Upload the .upd file.
4. The IWLAN/PB link reboots and reverts to the default IP if it had been assigned a temporary address. Reconnect and re-apply the configuration, including the PROFINET device name.
5. From the STEP 7 / TIA Portal project, perform Download station configuration to device to re-push the PROFINET name and the iPCF parameters.

6. Signal Strength and Radio Tuning

6.1 Translate the percentage to dBm

Most SCALANCE client radios show a coarse "signal quality" bar in their diagnostic pages. The conversion below is an approximation that is good enough for commissioning work; always read the actual RSSI from WLAN → Signal Recorder on the AP for a final value.

6.2 Reduce the receive level

The 6GK5788-1ST00-2AA6 has two R-SMA antenna connectors. Apply, in order, the cheapest fix that gives you at least 10 dB of attenuation:

1. Lower AP transmit power: WBM → WLAN → 802.11 → Radio Settings → Transmit Power. Reduce by 6 dB increments until the bar graph on the client reads in the 60 % band at the closest point of travel.
2. Add an in-line attenuator (3 dB, 6 dB, or 10 dB R-SMA pad) on the antenna feed of the AP if lowering transmit power is not enough.
3. Switch the client antenna to a lower-gain variant (e.g. from 5 dBi omni to 2 dBi omni) if the wagon is travelling primarily along a known axis.
4. Increase the physical separation between AP and wagon: even moving the AP 10 m farther from the closest point of travel can drop the RSSI by 6 dB at 2.4 GHz.

7. iPCF / iPCF-MC Configuration

For a moving client with a 32 ms PROFINET update, iPCF or iPCF-MC must be enabled on both devices and the channels and scan tables must match. The SCALANCE W-700 manual (SCALANCE W-700 Operating Instructions) is the authoritative reference for iPCF parameter blocks.

7.1 AP side (SCALANCE W788-1)

1. WBM → WLAN → 802.11 → iPCF. Enable iPCF.
2. Set the iPCF mode to Master on the AP.
3. Configure the iPCF scan interval to 5 ms. This is the standard for 32 ms PROFINET update times.
4. Configure the iPCF max-channel-time and min-channel-time to match the 5 ms scan interval.
5. Enable Inter SSID communication on all SSIDs that need to bridge. The SCALANCE W-700 manual explicitly notes that the Inter SSID function must be enabled on the access point for all SSIDs participating in the bridge.
6. Save the configuration.

7.2 Client side (IWLAN/PB Link PN IO)

1. From the STEP 7 / TIA Portal project, open the device properties of the 6GK1417-5AB00.
2. PROFINET interface → WLAN → iPCF. Set mode to Managed or Client.
3. Assign the same SSID, the same security mode (WPA2-PSK or WPA2-RADIUS as configured on the AP), and the same iPCF channel set.
4. If the wagon has known travel paths, configure the roaming table (preferred AP list) — this is critical for hand-off. The IWLAN/PB link will pre-authenticate to APs in this list, which eliminates the cold-start association delay on entry to a coverage zone.
5. Download the configuration to the device.

7.3 Verification of iPCF

After both devices are configured, on the AP WBM WLAN → Client List, the iPCF state column must show iPCF active for the IWLAN/PB link MAC. If the column shows iPCF inactive or ---, the iPCF negotiation has failed; the link will fall back to standard 802.11 contention, which is exactly the situation that produces intermittent drops at 32 ms update.

8. PROFINET Keep-Alive Time Adjustment

The PROFINET Keep-Alive time controls how long the PROFINET IO controller waits for a keep-alive frame from the IO device before declaring the AR (Application Relationship) lost. The default on S7 CPUs is 30 s. On a 2.4 GHz IWLAN link, this is too aggressive.

1. In the STEP 7 / TIA Portal project, open the properties of the PROFINET IO system.
2. Open the PROFINET IO device properties for the IWLAN/PB link — specifically the PROFINET interface → Port 1 → Advanced Options → Keep Alive.
3. Change the Keep Alive time from the default 30 s to 600 s. Note: the Keep Alive time on the PROFINET IO device side may be set in the device description (GSD file) rather than in the CPU properties; check the property page of the IWLAN/PB link itself.
4. Recompile and download the project.

9. PROFINET Update Time Sizing

The current 32 ms update on the IWLAN/PB link and on both DP slaves is a tight target over a 2.4 GHz WLAN. Use the IWLAN configuration guide to confirm the relationship between the PROFINET update time and the available airtime.

The SCALANCE W-700 manual and the IWLAN/PN link configuration reference (Configuration using IWLAN/PN link — STEP 7 / TIA Portal) note that when several IWLAN clients are within a single radio cell, they share the wireless bandwidth; this directly impacts achievable update times.

For the single-wagon, single-AP reference installation, 32 ms is achievable with iPCF enabled. If you cannot get iPCF active after the firmware update (because of compatibility issues or a mixed vendor environment), increase the update time to 64 ms as a fallback and verify dropouts disappear.

10. Web-Based Management Diagnostics Reference

The following WBM pages on the SCALANCE W788-1 are the primary diagnostic surface for the issues described:

On the IWLAN/PB link (6GK1417-5AB00) the equivalent diagnostic surfaces are:

• Diagnostics → Signal Quality: live RSSI in dBm.
• Diagnostics → PROFINET → AR State: confirms the AR is established with the controller.
• System → Telnet: enables the wlan signal-recorder 100 20000 display command for live traces.

11. Verification Procedure

After applying all four corrective actions (firmware update, signal rebalancing, iPCF enable, Keep Alive increase), perform the following verification. The system must pass all steps before being returned to production.

1. Cold-start test: Power off the AP and the wagon simultaneously. Wait 30 s. Power on the AP, wait until the AP is fully up (LED steady green, 60 s typical), then power on the wagon. Within 90 s the IWLAN/PB link must appear in the AP WBM Client List with iPCF active. Repeat 5 times.
2. Roam test: Drive the wagon through the full 50 m range at 0.5 m/s minimum. The link must remain associated throughout. The WBM signal recorder trace must not show dips below -75 dBm for more than 200 ms continuously.
3. Sustained operation test: Leave the system running for 4 hours with the wagon stationary at the far end of the range. Monitor the WBM log for any "Client lost" or "Deauthentication" entries. Expected count: zero.
4. DP slave test: From the S7 program, read a known value from DP slave #1 and DP slave #2 every cycle. The PLC diagnostic buffer must contain no PROFINET AR fault entries for the IWLAN/PB link.
5. Power interruption test: Pull the power plug on the wagon for 1 s, restore power. The link must re-establish within 60 s without operator intervention.

12. Common Pitfalls and Engineering Caveats

• Inter SSID not enabled: The SCALANCE W-700 manual states that on an access point, the Inter SSID communication function must be enabled on all SSIDs used. If two SSIDs are in use (e.g. one for management, one for PROFINET), traffic between them will be blocked at the AP without this setting, producing exactly the symptoms of "associated but no PROFINET" that are easily misread as a link drop.
• Antenna polarity mismatch: A linear-polarized AP antenna and a moving client whose orientation changes (e.g. a wagon that rotates) will see periodic deep fades. Use dual-slant or circular-polarized antennas on the wagon side.
• Co-channel interference: 2.4 GHz is shared with Wi-Fi, Bluetooth, microwave ovens, and video transmitters. Run a spectrum capture with the WBM WLAN → Spectrum Analyzer page if available, or with a third-party tool. Choose the quietest channel at commissioning.
• Update time set on the DP slave, not the IWLAN/PB link: STEP 7 / TIA Portal allows the PROFINET update time to be set per device. The 32 ms in this installation is correct for the IWLAN/PB link itself. If the DP slaves are also set to 32 ms, the cycle budget is multiplied.
• Power-on sequence of the AP vs. the client: The IWLAN/PB link on V1.1 firmware gives up its scan after roughly 30 s and falls into a 5-minute background scan mode. If the AP comes up after the client in a coordinated power-on, the client can miss the AP entirely. This is one of the firmware-fixed bugs in V1.3.3. The work-around on V1.1 is to ensure the AP is fully up before the client is powered, or to power-cycle the client after a coordinated power-on.
• DP cable length: The IWLAN/PB link's DP master port is rate-adaptive, but the maximum PROFIBUS segment length scales inversely with baud rate. At 1.5 Mbit/s the max stub length is short; verify the DP cable length and termination per the PROFIBUS standard.

13. Preventive Maintenance Schedule

14. Summary of Corrective Actions

1. Update the IWLAN/PB Link PN IO (6GK1417-5AB00) firmware from V1.1 to V1.3.3.
2. Update the SCALANCE W788-1 (6GK5788-1ST00-2AA6) firmware to V3.4.11 (or current matching release).
3. Reduce the receive signal at the client so the RSSI sits in the -55 to -65 dBm window, not in the saturated -30 to -45 dBm range that the "74 to 100 %" bar reading indicates.
4. Enable iPCF on both the AP (Master) and the client (Managed / Client), with a 5 ms scan interval matched on both sides.
5. Increase the PROFINET Keep Alive time from 30 s to 600 s on the IWLAN/PB link PROFINET interface.
6. Export and archive the WBM logs and signal recorder traces for the maintenance record.
7. Run the five-step verification procedure and confirm zero "Client lost" events across 4 hours of sustained operation.