Troubleshooting AS-AS Cross-Controller Communication and S7 Interconnection Failures in PCS 7

In SIMATIC PCS 7, when an AS-wide interconnection between two S7-400 or CPU 410-5H controllers fails to exchange data—despite NetPro indicating a healthy, running S7 connection (green setup status)—the issue typically stems from omitted chart compilation and downloads on the participating AS stations, execution within fast cyclic interrupt OBs exceeding buffer limits, or misaligned connection resource IDs (R_ID). NetPro only establishes the low-level transport layer; actual data transmission requires generating and executing Basis Library communication blocks (such as IK_SEND and IK_RCV). To resolve the failure, compile the Charts folder with 'Generate module drivers' and download to both controllers, verify that communication blocks execute in OB34 (200 ms) or slower (OB35/OB36), and confirm that connection IDs match across the multiproject.

Executive Summary

In multi-controller PCS 7 architectures, distributed process units must frequently exchange interlocks, permissive signals, and flow measurements across separate Automation Stations. PCS 7 provides the 'AS-wide Interconnection' feature, allowing control engineers to wire block pins between charts in different AS stations as if they resided in the same controller. However, commissioning teams frequently encounter deadlocks where NetPro reports a healthy green connection, yet interlocked values fail to transfer. Understanding the multi-layer dependency between NetPro transport routing, CFC communication driver block generation, and organization block scheduling resolves cross-controller communication halts.

Architecture and Fundamental Mechanics

The communication architecture of PCS 7 AS-wide cross-controller interconnections operates across three distinct system tiers:

The NetPro Transport Layer:
* NetPro configures the physical and logical S7 connection (configured over Industrial Ethernet ISO-on-TCP or S7 protocol via CP 443-1 or CPU internal PN ports).
* The green status indicator in NetPro confirms only that the bilateral connection handshake has succeeded and that Layer 4 transport sockets are established.
The CFC AS-Wide Interconnection Mechanism:
* When an engineer drags a signal line from Chart A (in AS 1) to Chart B (in AS 2), CFC does not use raw S7 communication SFCs directly in user code.
* During CFC compilation with Generate module drivers, the compiler automatically instantiates Basis Library communication driver blocks:
- IK_SEND (FB 1051) in the sending AS.
- IK_RCV (FB 1052) in the receiving AS.
- These blocks are assigned matching internal communication identifiers (R_ID).
- If the engineer downloads NetPro but forgets to compile and download the Charts folder on both AS stations, the communication runtime blocks do not exist in controller memory, preventing data flow.
Organization Block Execution Limits (OB34 vs. OB35):
* Siemens PCS 7 specifications strictly require that AS-wide communication driver blocks run in cyclic interrupt organization blocks with cycle times of 200 ms (OB34) or slower (OB35, OB36).
* If blocks are assigned to fast interrupt OBs (e.g., OB32 20ms or OB33 50ms), the S7 communication buffer saturates, triggering packet drops, communication jitter, and data transmission freeze.

Step-by-Step Implementation Procedure

Follow these procedures to diagnose, configure, and restore cross-controller AS-AS communication in PCS 7:

Verify S7 Connection Status in NetPro:
* Open NetPro in SIMATIC Manager.
* Select the CPU in AS 1; in the connection table, verify the S7 connection to AS 2.
* Select PLC > Activate Connection Status (online).
* Confirm that the connection status reports Set up successfully (green indicator).
* If disconnected, download connections via PLC > Download to Current Project > Selected Stations.
Verify Organization Block Execution in CFC Run Sequence:
* In SIMATIC Manager, open the CFC editor for AS 1.
* Open the Run Sequence editor (Edit > Run Sequence).
* Locate the automatically generated system charts (typically named @... or containing IK_SEND).
* Ensure these blocks are scheduled in OB34 (200 ms) or OB35 (100 ms / 500 ms). Communication blocks must never be scheduled in high-frequency OBs under 100 ms.
* Repeat this verification in AS 2 for IK_RCV.
Execute Full CFC Compilation with Driver Generation on Both AS Stations:
* In SIMATIC Manager, select the Charts folder of AS 1.
* Select Options > Charts > Compile....
* Set Scope: Entire program (or Changes only if drivers already exist).
* Ensure Generate module drivers is checked.
* Click OK.
* Repeat the exact compilation process for the Charts folder of AS 2.
Download Chart Changes to Both Controllers:
* Download the compiled S7 program to AS 1 in RUN mode (Changes only).
* Download the compiled S7 program to AS 2 in RUN mode (Changes only).
* Both controllers now have matching R_ID communication runtime blocks in active memory.
Check Communication Diagnostic Pins in CFC Test Mode:
* Open the chart in AS 2 receiving the cross-controller signal.
* Switch to CFC Test Mode (online).
* Observe the incoming signal pin. Verify that the value reflects the output of AS 1 with signal status 16#80 (Good).
* If problems persist, inspect output pin ERROR and STATUS on the internal IK_RCV block for communication error codes (e.g., STATUS = 16#0001 or 16#0013).

Verification

In AS 1, toggle the source digital value or force an analog setpoint in CFC test mode. Open the receiving chart in AS 2; confirm that the destination block registers the value change within 200 ms to 500 ms. Verify that the signal status pin reports 16#80 (Good) without assertions of communication fault alarms.

Limitations and Operational Precautions

AS-wide interconnections introduce small deterministic network latencies (typically 200 ms to 1 second depending on OB scheduling and plant bus load); they should not be used for high-speed turbine trips or millisecond-level machine safety (which require dedicated fail-safe or hardware wiring). Both AS controllers must reside within the same PCS 7 multiproject to support automated cross-AS compilation.

Engineer FAQ

Why did NetPro show green when no data was transferring?

NetPro validates the underlying network S7 connection; it does not verify that the CFC communication driver blocks (IK_SEND/IK_RCV) are compiled, loaded, and executing in both CPUs.

What OB should AS-wide communication blocks run in?

They should run in OB34 (200 ms) or slower (OB35/OB36); running in OBs faster than 100 ms causes buffer saturation and packet rejection.

Do I have to compile both AS stations when creating an AS-AS link?

Yes. Cross-AS interconnections generate coordinated send and receive blocks with shared R_ID parameters across both controllers; both Charts folders must be compiled and downloaded.