Identifying Replacements for Obsolete FB205 and FB206 During PCS 7 V5.2 to V6.1 Migration

In legacy SIMATIC PCS 7 migrations from V5.2 to V6.1, function blocks FB205 and FB206 were early first-generation communication and driver interface blocks belonging to the obsolete PCS 7 V5 Standard Library (specifically dedicated to legacy serial or telecontrol communication interfaces). When migrating to PCS 7 V6.1, Siemens completely restructured the standard driver library and removed FB205 and FB206 from official distribution. Depending on their specific usage in the legacy CFC charts, these blocks are superseded by either the standardized communication blocks from the PCS 7 Standard Library (FB80 MODB341 / FB8 P_SND_RK for serial communication) or the modern channel driver blocks (CH_AI, CH_DI). Engineers must review the original V5 CFC symbol table to identify the block's exact symbolic name and replace the obsolete block instances with the designated V6.1 equivalents.

Historical Context: Function Blocks FB205 and FB206 in PCS 7 V5.x

During the era of SIMATIC PCS 7 V5.0 to V5.2, the automation software relied on the 'PCS 7 Standard Library' (often designated Lib_V50 or Lib_V52). In this release, block numbers were assigned statically across arbitrary ranges. Blocks FB205 and FB206 were utilized in specialized subroutines, such as serial communication buffer management for CP 341/CP 441 modules or proprietary telecontrol protocols.

When Siemens engineered PCS 7 V6.0 and V6.1, the entire driver architecture was redesigned around the automated driver generator and standardized communication interfaces. Legacy blocks that did not support the new diagnostic structures or that used non-standard process images were discontinued and deleted from the V6.1 library media.

Identifying Block Functionality via the Legacy Symbol Table

Because block numbers (like FB205) can be reused across custom project libraries, an engineer must verify the exact function before replacing them:

Inspect V5 Symbol Table: In the original unmigrated PCS 7 V5.2 project, open the Symbol Table (Symbols). Locate FB205 and FB206.
Check Symbol and Comment: Identify the symbolic identifier (e.g., SEND_341, RCV_341, or custom driver name) and author signature.
Inspect CFC Instances: Open the CFC charts where FB205 and FB206 are placed. Review the connected input/output pins, hardware addresses (LADDR), and downstream controller blocks.

Superseding Modern Blocks in PCS 7 V6.1 Standard Library

Once the block's functional role is established, map it to the modern PCS 7 V6.1 equivalent:

Serial Point-to-Point Communication: If FB205/FB206 were used for serial communication over CP 341 or CP 441, replace them with official CP 341 communication blocks (FB7 P_RCV_RK and FB8 P_SND_RK) or the Modbus Master driver block (FB80 MODB341).
Analog / Digital Channel Processing: If the blocks were custom signal conditioners, replace them with standard PCS 7 V6.1 channel blocks (CH_AI, CH_DI, CH_AO, CH_DO).
Telemetry / Remote Telecontrol: If used for SINAUT or telemetry, install the official PCS 7 TeleControl library.

Step-by-Step Procedure to Replace Obsolete Blocks in CFC

To eliminate obsolete blocks during migration:

Document Chart Connections: Print or take screenshots of all CFC sheets containing FB205 and FB206, recording all parameter values and signal routings.
Delete Legacy Instances: In the CFC editor, delete the instances of FB205 and FB206.
Clean Up Block Types: In CFC, select Options > Block Types.... Select FB205 and FB206 in the left pane and click Clean Up to purge their definitions from the chart database.
Insert Modern Replacements: From the PCS 7 V6.1 Library, drag the replacement blocks into the CFC chart.
Rewire Signals: Reconnect the input/output operands according to the documented baseline.
Compile and Verify: Execute a full CFC compilation with module driver generation.

Comparison of Obsolete V5 Blocks and Modern V6.1 Counterparts

The following table compares legacy V5.x blocks with their standardized V6.1 replacements:

Legacy V5 BlockFunctional PurposeModern PCS 7 V6.1 ReplacementAdvantages of Modern EquivalentFB205 / FB206 (Serial)CP 341 / CP 441 Telegram HandlingP_SND_RK / P_RCV_RK or MODB341Buffer overflow protection, full diagnostic wordsFB205 (Custom Driver)Legacy Peripheral Signal FormattingCH_AI / CH_DI Channel BlocksNative driver generation, automatic quality codesLegacy PI/PA BlocksPROFIBUS PA Instrument DriversPA_AI / PA_DI Channel BlocksIEEE 754 floating point decoding, NE 107 diagnosticsObsolete Math BlocksRaw Scaling and ConditioningStandard APL / Standard Math BlocksDeterministic IEEE math, zero overflow traps

Verification

Verify the replacement of obsolete blocks in PCS 7 V6.1:
1. In SIMATIC Manager, open the offline block folder; confirm that FB205 and FB206 no longer exist.
2. Open the CFC editor; verify that the chart folder contains no yellow question-mark placeholder blocks.
3. Execute 'Chart > Compile > As Program' with scope 'Entire program'; confirm that the compiler finishes with '0 Errors'.
4. Download to S7-PLCSIM; verify that the newly wired communication or channel blocks process signals dynamically without assertion faults.

Limitations and Operational Precautions

Never copy obsolete V5.x block binaries (.bin / compiled DB/FB files) forward into a PCS 7 V6.1 or V7.x project directory to 'force' compatibility. Doing so introduces 16-bit addressing bugs, causes memory boundary overlaps in S7-400 CPUs, and invalidates WinCC OS compilation. Always replace discontinued blocks with certified library successors.

Engineer FAQ

Why did Siemens delete blocks between PCS 7 V5 and V6?

PCS 7 V6 introduced modern 32-bit driver generation, process image partitioning, and Microsoft SQL Server integration. Legacy V5 blocks lacked the required interfaces and attributes, necessitating a clean library restructuring.

How do I find out what an unknown FB in a legacy project does if there are no comments?

Open the block in the Step 7 LAD/FBD/STL editor (or SCL if source is present). Inspect the input parameter names and system function calls (SFCs/SFBs) called inside the block to deduce its functional purpose.