Configuring CFC AS-Wide Interconnections Between S7-400 Stations

Overview

Continuous Function Chart (CFC) on SIMATIC S7 normally confines interconnections to blocks that live inside the same CPU. When two or more Automation Stations (AS, i.e. S7-400 CPUs) belong to the same STEP 7 project or multiproject, CFC supports an AS-wide interconnection: a chart output on Station A can drive a chart input on Station B as if both blocks were installed in the same CPU. The runtime system of each AS keeps the value flow synchronized through the configured S7 connection.

AS-wide interconnections are not a replacement for high-performance peer-to-peer communication. They are an engineering convenience for distributing complex control strategies across multiple CPUs while keeping the chart topology readable. Siemens imposes a strict envelope on this feature: only S7-400 CPUs are supported, the cycle time of the underlying cyclic interrupt OB must be at least 100 ms, no fail-safe blocks may participate, and only elementary data types are allowed on the wire. Understanding and respecting these limits is the difference between a chart that compiles and downloads cleanly and a project that refuses to go online.

This reference documents every official prerequisite, walks through a working configuration, and provides a troubleshooting matrix for the failures most often observed when interconnecting two CFC charts across two S7-400 stations.

Hardware and Software Prerequisites

Before opening a CFC chart, verify that the project satisfies every condition in the table below. The CFC editor enforces most of these rules at compile time, but the failure messages are easier to interpret when the underlying constraint is known. See the CFC for SIMATIC S7 Function Manual for the canonical list.

Project and Network Configuration Requirements

An AS-wide interconnection rides on top of an S7 connection between the two station objects in NetPro. Without that connection the runtime system has no path to exchange the value at the end of each cycle.

1. Insert both S7-400 stations into the same STEP 7 project (or into stations that are part of the same multiproject).
2. Open NetPro and right-click the first CPU. Select Insert New Connection > S7 Connection, then choose the partner CPU. NetPro writes the connection resources on both sides automatically.
3. Confirm that the connection type shown is S7 connection, not ISO-on-TCP-only or TCP-only. AS-wide interconnections require the full S7 protocol stack.
4. Compile and download the NetPro configuration to both CPUs. The download writes the connection DBs and the connection table to each station's system data.
5. Verify the connection using PLC > Diagnostics/Set > Check Connection Status in NetPro. A green status indicates the S7 connection is established.

If you skip NetPro and try to draw the AS-wide link directly in CFC, the editor will offer to create the connection for you. Accepting this prompt is acceptable, but it is safer to verify the connection works in isolation before introducing CFC complexity.

Runtime Group and OB Cycle Timing

Every CFC chart is executed inside a runtime group, which is bound to a cyclic interrupt OB (OB30 through OB38). For AS-wide interconnections, the runtime group on both stations must satisfy two rules:

1. The cycle time of the OB must be at least 100 ms. The fastest legal cycle is therefore 100 ms; anything shorter is rejected.
2. The priority of the OB must be consistent with its cycle time: the faster the cycle, the higher the priority. Mismatched priorities cause sporadic connection timeouts because the slower-priority OB on one side never gets a guaranteed time slice to send the update.

Recommended OBs for AS-wide interconnections:

The relationship between cycle time T and priority P can be expressed as:

T_station_A <= T_station_B ⇒ P_station_A >= P_station_B

In other words, the station with the faster cycle must run at higher OB priority, otherwise the slower station will not be scheduled often enough to keep its side of the link updated. If station A runs in OB35 (100 ms, priority 9) and station B runs in OB33 (200 ms, priority 8), the configuration is valid. If the priorities are reversed the link will compile but fail intermittently online.

Supported Data Types and Chart Constraints

Not every data type can ride an AS-wide interconnection. The runtime system serializes the value into the S7 connection, and certain compound or variable-length types cannot be packed into the fixed-length telegram expected by the partner.

Additional chart-level restrictions:

• The I/Os of nested CFC charts cannot be interconnected AS-wide. Only the I/Os of blocks (FB, FC, multi-instance blocks, library blocks) at the top level of the chart may participate.
• The interconnection cannot point at a runtime group itself; it must target a block I/O pin.
• The interconnection cannot point at an absolute address (e.g. DB100.DBD0) on the partner station. The target must be a named chart I/O.
• Connectors (signal flags) on the partner chart are also forbidden. Only chart block I/Os are valid endpoints.

System Capacity and Topology Limits

Two hard numerical limits cap the use of AS-wide interconnections at the project and CPU level:

For projects that need to exceed 200 AS-wide links on a single CPU, the recommended pattern is to introduce an interface chart: one CFC chart on each CPU acts as a broker, aggregating multiple partner values into a single block whose output is then fanned out to local charts. This keeps the visible link count under the 200-element ceiling while preserving the overall topology.

Step-by-Step Configuration Procedure

The following procedure connects one output of a CFC chart on Station A (an S7-400 CPU) to one input of a CFC chart on Station B (a second S7-400 CPU) in the same project.

1. Confirm the prerequisites. Both stations must be S7-400 CPUs in the same STEP 7 project, NetPro must show an S7 connection between them, and the chart targets must be top-level blocks with elementary data type I/Os.
2. Open Station A's CFC chart that contains the source block. Identify the output pin you intend to expose.
3. Open Station B's CFC chart that contains the destination block. Identify the input pin that should receive the value.
4. Right-click the source output on Station A and choose Insert Interconnection. The cursor changes to a connector symbol.
5. Click the destination input on Station B. CFC inspects the partner AS, verifies the rules above, and either accepts the link or displays a dialog explaining why it cannot be created.
6. Set the runtime group on both charts. Open Chart > Runtime Properties, then assign each chart to OB33 (or another OB30–OB38 with cycle ≥ 100 ms and consistent priority).
7. Compile both charts. Use Chart > Compile > Charts As Program on each station separately. The compiler writes a system data block describing the AS-wide links on each side.
8. Download both programs. The download order does not matter; the runtime system establishes the link after both sides are present.
9. Go online on Station A, open the chart, and watch the source output. Force a value change and observe whether the value arrives at the destination input on Station B.
10. Check the partner station's diagnostic buffer (Station B > PLC > Diagnostics/Set > Diagnostic Buffer). Look for events of type S7 connection established/lost or AS-wide link failure.

Verification and Diagnostics

After download, perform the following checks in order:

1. Online view of both charts. In CFC online mode, every AS-wide interconnection shows a small icon overlay. Hover the cursor over the link to display the partner station name, the partner block, and the partner I/O. A red exclamation mark indicates that the runtime system cannot deliver the value.
2. SFC visual instance for the S7 connection. In NetPro online, right-click the S7 connection and select Connection Status. The connection must show Established at both ends.
3. Diagnostic buffer. Look for W#16##8x5 entries on both CPUs. Event ID 0x8x54 typically indicates that an AS-wide link could not be activated because the partner connection is down; W#16#4301 indicates an S7 connection establishment failure.
4. Force and observe. Force the source value to a known value on Station A, then read the destination input on Station B. If the values do not match, the runtime group on at least one side is not running, or the cycle/priority mismatch described above is in effect.
5. Check OB1 scan time. A saturated OB1 can starve the cyclic interrupt OB. Verify that OB1 completion time stays below the configured OB30–OB38 period, otherwise the cyclic interrupt is delayed past its scheduled slot.

Troubleshooting Matrix