Historical Deprecation and Migration of Ethernet Communication Blocks FB205 and FB206 Across PCS 7 Versions

In legacy SIMATIC PCS 7 systems, function blocks FB205 (FR_AGSEN) and FB206 (FR_AGRCV) originated in the obsolete PCS 7 V5.2 Communication Library to provide a simplified CFC interface for CP 443-1 Ethernet communications. These wrapper blocks are absent from all modern PCS 7 libraries (V6.0, V7.x, V8.x, V9.x) because Siemens discontinued the dedicated Communication Library in favor of direct utilization of standard SIMATIC NET communication functions FC5 (AG_SEND) and FC6 (AG_RECV) from the SIMATIC_NET_CP library, or native S7 fault-tolerant connections (BSEND/BRCV). Furthermore, in modern PCS 7 Basis Libraries, block numbers FB205 and FB206 were reassigned to completely different system blocks (OB_BEGIN_HPN and MOD_CENTRAL), making migration to standard FC5/FC6 mandatory to prevent compiler collisions.

Historical Context: The PCS 7 V5.2 Communication Library

During the late 1990s and early 2000s, SIMATIC PCS 7 V5.0 to V5.2 introduced Ethernet-based controller networking via CP 443-1 communication processors. In that era, the Step 7 standard communication functions AG_SEND (FC5) and AG_RECV (FC6) required engineers to manage manual pulse triggering, ANY pointers, and complex error return codes in STL.

To simplify engineering within Continuous Function Chart (CFC), Siemens authored the 'PCS 7 Communication Library', supplying two wrapper blocks:
1. FB205 FR_AGSEN: Handled cyclic scheduling and transmission buffer pointers to the CP 443-1.
2. FB206 FR_AGRCV: Handled reception buffer pointers and incoming telegram decoding.

Why FR_AGSEN and FR_AGRCV Were Removed in Modern Releases

When PCS 7 V6.0 was released, Siemens overhauled the entire communication and driver philosophy:

Modernized CFC Compiler: The CFC compiler gained native support for elementary functions (FCs), allowing FC5 and FC6 to be wired directly into charts without requiring wrapper FBs.
Shift to S7 Configured Connections: For plant-wide automation, Siemens established S7 Connections using system function blocks SFB 12 (BSEND) and SFB 13 (BRCV), which provide deterministic, high-throughput, fault-tolerant data exchange integrated with the OS Server.
Retirement of the Library: The separate Communication Library was eliminated from the installation media to enforce standardization.

Block Number Reassignment in Modern PCS 7 Basis Libraries

A dangerous trap for engineers migrating legacy projects into PCS 7 V8 or V9 is block number reassignment:

FB205: Reassigned in modern PCS_7_BASISLIBRARY to OB_BEGIN_HPN (H-CPU diagnostic function block).
FB206: Reassigned in modern PCS_7_BASISLIBRARY to MOD_CENTRAL (Non-diagnostic central module supervisor).

If an engineer attempts to compile a migrated V5 project containing legacy FR_AGSEN while the modern Basis Library is present, the Step 7 compiler detects an irreconcilable symbol clash, aborting compilation.

Migration Workflow to Standard FC5 and FC6 (SIMATIC_NET_CP)

To modernize legacy communication charts:

Open the project in SIMATIC Manager on the updated Engineering Station.
Open the SIMATIC_NET_CP library under CP 400 > Blocks.
Copy FC5 (AG_SEND) and FC6 (AG_RECV) into the S7 program's offline block folder.
Open the CFC charts. Delete the obsolete FB205 and FB206 instances.
In CFC, select Options > Block Types... and clean up FB205 and FB206 from the chart database.
Place FC5 and FC6 into the chart. Reconnect the Connection ID (ID), Hardware Address (LADDR), and Data Pointer (DATA).
Recompile the CFC charts with scope 'Entire program'.

Comparison of Communication Block Architectures Across PCS 7 Eras

The following table compares Ethernet communication approaches across different generations of SIMATIC PCS 7:

Era / VersionCommunication Software EntityUnderlying System FunctionsBlock FootprintS7 Redundancy IntegrationPCS 7 V5.0 - V5.2FR_AGSEN / FR_AGRCV (FB205/206)Wraps FC5 / FC6Consumes FB Instance DBsNone (Single CP only)PCS 7 V6.x - V9.x (Raw TCP)Standard AG_SEND / AG_RECVDirect FC5 / FC6 callsLightweight FC (No DB)Manual switchover logicPCS 7 V6.x - V9.x (S7 DCS Standard)BSEND / BRCV (SFB 12 / SFB 13)Native S7-400 CPU FirmwareHighly Optimized100% Bumpless Fault-Tolerant S7 ConnectionModern Basis LibraryOB_BEGIN_HPN / MOD_CENTRALSystem Diagnostic DriversDiagnostic System FBsCore S7-400H Supervision

Verification

Verify modern communication function execution in CFC:
1. In SIMATIC Manager, verify that the offline block folder contains FC5 and FC6 and no longer references legacy FB205/FB206 wrappers.
2. Open the CFC chart online in Test Mode; confirm that AG_SEND (FC5) pulses DONE = 1 and ERROR = 0 on each transmission trigger.
3. Confirm that the remote partner receives the data payload accurately.
4. Verify that AG_RECV (FC6) updates the target DB buffer with valid process bytes upon packet arrival.

Limitations and Operational Precautions

Never attempt to import archived V5.2 .bin block files for FB205/FB206 into a modern PCS 7 project. Doing so overwrites the modern Basis Library's OB_BEGIN_HPN block, causing catastrophic controller driver generation failures during AS compilation. If communicating with another SIMATIC S7 station, always prioritize configured S7 connections (BSEND/BRCV) over raw AG_SEND/AG_RECV to take full advantage of controller redundancy and diagnostic logging.

Engineer FAQ

Why did Siemens delete the PCS 7 Communication Library after V5.2?

It was redundant. Step 7 introduced native CFC support for FCs, allowing engineers to call the official SIMATIC NET blocks (FC5/FC6) directly without extra wrapper layers.

Can I use FC5 and FC6 with redundant S7-400H CPUs?

Yes, but you must configure redundant CPs and program application-level connection monitoring. For redundant systems, configured S7 Fault-Tolerant Connections using BSEND/BRCV are strongly recommended because switchover is handled automatically in hardware.