S7-1500 Fault 16#39C3 — PROFINET IO Communication Error

Overview

The diagnostic address event 16#39C3 on a Siemens S7-1500 PLC indicates a PROFINET IO communication error, specifically a loss of the Application Relation (AR) between the CPU (configured as the PROFINET controller) and a distributed IO device. This code represents a communication connection failure where consecutive cyclic data packets are missed beyond the configured watchdog limit, causing the controller to mark the remote node as offline. It points directly to a layer 1 (physical) or layer 2 (data link) failure in your industrial Ethernet infrastructure.

Symptoms

When standard or safety-critical industrial networks trigger the 16#39C3 fault, several immediate physical and software symptoms present themselves across the plant floor:

• Red LED Indicators on the CPU: The SF (System Fault) or ERROR LED on the Siemens S7-1500 CPU lights up solid or flashes red.
• Flashing LEDs on I/O Stations: The BF (Bus Fault) LED on distributed interface modules (such as ET 200SP, ET 200MP, or ET 200AL) flashes red, indicating they are no longer in cyclic communication with the master PLC.
• TIA Portal Diagnostics Buffer Entry: An active entry in the diagnostic buffer reading: "IO device failure - Application Relation (AR) closed" accompanied by the Hex error code 16#39C3.
• Unexpected Process Shutdowns: Outputs connected to the failed node instantly drop to their configured "Safe State" (fail-safe mode, typically 0V DC), halting machinery or triggering safety interlocks.
• CPU Mode Switch (Optional): If diagnostic Organization Blocks (OBs)—specifically OB86 (Loss of Rack/Station) or OB122 (I/O Access Error)—are not compiled and loaded into the S7-1500 CPU, the processor will immediately drop from RUN to STOP mode, halting the entire control system.

Possible Causes

Identifying the root cause of the 16#39C3 diagnostic code requires examining both physical physical components and software configuration parameters. Common causes include:

• EMI / Electrical Interference: High-voltage cable trays (480V AC motor lines or VFD output wiring) run parallel to green PROFINET Ethernet lines without adequate physical spacing or shielding, distorting high-frequency Ethernet frames.
• Faulty or Poorly Terminated Connectors: Loose RJ45 FastConnect plugs, broken retaining tabs, or poor shielding connection to the metal connector body at the IO module or switch port.
• Exceeded PLC Watchdog Limits: The configured update time (e.g., 2ms) combined with a low watchdog factor (e.g., 3 missed cycles) is too tight for the network topology, causing a timeout during brief periods of high network traffic.
• Mismatched IP Addresses or Device Names: The PROFINET Device Name assigned in the TIA Portal hardware configuration does not match the actual string name written permanently to the IO device's memory.
• Faulty Industrial Network Switches: Poorly configured or non-conformance-class managed switches dropping multicast or PROFINET cyclic Real-Time (RT) frames.
• Mechanical Cable Fatigue: Damaged copper cores or micro-fractures in fiber-optic lines on drag chains, robotic joints, or rotary unions.

Step-by-Step Troubleshooting

To safely isolate and repair the issue causing error 16#39C3, execute these structured troubleshooting steps in sequence:

Step 1: Examine the TIA Portal Diagnostic Buffer

Connect your field pg or laptop to the S7-1500 CPU via PG/PC interface. Open TIA Portal, select your CPU in the project tree, and choose Online & Diagnostics. Open the Diagnostic Buffer tab. Locate the event with the code 16#39C3. Double-click the event to read the exact details: note the hardware identifier (HW ID), the IP address, and the precise timestamp. This pinpointing identifies exactly which PROFINET IO device (e.g., an ET 200SP IM module at node 5) dropped off the network.

Step 2: Extract Network Diagnostics via PRONETA

Download and run the free Siemens PRONETA software tool on your programming laptop. Connect directly to the plant network. Run a network analysis to view the actual online network topology. PRONETA will instantly show you if the target node is accessible via Ping, what its current IP address is, and whether the programmed Device Name is identical to the one currently assigned.

Step 3: Inspect Physical Layer Termination and Earth Shielding

Shut down high-voltage power lines nearby and safely open the control panel housing the disconnected IO device.

• Inspect the RJ45/M12 plug on both ends. Ensure the Siemens FastConnect stripping tool was used correctly; the braided copper shield of the industrial Ethernet line must make clean contact with the metal housing of the RJ45 jack.
• Check for tight bending radiuses (minimum bend radius for solid standard PROFINET cable is 30mm for nested runs; 150mm for flexible/moving runs).
• Verify that functional earth (FE) rails are connected properly to the DIN rails, and shield clamps are installed at input points to block stray electromagnetic fields.

Step 4: Analyze Network Port Statistics

If your network incorporates managed switches (such as Scalance series), log into the switch's web-based management (WBM) interface. Navigate to the port diagnostic pages. Track the error frame counters (CRC Errors, Alignment Errors, and Discards). A steadily climbing counter on a specific port shows you exactly which cable segment is physically corrupted.

Step 5: Adjust the Communication Update Times in TIA Portal

If your network has a high line depth (multiple IO devices daisy-chained inline), the default update times might be too demanding.

1. Open TIA Portal, navigate to Device Configuration -> Network View.
2. Select the PROFINET line and locate the failing device properties.
3. Under PROFINET Interface [X1] -> Advanced Options -> Real-time settings, change the update time configuration from "Automatic" to "Manual".
4. Increase the Update Time (e.g., from 1ms to 4ms or 8ms) and increase the Watchdog Factor (e.g., from 3 cycles to 6 or 8). This gives the PROFINET network breathing room to handle momentary signal jitter without dropping connection.

Recommended Actions

To prevent recurrence of the 16#39C3 communication error, implement these operational practices:

• Isolate Communication Lines: Never route green PROFINET cabling alongside 400V/480V AC motor power cables or frequency converter feeds. Maintain at least a 10cm to 20cm air gap, or use solid metal partition conduit layouts.
• Install Diagnostic OBs: Save your system from unannounced downtime. Compile and download OB86 (Submodule/Station failure) and OB122 (I/O access error) to your CPU program. This ensures that when a node drops (fault 16#39C3), the CPU records the fault and continues running other unaffected processes instead of entering STOP mode.
• Standardize Class B/C Switches: Avoid cheap, unmanaged office-grade Ethernet switches. Standardize on managed industrial switches that support PROFINET Conformance Class B or C to guarantee prioritization of Real-Time (RT) PROFINET traffic.

Recommended Replacement Parts

When a hardware component degrades and causes chronic communication drops, replace it with these tested industrial parts:

• Siemens ET 200SP Interface Module: 6ES7155-6AU01-0CN0 (IM 155-6 PN High Feature) – Ideal for high-vibration spots demanding stable throughput.
• Industrial Ethernet FastConnect RJ45 Plug: 6GK1901-1BB10-2AA0 (IE FC RJ45 Plug 180 2x2) – Standard 180° metal-housed connection.
• Siemens FastConnect Stripping Tool: 6GK1901-1GA00 – Ensures shields are cut to standard industrial scale every time without slicing signal wires.
• S7-1500 Standard CPU 1515-2 PN: 6ES7515-2AM02-0AB0 – Robust dual-interface controller for dynamic networks.

Related Articles

• /knowledge/replacement/siemens-et200sp-im155-replacement-guide
• /knowledge/compatibility/profinet-copper-vs-fiber-industrial-automation
• /knowledge/guide/tia-portal-diagnostic-buffer-analysis

FAQ

Q: Can a duplicate PROFINET Name cause error 16#39C3?

A: No, but it prevents the initial connection entirely. If a duplicate name is written to another device online, the controller might become confused during startup, triggering startup errors. A 16#39C3 error usually happens on a running, configured network where an existing connection drops due to signal quality issues.

Q: What is the recommended PROFINET watchdog factor for standard networks?

A: For standard industrial plants with low electromagnetic interference and standard switch layouts, a watchdog factor of 3 (meaning 3 missed cycles) is normal. On larger networks with deeper lines or wireless segments (like slip-rings power paths), increasing the watchdog limit to 6 or 8 avoids spurious faults.

Q: Why does the CPU go to STOP when 16#39C3 occurs, and how can I prevent it?

A: By default, if the S7-1500 CPU tries to read or write cyclic I/O data from a missing device and the diagnostic Organization Blocks (OB86 and OB122) are not loaded in the user program, the CPU immediately shuts down to a safe state (STOP). Adding these OBs to your TIA Portal project and downloading them to the CPU prevents this behavior.

Q: Can a bad PLC power supply cause a 16#39C3 PROFINET error?

A: Yes. If the 24V DC auxiliary power supplying the PROFINET remote interface module dropped below minimum operating voltage (typically 19.2V DC), the module will power down and drop the network link, triggering error 16#39C3 on the host controller.