Demystifying Named Connections in PCS 7: Why BSEND and BRECV Are Not Required for WinCC

In SIMATIC PCS 7, creating a Named Connection in NetPro between an S7-400 automation station and a WinCC OS Server does NOT require placing communication blocks (BSEND, BRECV, USEND, or URCV) in the PLC program. Named Connections utilize symbolic connection identifiers compiled into an XDB database by the OS Mapper tool, allowing the WinCC S7 Protocol Suite driver to access PLC data directly via uncoordinated S7 communication firmware calls (GET/PUT protocol mechanisms handled entirely by CPU operating system firmware). The system function blocks BSEND (SFB 12) and BRECV (SFB 13) are strictly required for AS-to-AS cross-controller communication or specialized raw data stream archiving, not for standard HMI process tags, faceplates, or alarms.

The Architecture of Named Connections in SIMATIC PCS 7

In SIMATIC PCS 7, communication between operator stations (OS) and automation stations (AS) is established using Named Connections:

Symbolic Addressing in NetPro: Instead of hardcoding static IP addresses into SCADA drivers, engineers configure a symbolic connection name in NetPro (e.g., OS_to_AS01).
The Mapper Tool and XDB Database: During OS compilation, the PCS 7 PLC/OS Engineering Tool (Mapper) compiles connection parameters, routing tables, and application endpoints into an XML-based database (*.XDB).
Decoupling Hardware from Engineering: If a physical CP 443-1 module is replaced or its IP address changes, the symbolic Named Connection ensures that WinCC Tag Management references remain valid without requiring manual tag retargeting.

How WinCC Accesses PLC Data via Firmware Communication

The fundamental reason why no PLC blocks are required for WinCC communication lies in the architecture of the SIMATIC S7 Protocol Suite:

Firmware-Level Execution: The S7-400 and CPU 410-5H operating system firmware natively includes an S7 Communication Server.
Uncoordinated Read/Write Services: When WinCC needs to display an analog level or write an operator setpoint, the WinCC communication channel sends standard S7 PDU (Protocol Data Unit) read/write requests over Industrial Ethernet.
CPU Background Processing: The CPU firmware handles these requests asynchronously in dedicated communication time slices between cyclic OB execution.
The user application program (CFC / SCL / STL) is completely unaware of these reads and writes; data is accessed directly from data blocks (DBs) and process images without executing any user function blocks.

Why BSEND and BRECV Are Unnecessary for HMI Tags

Engineers migrating from non-Siemens DCS platforms or legacy PLC systems often assume that every network conversation requires explicit send and receive blocks:

BSEND (SFB 12) and BRECV (SFB 13) are coordinated, block-oriented communication blocks. They require user code on both ends to initiate transfers, manage buffers, toggle request flags (REQ), and acknowledge receipt (NDR).
Attempting to place BSEND and BRECV in an AS program for WinCC communication is redundant and unworkable for standard tags because the WinCC S7 Protocol Suite driver does not implement matching BRECV endpoints for cyclic tags.
WinCC communicates directly with the data blocks generated by the CFC compiler (DB numbers assigned during CFC compilation).

When BSEND/BRECV and Raw Data Blocks Are Actually Required

There are only two specific scenarios in PCS 7 where BSEND/BRECV blocks are utilized:

1. AS-to-AS Cross-Controller Communication

When Controller A must transmit large data arrays or recipes to Controller B without routing through an intermediate SCADA server, BSEND on AS 1 pairs with BRECV on AS 2 over an S7 connection in NetPro.

2. Specialized Raw Data Archiving (AR_SEND / BATCH)

AR_SEND (SFB 37): Used when an AS controller buffers high-speed historical samples locally (e.g., turbine vibration bursts) and sends a single pre-timestamped raw archive packet directly into WinCC Tag Logging.
SIMATIC BATCH Raw Streams: Internal system blocks stream electronic batch recipe execution logs to the BATCH Server using specialized communication channels.

Communication Method and Block Requirement Comparison Matrix

The following table summarizes communication types, required PLC blocks, and their primary operational purposes:

Communication ArchitectureRequired AS BlocksRequired WinCC DriverPrimary Operational PurposeStandard HMI Tags & FaceplatesNone (Firmware handled)SIMATIC S7 Protocol Suite (Named)Real-time monitoring, setpoint control, alarmsAS-to-AS Cross-ControllerBSEND (SFB 12) / BRECV (SFB 13)N/A (Controller-to-Controller)Inter-PLC data block and recipe transferHigh-Speed Alarm ArchivingAR_SEND (SFB 37)WinCC Process Value ArchivingHigh-frequency burst data with AS timestampsAS-Wide InterconnectionsIK_SEND / IK_RCV (Auto-generated)N/A (CFC Compiler Managed)Cyclic process interlocks between controllers

Recognizing that Named Connections operate at the firmware level eliminates unnecessary block programming in CFC.

Verification

To verify that WinCC communicates over Named Connections without PLC blocks:
1. In NetPro, verify that the S7 Named Connection shows 'Active' between the OS and AS.
2. In SIMATIC Manager, compile the OS; confirm that structure tags are generated in WinCC Tag Management under Named Connections.
3. Activate WinCC Runtime.
4. Launch WinCC Channel Diagnosis; navigate to SIMATIC S7 Protocol Suite -> Named Connections.
5. Verify that the connection status indicates 'Ready / OK' and that read/write cycle counters increment continuously while monitoring process faceplates.

Limitations and Operational Precautions

Do not place BSEND or BRECV blocks in CFC intending to connect to standard WinCC process tags; this consumes communication work memory needlessly.
Ensure the Engineering Station runs the OS Compilation Mapper to keep the XDB connection database synchronized with NetPro.
On redundant S7-400H controllers, ensure the Named Connection is configured as an 'S7 fault-tolerant connection' to enable automatic path failover.
Always validate final license quantities, module availability, and MLFBs against the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

What does the Mapper tool do during OS compilation?

The Mapper tool scans NetPro network configurations and CFC charts, generating the symbolic tag connection database (*.XDB) that allows WinCC to address AS data blocks without manual mapping.

Can WinCC read data from DBs that were not created in CFC?

Yes. WinCC can access shared DBs created in standard STEP 7 (e.g., DB1, DB100) using direct DB addressing (DB100,DBD0), though standard PCS 7 engineering recommends using CFC blocks with OCM enabled.

What is the difference between an S7 Connection and a Named Connection?

An S7 Connection references specific hardware IP addresses and rack/slot numbers. A Named Connection abstracts these parameters into a symbolic name managed through an XDB configuration database.