Resolving S7 Connection Overload Recipient Alarms in PCS 7 AS-Wide Interconnections

In SIMATIC PCS 7, persistent alarms reporting 'S7 connection overload recipient' across AS-wide interconnections between S7-400 controllers occur when the receiving CPU's communication processing capacity is overwhelmed by high-frequency transmission blocks. Even if NetPro shows healthy connections, execution scan rate mismatches or insufficient communication load allocation trigger buffer overflows on the receiver. To resolve this, increase the CPU's Communication load setting to 40-50% in Hardware Config, harmonize cyclic OB execution rates between sender and receiver, and bundle interconnections.

Mechanics of AS-Wide Interconnections in SIMATIC PCS 7

In SIMATIC PCS 7 multiproject architectures, controllers (such as CPU 414-5H MLFB 6ES7414-5HM06-0AB0 or CPU 410-5H MLFB 6ES7410-5HX08-0AB0) frequently need to share real-time process data across plant units. In CFC, engineers establish AS-wide interconnections by connecting the output of a block in one controller (e.g., AS1/ChartA/Block1.Out) directly to the input of a block in another controller (e.g., AS2/ChartB/Block2.In).

During compilation, the CFC compiler automatically creates the underlying communication architecture:
1. It instantiates internal communication driver blocks (IK_SEND and IK_RCV).
2. It generates S7 connection resources in NetPro over the Plant Bus (Industrial Ethernet).
3. It packages signals into coordinated communication telegrams transmitted cyclically.

When the receiving controller is inundated with incoming S7 communication frames faster than its operating system can transfer them into the receiving data blocks, the communication buffer overflows. The operating system triggers the diagnostic alarm:
S7 connection overload recipient

This condition occurs even when NetPro reports that the S7 connection is fully established and redundant, because NetPro only verifies physical layer link connectivity, not runtime FIFO buffer saturation.

Communication Parameters and Overload Threshold Matrix

Evaluate how controller scan rates and communication allocations impact connection stability:

Controller Configuration ParameterDefault System SettingRecommended Setting for Heavy Cross-AS TrafficImpact on Communication StabilityCPU Cycle: Communication Load20%40% to 50%Increases CPU time dedicated to processing incoming S7 packetsSending Task Execution CycleFast cyclic OB (e.g., OB 32: 100 ms)Coordinated OB (e.g., OB 34: 200 ms)Throttles packet generation to match receiver processing rateReceiving Task Execution CycleSlow cyclic OB (e.g., OB 33: 500 ms)Fast/Matching OB (e.g., OB 34: 200 ms)Prevents receiver FIFO buffer from accumulating stale framesSignal Bundling StrategyIndividual scalar interconnectionsStructured arrays or bundled clustersReduces packet header overhead and connection multiplexingMaximum Number of InterconnectionsUnlimited per projectMaximum 200 to 500 per AS pairExceeding limits saturates S7 communication credit limits

System configuration notice: Higher communication load settings slightly extend OB 1 background scan times.

Adjusting CPU Communication Load and Cycle Clock Settings in HW Config

To allocate sufficient operating system processing time to incoming communications:

Open Hardware Configuration on Receiving CPU:
* In SIMATIC Manager, open HW Config for the recipient controller.
* Double-click the CPU module to open Properties - CPU.
Increase Communication Load Allocation:
* Navigate to the Cycle/Clock Memory tab.
* Locate the parameter: Scan cycle load from communication [%] (Zyklusbelastung durch Kommunikation).
* By default, this is configured at 20%.
* Increase the value to 40% (or up to 50% for controllers with extensive cross-AS data traffic).
* This reserves guaranteed CPU execution slices between cyclic interrupt OBs to empty the communication FIFO buffers.
Verify OB Scan Execution Priorities:
* Under the Cyclic Interrupts tab, verify that the execution priority of the receiver OB (e.g., OB 34) is higher than background tasks.
Save, Compile, and Download:
* Save and compile HW Config.
* Download the updated configuration to the CPU in STOP or via CiR.

Harmonizing Sender and Receiver Cyclic OB Rates and Bundling Interconnections

The primary cause of receiver overload is execution scan rate mismatch:
1. Match Sender and Receiver Runtime Groups:
* Open CFC Editor on the sending AS. Inspect the runtime group of IK_SEND.
* If IK_SEND executes in OB 32 (100 ms), the sender transmits packets every 100 ms.
* If IK_RCV on the recipient AS resides in OB 33 (500 ms), the receiver only empties the buffer once every five incoming packets, causing a persistent queue overflow.
* Resolution: Move the sending and receiving runtime groups into identical cyclic interrupt OBs (e.g., both in OB 34 executing every 200 ms).
2. Bundle Scalar Signals into Structured Types:
* Replace dozens of individual binary interconnections with a single packed DWORD or an APL structured signal.
* Bundling consolidates multiple individual communication transactions into a single S7 packet, reducing CPU interrupt overhead by up to 80%.

Verification

Verify that connection overload is resolved:
1. Open the WinCC Alarm Logging list on the Operator Station.
2. Confirm that the alarm S7 connection overload recipient transitions from Incoming to Outgoing and clears from the active alarm screen.
3. In SIMATIC Manager, select the receiving AS, open PLC > Diagnostic/Setting > Module Information > Diagnostic Buffer.
4. Confirm zero recurrence of communication overflow events.
5. In CFC, monitor cross-AS values online: Confirm that variables update continuously with zero packet loss or lag.

Limitations and Operational Precautions

Never set the CPU communication load above 50%; allocating more than 50% to communication can cause watchdog trip timeouts on time-critical cyclic interrupt organization blocks (OB 30-38).
In fault-tolerant S7-400H systems, AS-wide interconnections must use fault-tolerant S7 connections configured in NetPro to ensure communication survives a CPU switchover.
Modifying cyclic interrupt execution times in HW Config requires recompiling the CFC charts and executing a program download.

Engineer FAQ

Why does NetPro show connection OK while the CPU logs an overload alarm?

NetPro monitors the physical layer and transport protocol connection state (TCP/IP handshake). The 'overload recipient' alarm is an application-layer FIFO buffer overflow inside the CPU operating system.

Can I use BSEND/BRCV instead of AS-wide interconnections?

Yes. For transmitting large blocks of telemetry or batch recipes, using BSEND/BRCV with custom handshakes provides superior throughput and buffer management compared to AS-wide interconnections.