C135 PCS 7 (PCS7)
6C
6 Cielo Ciencia y Tecnología
·Publicada en C135 PCS 7 (PCS7)

Diagnosing S7 Connection Rejection (Error 0xFFDF 4116) on Redundant WinCC OS Servers

Diagnosing S7 Connection Rejection (Error 0xFFDF 4116) on Redundant WinCC OS Servers

In SIMATIC PCS 7 redundant Operator Station (OS) architectures, when one OS server connects successfully while the partner server displays grayed-out controls, bad quality codes, and logs 'Cannot connect to S7$Program: Errorcode 0xFFDF 4116! (L4_CONN_REJECT)', the controller has actively rejected the S7 connection request. This error is caused by exceeding the maximum number of permissible configured S7 connections in the S7-400 CPU communication properties, or by mismatched rack/slot configuration between Station Configuration Editor and NetPro. To resolve this, increase the CPU's reserved S7 connection resources in HW Config, verify Named Connections in NetPro, and re-download connection routing.

Table of Contents

Architecture of Redundant Named Connections in SIMATIC PCS 7

In SIMATIC PCS 7, the plant bus links the Automation Stations (S7-400 or CPU 410-5H) to redundant Operator Stations (OS Server Main and OS Server Standby). Communication does not rely on generic polling; it operates using Named Connections managed by the SIMATIC S7 Protocol Suite:

  • Independent Connection Channels: Each OS server maintains its own independent physical and logical S7 communication channel to every S7-400 CPU. If a plant has one S7-400 AS and a redundant OS server pair, the CPU must sustain at least two separate S7 connections (one for OS Server 1, one for OS Server 2).
  • Named Connections (S7$Program): In NetPro, connections are defined between the SIMATIC PC Station application (WinCC Appl.) and the CPU's S7 Program. Connection endpoints, TSAP identifiers, and routing tables are compiled and downloaded to both the PC Stations and the controllers.
  • The Failure Symptom: In runtime, OS Server 2 connects normally and displays live values. On OS Server 1, however, all process graphics appear grayed out with yellow warning triangles (Quality Code 0x00 or 0x4C), and buttons cannot be operated. The diagnostic log SIMATIC_S7_PROTOCOL_SUITE_01.LOG reports fatal connection rejection.

Anatomy of Error 0xFFDF 4116 (L4_CONN_REJECT)

When WinCC initializes its communication channel, it issues an ISO transport connection request (Layer 4) to the controller. If the connection cannot be established, the driver records:

ERROR Cannot connect to 'S7$Program': Errorcode 0xFFDF 4116! Error 4116 - L4_CONN_REJECT

This error code indicates that the controller's communication processor or operating system received the connection request packet, parsed the header, and actively sent back a rejection frame. The two distinct technical causes are:
1. Connection Resource Limit Exceeded: The CPU has reached the ceiling of simultaneous S7 connections supported by its firmware or configured in its properties table.
2. Addressing / Parameter Mismatch: The request reached an S7-400 communication interface (e.g., CP 443-1), but the target CPU rack/slot addressing specified in the WinCC connection parameter does not match the physical hardware configuration.

The S7-400 Connection Resource Budget and Exhaustion Limits

Every SIMATIC S7-400 controller possesses a finite number of communication connection resources (e.g., 32 resources on CPU 414-3, 64 on CPU 416-3, and up to 120+ on CPU 417-4 / CPU 410-5H).

In HW Config > CPU Properties > Communication, resources are segregated into:
* PG Communication: Reserved for Engineering Stations / programming devices (typically 1 or 2).
* OP Communication: Reserved for local operator panels (Comfort Panels, mobile panels).
* S7 Basic Communication / Standard S7 Connections: Used for cyclic DCS communications.

Why the Redundant Server Fails:
Consider an S7-400 CPU with a limit of 32 connections. If 31 connections are consumed by Engineering Stations, other controllers, batch servers, and OS Server 2, then when OS Server 1 boots up and requests a connection, the CPU has zero available connection slots. It immediately issues 0xFFDF 4116 (L4_CONN_REJECT). OS Server 2 continues operating normally because its connection was negotiated first.

Station Configuration Editor and NetPro Slot Mismatches

A secondary, highly prevalent cause of 4116 L4_CONN_REJECT is local configuration desynchronization on the affected OS Server:

  • The Virtual Rack: The Station Configuration Editor on the physical OS Server machine acts as a virtual S7 rack. The WinCC Application is typically placed in Slot 1, and the network card (IE General or CP 1623) resides in Slot 3 or Slot 4.
  • The NetPro Configuration: In the engineering multi-project, the SIMATIC PC Station must mirror this exact slot numbering and station name.
  • The Failure Mechanism: If an engineer configured IE General in Slot 3 in Station Configuration Editor, but NetPro was compiled with IE General in Slot 4, the communication endpoint TSAP will be miscalculated. The S7-400 CPU rejects the connection because the requesting application credentials do not match any authorized connection in the compiled AS connection table.

Diagnostic Error Code Matrix for SIMATIC S7 Protocol Suite

The following table lists common error codes found in SIMATIC_S7_PROTOCOL_SUITE_01.LOG and their root causes:

Error CodeError DescriptionRoot Cause0xFFDF 4116L4_CONN_REJECTCPU connection resources exhausted, or rack/slot addressing mismatch0xFFDF 410EL4_CONN_TIMEOUTPhysical network cable disconnected, firewall blocking TCP port 102, or wrong IP0xFFDF 4104L4_ACCESS_DENIEDCPU protection level active; read/write access password required0xFFDF 4001L4_NO_MEMORYWinCC host PC ran out of internal non-paged pool memory for S7 drivers0xFFDF 4115L4_PORT_UNREACHABLES7ONLINE access point misconfigured in Set PG/PC Interface

Step-by-Step Procedure to Resolve S7 Connection Rejections

Follow this verified engineering procedure to resolve 4116 L4_CONN_REJECT errors:

  1. Audit Available CPU Connections in SIMATIC Manager:
    * In SIMATIC Manager, select the CPU and press Ctrl+D (or right-click > PLC > Module Information).
    * Navigate to the Communication tab.
    * Inspect the table: Compare Configured connections and Maximum connections.
    * If available connections are at maximum, connection exhaustion is confirmed.
  2. Increase CPU Connection Resources in HW Config:
    * Open HW Config for the S7-400 AS.
    * Double-click the CPU to open Object Properties > Communication tab.
    * Adjust connection quotas to ensure sufficient unreserved or configured connections exist for all OS servers.
    * Save, compile, and download HW Config to the CPU in a planned maintenance window.
  3. Verify Station Configuration Editor Matching:
    * On the failing OS Server machine, open Station Configuration Editor.
    * Verify the exact slot numbers:
    • Slot 1: WinCC Appl.
    • Slot 3: IE General (or CP 1623)
    • Verify that the Station Name matches the PC Station name in SIMATIC Manager exactly (case-sensitive).
  4. Verify Set PG/PC Interface:
    * Open Set PG/PC Interface on the OS Server.
    * Ensure the S7ONLINE access point is set to PC internal.local.1.
  5. Recompile NetPro Connections and Download:
    * In SIMATIC Manager, open NetPro.
    * Select Network > Save and Compile > Check and compile all.
    * Select the failing OS PC Station, click Download to Current Project > Selected Stations.
    * Open WinCC Explorer on the OS Server and re-activate Runtime.

Verification

Verify that communication is healthy across both redundant servers:
1. On OS Server 1, confirm that all process graphics display active dynamic values and all gray overlays disappear.
2. Open WinCC Channel Diagnosis (WinCCChannelDiagnosis.exe):
* Select the Named Connections channel.
* Verify that every AS connection displays a green checkmark with status "Ready" or "Connected".
3. Inspect <WinCC_Installation>\Diagnostics\SIMATIC_S7_PROTOCOL_SUITE_01.LOG:
* Confirm that new log entries show Connection to 'S7$Program' established successfully with zero 4116 rejection errors.
4. Verify WinCC Redundancy synchronization: Confirm that both servers report runtime status Normal without synchronization alarms.

Limitations and Operational Precautions

  • CPU Connection Ceiling: Physical S7-400 hardware models have an absolute, non-expandable limit on communication resources (e.g., CPU 414 cannot exceed 32 connections regardless of settings). If your plant requires more connections than the CPU hardware permits, you must upgrade the CPU (e.g., to CPU 416, 417, or CPU 410-5H) or route connections through CP 443-1 communications processors.
  • Downloading HW Config (STOP Mode): Changing CPU connection property tables in HW Config cannot be downloaded in RUN mode on standard controllers; it requires transitioning the CPU to STOP mode. Plan downtime accordingly.
  • Redundant Server Independence: Never attempt to share a single Named Connection between two servers. Redundant servers strictly require dedicated connection instances in NetPro.
  • Commercial Accuracy: Ensure that the S7-400 hardware (6ES7410-5HX08-0AB0) and WinCC Redundancy package (6AV6371-1CF00-0AX0) are licensed according to manufacturer specifications. Obtain quotation verification from an authorized Siemens distributor.

Engineer FAQ

Why did the redundant server connect while the main server was rejected?

The redundant server initiated its network connection first, seizing the last available S7 communication resource in the CPU. When the main server attempted to connect moments later, the CPU had zero free resources remaining, rejecting the connection.

Can CP 443-1 communications processors offload S7 connections from the CPU?

Yes. An industrial Ethernet CP 443-1 module (6GK7443-1EX30-0XE0) handles low-level transport protocols and provides additional multiplexed S7 connections, relieving the main controller backplane of network processing overhead.

What does the Quality Code 0x4C mean on WinCC graphic elements?

Quality Code 0x4C represents 'Uncertain / Initial Value'. It indicates that WinCC has established the tag in memory but has never received an initial valid cyclic data response from the PLC, typically because the channel connection is severed or rejected.

Does S7-PLCSIM have connection resource limits like physical hardware?

S7-PLCSIM emulates communication connections, but does not strictly enforce the same hardware connection ceilings as physical CPUs, which is why connection exhaustion problems often remain hidden until commissioning on physical hardware.

Technical References

Related Articles

Related Tags

  • Named Connections
  • S7 Protocol Suite
  • Error 4116
  • L4_CONN_REJECT
  • WinCC Redundancy
  • S7-400 Communication
6 respuestas