Diagnosing S7 Connection Rejection and Connection Resource Exhaustion on S7-400 CPUs in PCS 7

When replacing an S7-400 CPU (e.g., upgrading from CPU 414-2DP to CPU 414-3DP or CPU 416) in a PCS 7 or CEMAT plant, an Operator Station (OS) or Server failing to connect while all other stations connect normally is caused by exceeding the maximum permitted AS connection resources or misallocating reserved connection quotas in NetPro and HW Config. Each S7-400 CPU possesses a strict hardware limit of simultaneous S7 connections (typically 32 on CPU 414 and 48 to 64 on CPU 416), partitioned between PG/ES, OP/OS, and S7 basic communication. To resolve this, inspect the CPU Diagnostic Buffer for error 16#4521 / 16#2522, verify configured connections in NetPro, adjust the reserved OP communication quota in CPU properties under 'Communication', and re-download the hardware configuration.

S7-400 Connection Resource Architecture and Hardware Limits

In SIMATIC S7-400 controllers, inter-station communication—including Operator Stations (WinCC OS Servers/Clients), Engineering Stations (ES), and peer-to-peer PLC communication—is executed over S7 Connections managed by the CPU operating system.

Every S7-400 CPU possesses a fixed hardware budget of communication connection instances (also referred to as connection resources or connection endpoints). This maximum is governed by the CPU hardware design, processor performance, and firmware version.

The total pool of connection resources is divided into two categories in the CPU configuration:
1. Reserved Communication Resources: Dedicated connection instances reserved exclusively for specific device classes to ensure critical nodes cannot be locked out:
- PG Communication: Reserved for Engineering Stations (STEP 7, CFC, monitoring).
- OP Communication: Reserved for Operator Panels and WinCC OS Stations.
- S7 Basic Communication: Reserved for non-configured communication blocks (I_PUT, I_GET).
2. Free (Unreserved) Resources: The remaining unallocated pool of connections available dynamically on a first-come, first-served basis to any configured S7 Named Connection or open communication channel.

Mechanism of Connection Exhaustion and Rejection

When an engineer replaces a CPU (e.g., replacing a legacy 6ES7414-2XJ01-0AB0 with 6ES7414-3XJ00-0AB0 or 6ES7416-3XR05-0AB0), connection rejections typically emerge due to three configuration oversights:

Failure to Compile and Download NetPro: When replacing a CPU in HW Config, the network routing tables in NetPro must be recompiled. If NetPro is not compiled, the newly inserted CPU does not receive the pre-configured S7 Named Connection table, forcing WinCC stations to compete for dynamic, unreserved connections.
Exceeding Total Hardware Connection Limits: In architectures such as CEMAT, redundant OS servers, engineering stations, and local operator workstations often connect directly to the CPU. If the total number of connection requests exceeds the CPU's hard limit, the last station to initialize after a reboot is rejected:
- Error code in WinCC: Connection disconnected or Channel Error: 0x0001.
- Error code in CPU Diagnostic Buffer: Event ID 16#4521: Communication connection cannot be established - Resource limit exceeded.
Misconfigured Architecture (Client Direct Connection): In standard PCS 7 and CEMAT architectures, OS Clients must connect to OS Servers over the Terminal Bus. If an OS Client is incorrectly configured to communicate directly with the AS controller over the Plant Bus, it consumes an additional AS connection resource, starving redundant OS servers of communication slots.

Step-by-Step Diagnostic and Resolution Procedure

To diagnose connection resource exhaustion and restore communication across all PC stations:

Step 1: Inspect the CPU Diagnostic Buffer

In SIMATIC Manager, select the AS station.
Select menu PLC > Module Information.
Select the Diagnostic Buffer tab.
Filter for communication events. Look for:
- Event ID 16#4521 or 16#4522: S7 connection establishment refused due to resource exhaustion.
- Note the exact local and remote connection IDs indicated in the event details.

Step 2: Check Connection Allocations in NetPro

In SIMATIC Manager, open NetPro.
Select the CPU in the graphical network view.
In the lower table, inspect the list of all configured S7 connections.
Check the Status column: Connected stations appear green; disconnected or faulted connections show error indicators.
Review the total count of configured connections against the CPU's rated capacity.

Step 3: Adjust Reserved Connection Resources in HW Config

Open HW Config for the AS station.
Double-click the CPU to open its Object Properties.
Select the Communication tab (or Cycle/Clock Memory on older firmware).
Review the connection allocation table:
- Increase Reserved for OP communication to match the total number of connected WinCC OS servers and single stations (e.g., set to 16 or 20).
- Ensure Reserved for PG communication has at least 1 or 2 instances reserved for the Engineering Station.
- Reduce S7 Basic Communication to 0 if non-configured blocks (I_PUT/I_GET) are not utilized.
Click OK, then select Station > Save and Compile.

Step 4: Full NetPro Compilation and Download

In NetPro, select menu Network > Save and Compile....
Select Compile and check everything. Ensure zero errors.
Select the AS station in NetPro and execute PLC > Download to Current Station > Selected Stations.
Restart the disconnected OS station's WinCC runtime channel to re-establish the link.

CPU Communication Resource Parameter Matrix

The following matrix compares connection capacities across common S7-400 CPUs used in PCS 7 and CEMAT plants:

S7-400 Controller MLFBStandard ModelMaximum Total S7 ConnectionsDefault Reserved OP ConnectionsDiagnostic Buffer Event on Overrun6ES7414-2XJ01-0AB0CPU 414-2DP (Legacy)32 connections1 reserved16#4521 / 16#45226ES7414-3XJ00-0AB0CPU 414-3DP32 connections1 reserved16#4521 / 16#45226ES7416-2XN05-0AB0CPU 416-2DP48 connections1 reserved16#4521 / 16#45226ES7416-3XR05-0AB0CPU 416-3DP64 connections1 reserved16#4521 / 16#45226ES7410-5HX08-0AB0CPU 410-5H Process AutomationUp to 120 connectionsScalable based on System Expansion Card16#4521 / 16#4522

Verification

To confirm that connection resource conflicts are permanently resolved:
1. Open NetPro online (Network > Open Online).
2. Select the CPU: In the connection table, verify that all S7 connections to all OS Servers, Clients, and ES stations show status Configured: OK (Green icon).
3. On the previously disconnected operator station (C_OWS1254), open WinCC Explorer and check the channel status under Tag Management > SIMATIC S7 Protocol Suite > Named Connections.
4. Confirm that the connection status indicates a green checkmark (Connected) with continuous packet exchanges.
5. In SIMATIC Manager, check the CPU Diagnostic Buffer: Ensure no new communication error events are logged during station startup.

Limitations and Operational Precautions

Observe the following engineering constraints:
- NetPro Changes Require Controller Download: Modifying connection tables in NetPro or altering reserved connection quotas in HW Config requires downloading system data blocks (SDBs) to the CPU. In non-redundant systems, this may require a brief CPU STOP.
- Redundant OS Servers Consume Two Connections: In redundant OS architectures, each server in the redundant pair (OS Server Master and OS Server Standby) establishes an independent S7 connection to the AS controller. A redundant server pair consumes twice the connection resources of a single station.
- CP 443-1 Multiplexing Limits: If communication passes through an Industrial Ethernet communications processor (CP 443-1), verify that the CP model also supports the required total connection quantity.

Engineer FAQ

Why did the new CPU 414-3DP reject the connection when the old CPU 414-2DP worked?

Both CPU 414 models share a hardware limit of 32 connections. However, when the new CPU was inserted, the reserved OP communication parameters reverted to factory defaults (1 reserved OP), leaving fewer unreserved resources for dynamic stations.

How many S7 connections does an OS Client consume?

In a properly architected PCS 7 multi-client system, an OS Client connects to the OS Server over the Terminal Bus and consumes ZERO connections on the AS CPU. If an OS Client is consuming AS connections, it was incorrectly configured as a Single Station.

Can I add a CP 443-1 to increase the CPU's total connection capacity?

No. While a CP 443-1 offloads network processing from the CPU backplane, the total number of simultaneous S7 connections is capped by the CPU operating system's internal connection table.