C135 PCS 7 (PCS7)
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Configuring AS-Wide Interconnections in CFC vs Configured S7 Connections in PCS 7

Configuring AS-Wide Interconnections in CFC vs Configured S7 Connections in PCS 7

In SIMATIC PCS 7, engineers can exchange process variables between separate automation stations (AS) using either native AS-Wide Interconnections in CFC or configured S7 Connections in NetPro. AS-Wide Interconnections provide seamless, chart-to-chart signal linking without requiring manual NetPro connections or communication blocks (BSEND/BRECV). When an engineer drags an output from an AS 1 chart to an input on an AS 2 chart, the PCS 7 CFC compiler automatically instantiates background communication blocks (IK_SEND, IK_RCV) and reserves connection resources in the background. Understanding when to deploy lightweight cyclic AS-Wide Interconnections versus high-capacity NetPro S7 Connections is vital for optimizing controller communication loads.

Table of Contents

Architectural Overview: AS-Wide Interconnections vs S7 NetPro Links

Process automation architectures frequently distribute plant operations across multiple controllers (e.g., Boiler AS, Reactor AS, Utilities AS). Sharing signals between these controllers requires selecting the appropriate communication method:

1. AS-Wide Interconnections (Recommended for Process Interlocks)

  • Design Philosophy: Direct, transparent signal wiring between CFC charts across project boundaries.
  • Engineering Workflow: Drag-and-drop linking in CFC. The compiler generates all background blocks and handles cyclic transmission.
  • Data Types: Elementary variables (BOOL, REAL, BYTE, INT, DINT, WORD) and standard APL structured signals (Struct containing Value and Quality Status ST).
  • Update Frequency: Synchronized with the calling cyclic interrupt OB (e.g., OB35 / 100 ms).

2. Configured S7 Connections via NetPro (BSEND/BRECV)

  • Design Philosophy: High-capacity block and recipe transfer.
  • Engineering Workflow: Explicit connection creation in NetPro and programming of SFB 12/13 blocks in CFC/SCL.
  • Data Types: Complete data blocks, recipe UDTs, large arrays up to 64 KB.

How the CFC Compiler Implements AS-Wide Interconnections

When an engineer compiles CFC charts containing cross-AS interconnections:

  1. Automatic Block Insertion: The compiler automatically instantiates internal communication blocks in system @ charts:
    * IK_SEND: Placed in the sending AS runtime group.
    * IK_RCV: Placed in the receiving AS runtime group.
  2. Dynamic Connection Provisioning: The compiler checks available S7 communication endpoints on the CP 443-1 or CPU 410-5H PROFINET interfaces.
  3. Quality Status Preservation: Unlike raw communication blocks, AS-Wide Interconnections preserve the APL signal status byte (ST). If the sending AS transitions to STOP or the network cable drops, the receiving block automatically detects the failure and sets ST = 16#00 (Bad), triggering downstream interlocks safely.

Configuring AS-Wide Chart Interconnections Step-by-Step

Follow this engineering sequence to establish AS-Wide Interconnections across a multiproject:

Step 1: Verify Multiproject and Network Consistency

  1. In SIMATIC Manager, ensure all participating AS stations reside in the same Multiproject.
  2. Open NetPro and verify that all AS stations are attached to the same physical Industrial Ethernet subnet (e.g., PlantBus).
  3. Save and compile network connections in NetPro.

Step 2: Create Interconnections in CFC

  1. Open the source CFC chart in AS 1.
  2. Open the destination CFC chart in AS 2 side-by-side.
  3. Click the output pin in AS 1 (e.g., Permit_Out on an interlock block).
  4. Drag and drop the signal connection onto the input pin in AS 2 (e.g., Permit on MotL).
  5. A green inter-AS connector symbol appears in the sheet margin indicating an active AS-Wide link.

Step 3: Compile and Download Both Controllers

  1. Compile charts on AS 1 (select Changes Only).
  2. Compile charts on AS 2 (select Changes Only).
  3. Download changes to both controllers to activate communication.

Communication Resource Limits and Diagnostic Monitoring

While AS-Wide Interconnections provide unmatched engineering convenience, controllers have physical resource boundaries:

  • S7 Connection Limits: Each AS-to-AS partner pair consumes at least one underlying S7 connection resource on the CPU / CP card.
  • Telegram Sizing: Multiple cross-AS signals between the same two controllers are automatically packed into common telegram frames to conserve bus bandwidth.
  • Monitoring Connection Health:
  • In CFC, the receiving block output Subn1Id / Subn2Id and signal status indicate link health.
  • In the AS operating system, diagnostic buffer events log communication timeouts (Event 16#45xx).

Cross-Controller Communication Method Comparison Matrix

The following table compares AS-Wide Interconnections and NetPro S7 Connections:

Feature / CriterionAS-Wide Interconnections in CFCConfigured S7 Connection (BSEND/BRECV)Configuration EffortZero coding (Drag-and-drop in CFC)Manual NetPro setup + block programmingSignal LatencyLow (Fast cyclic execution in OB)Variable (Asynchronous background slice)APL Quality Status (ST)Natively preserved (Value + ST)Must be manually packed and parsedMaximum Data VolumeTypically individual signals / small structsUp to 64 KB per transmissionConnection RecoveryFully automatic fail-safe fallbackMust handle error codes in user logicSimulation in PLCSIMSupported (via NetPro single links)Supported

Deploying AS-Wide links for process interlocks and BSEND/BRECV for large datasets ensures balanced controller performance.

Verification

To verify that AS-Wide Interconnections function correctly:
1. In SIMATIC Manager, confirm that both AS 1 and AS 2 compile with 0 Errors and 0 Warnings.
2. Put both controllers in RUN mode.
3. In CFC test mode, force the sending block output to TRUE in AS 1.
4. Open the receiving chart in AS 2; verify that the input transitions to TRUE within one cyclic OB execution cycle (< 100 ms).
5. Verify that the receiving signal status indicates 'Good' (ST = 16#80).
6. Place AS 1 in STOP; confirm that AS 2 detects the outage and asserts bad signal quality (ST = 16#00).

Limitations and Operational Precautions

  • AS-Wide Interconnections require all participating controllers to reside within the same STEP 7 Multiproject.
  • Do not exceed the maximum number of configured communication connections supported by the CPU 410-5H or CP 443-1.
  • Modifying AS-Wide connections requires downloading changes to both participating controllers.
  • Always validate final license quantities, module availability, and MLFBs against the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

What happens to an AS-Wide input if the network cable is unplugged?

The background IK_RCV block detects the communication timeout, freezes or sets the value to a configured fallback state, and sets signal status ST = 16#00 (Bad), tripping any safety interlocks immediately.

Can I create an AS-Wide connection between an S7-300 and an S7-400?

No. Native PCS 7 AS-Wide Interconnections are exclusively supported between S7-400 and AS 410 controllers in PCS 7 multiprojects.

Do AS-Wide Interconnections consume Process Object (PO) licenses?

No. AS-Wide Interconnections consume internal controller communication memory and connection resources, but do not consume additional Process Object licenses.

Technical References

Related Articles

Related Tags

  • AS-Wide Interconnections
  • CFC Cross-AS Links
  • IK_SEND IK_RCV
  • Signal Quality Status
  • NetPro S7 Connection
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