Configuring Cyclic Organization Block Priorities and Resolving Cross-AS Interconnection Speed Limits in PCS 7

In SIMATIC PCS 7 multiprojects, establishing cross-AS chart interconnections between separate automation stations can trigger the compilation error 'fastest inter-AS connection is 200ms' when source or destination charts execute in high-frequency cyclic interrupt OBs (such as OB36 at 50 ms or OB38 at 10 ms). Resolving this restriction requires configuring OB execution intervals in CPU Hardware Configuration and moving cross-AS communication blocks into supported cyclic interrupt OBs (OB32 at 1.0 s or OB35 at >= 200 ms).

Mechanics of Cross-AS Interconnections in SIMATIC PCS 7

SIMATIC PCS 7 supports seamless graphical cross-AS interconnections, allowing an output pin of a CFC block in Automation Station 1 (AS1) to connect directly to an input pin in Automation Station 2 (AS2):
* Automated Communication Blocks: When you compile the multiproject, the CFC compiler automatically instantiates internal communication blocks (IK_SEND / IK_RCV or BSEND / BRCV pairs) in both controllers.
* S7 Connection Binding: The compiler generates bilateral S7 connections in NetPro to transmit data telegrams across the Industrial Ethernet plant bus.
* Deterministic Execution: The data exchange blocks execute within the cyclic interrupt Organization Block (OB) where the interconnected CFC chart is placed.

The 200 ms Limit: Plant Bus Bandwidth and Execution Constraints

During multiproject chart compilation, engineers often encounter the compiler error:

The fastest inter-AS connection is 200ms.

This limitation is enforced by the CFC engineering compiler for critical system stability reasons:
* Industrial Ethernet Latency: Transferring data packets across the Industrial Ethernet plant bus (via CP 443-1 or CPU 410 integrated PN ports), through network switches, and processing them in the partner CPU requires deterministic protocol handshakes.
* Communication Processing Overrun: Attempting to execute cross-AS network communication in ultra-fast cyclic interrupt OBs (e.g., OB36 at 50 ms, OB37 at 20 ms, or OB38 at 10 ms) exhausts controller communication buffers, triggering communication starvation or CPU cycle overruns.
* Compiler Rule: The CFC compiler mandates that all automatic cross-AS communication blocks execute in an OB with a cycle time $\ge 200 ext{ ms}$.

Configuring Cyclic Interrupt OBs (OB30 to OB38) in HW Config

To establish supported execution intervals:
1. Open SIMATIC Manager and open the Hardware configuration for each participating S7-400 or CPU 410 station.
2. Double-click the CPU in the rack to open Properties - CPU.
3. Select the Cycle/Clock Memory tab.
4. Review the configured execution times for cyclic interrupt organization blocks:
* OB35 (Default): Typically set to 100 ms in PCS 7. If set to 100 ms, it cannot host cross-AS interconnections directly without adjustment.
* OB34: Default 200 ms.
* OB33: Default 500 ms.
* OB32: Default 1000 ms (1.0 second).
* OB31: Default 2000 ms (2.0 seconds).
5. If OB35 is required for cross-AS links, increase its execution time to 200 ms (or reassign the specific chart to OB34 at 200 ms or OB32 at 1000 ms).
6. Save and compile HW Config, and download hardware changes to the controllers during a planned shutdown window.

Step-by-Step Procedure to Resolve Cross-AS Compiler Errors

To eliminate the 200 ms compiler error without altering hardware settings:
1. Identify the Interconnected Charts: Open the CFC cross-reference list to locate the specific charts containing cross-AS connections.
2. Move Charts to a Supported OB:
* In SIMATIC Manager or CFC, open the Chart -> Edit Insertion Pointer (or select the block in the chart and press Ctrl + B to open the Run Sequence).
* Relocate the interconnected blocks or the entire chart from OB35 (100 ms) or OB36 (50 ms) to OB34 (200 ms), OB33 (500 ms), or OB32 (1000 ms).
3. Dedicated Inter-AS Buffer Chart (Best Practice):
* Instead of moving entire process control loops, create a dedicated buffer chart (e.g., @InterAS_Comm) assigned to OB32 (1000 ms).
* Connect internal fast signals to this buffer chart within the local AS, and route only the cross-AS interconnections through the buffer chart.
4. Compile Multiproject:
* Select the multiproject root in SIMATIC Manager.
* Execute PLC -> Compile and Download Objects.
* Compile Charts with Changes Only. The compiler successfully generates IK_SEND/IK_RCV blocks without error.

Verification

In SIMATIC Manager, execute a complete consistency check on the multiproject: confirm that no inter-AS cycle time errors are reported.
Download chart changes to both AS1 and AS2 in RUN mode.
In CFC dynamic test mode, observe the signal in AS1 and verify that the corresponding input in AS2 updates within the configured OB interval (e.g., within 200 ms to 1000 ms).
Open Module Information -> Communication in SIMATIC Manager: confirm that communication load remains well within normal operational thresholds (< 20%).

Limitations and Operational Precautions

Cross-AS communication is not designed for fast sub-50 ms safety trips or fast turbine shutdown loops; hardwire critical inter-AS trips or use dedicated peer-to-peer fieldbus couplers.
Changing cyclic interrupt OB execution times in HW Config requires downloading System Data Blocks (SDB), which requires putting the controller in STOP mode.
Moving blocks between OBs in the CFC Run Sequence can be downloaded in RUN mode, avoiding plant downtime.

Engineer FAQ

Why does PCS 7 enforce a 200 ms minimum cycle time for cross-AS connections?
To guarantee network communication stability across the plant bus and prevent communication buffer overruns in the S7 CPUs.
What organization block is most commonly used for cross-AS telemetry in PCS 7?
OB32 (configured at 1000 ms / 1 second) is standard for non-critical supervisory data; OB34 (200 ms) is used for faster interlocks.
Can I keep my PID loop in OB35 (100 ms) and still send its output to another AS?
Yes, connect the PID output to an intermediate buffer block executing in OB32 or OB34, and route the cross-AS connection from the buffer block.
What blocks are generated by the CFC compiler for cross-AS connections?
The compiler automatically creates IK_SEND and IK_RCV blocks linked to internal BSEND and BRCV system communication blocks.
Can cross-AS connections be established across redundant S7-400H controllers?
Yes, the compiler automatically binds cross-AS connections to redundant S7 fault-tolerant connections in NetPro.
Does changing a chart's insertion OB require a CPU STOP?
No, reordering block execution in the CFC Run Sequence can be compiled and downloaded in RUN mode.
What happens to the receiving block if the cross-AS network connection fails?
The receiving block retains its last valid value or asserts a communication fault status depending on block configuration.
What is the maximum number of cross-AS connections supported in a PCS 7 multiproject?
While technically bounded by CPU connection resources, typical architectures support hundreds of cross-AS connections when grouped cleanly into OB32.
Can I send analog and digital signals across the same cross-AS link?
Yes, the compiler automatically aggregates signals between the same station pairs into consolidated communication packets.
Where in NetPro can I view the cross-AS connections?
Open NetPro; cross-AS links appear in the connection table as S7 connections with application type "CFC Cross-AS".

Technical References

SIMATIC PCS 7 CFC for S7 / PCS 7 Manual (V9.0 / V9.1)
SIMATIC PCS 7 Engineering System Configuration Manual
Siemens Industry Online Support Entry: Cross-AS Interconnection Cycle Limits (Entry ID 298983)
SIMATIC S7-400 CPU Data Reference Manual

Related Tags

PCS 7, Cross-AS, CFC, Organization Blocks, OB35, OB32, OB34, Cycle Time, NetPro, Interconnection

Configuring Cyclic Organization Block Priorities and Resolving Cross-AS Interconnection Speed Limits in PCS 7