Locating and Integrating Legacy FB51 (MEAS_MON) in SIMATIC PCS 7 - Classic Library vs Advanced Process Library

In SIMATIC PCS 7 V8.0, V8.1, V8.2, and V9.0, function block FB51 (MEAS_MON - Measured Value Monitoring) is not missing due to a corrupted installation; rather, it belongs to the legacy 'PCS 7 Standard Library' which was superseded by the Advanced Process Library (APL) starting with PCS 7 V8.0. In modern PCS 7 releases, analog monitoring is performed by the APL block MonAnL (FB1845). To utilize legacy FB51 in newer PCS 7 projects (for brownfield expansion or maintenance continuity), engineers must install the 'PCS 7 Classic Library' from the 'Additional Products' folder on the official PCS 7 installation media or download it from Siemens Industry Online Support (SIOS Entry 109480136). While supported for defect maintenance, the classic library receives no new features.

Architectural Evolution: PCS 7 Classic Library to APL

For over a decade (from PCS 7 V4.0 through V7.1), the foundational function block collection in SIMATIC PCS 7 was the 'PCS 7 Standard Library' (often termed the Classic Library). Core technological blocks in this suite included MEAS_MON (FB51) for analog monitoring, CTRL_PID (FB61) for control loops, and MOTOR (FB66) for direct drives.

Starting with PCS 7 V8.0, Siemens introduced the Advanced Process Library (APL) as the mandatory default standard library. APL introduced structured data types containing IEEE 754 floating-point values paired with NAMUR NE 107 quality bytes, standardized operating modes, and high-performance ergonomic faceplates. Consequently, the classic blocks were retired from the primary catalog and relocated to an optional legacy compatibility package.

Comparing Legacy FB51 (MEAS_MON) with Modern APL FB1845 (MonAnL)

Understanding the differences between FB51 and its modern replacement highlights why modernization is recommended:

Feature / ParameterLegacy FB51 (MEAS_MON)Modern APL FB1845 (MonAnL)Signal InterfaceFlat REAL values (U, V)Structured Val (Value: REAL + ST: BYTE quality code)Diagnostic StandardBasic wire-break bit (CSF)Standardized NAMUR NE 107 status flags and flutter suppressionHMI FaceplateClassic WinCC faceplate (@pg_meas_mon.pdl)Modern high-resolution APL faceplate (@pg_monanl.pdl)Alarm IntegrationUses Alarm_8P with basic message classesAdvanced multi-limit alarm management with priority coloringMaintenance StatusFrozen at V7.1 SP4 (Bug fixes only; no new features)Fully active lifecycle in PCS 7 V8.x and V9.xOS Compile IntegrationSupported via legacy AS-OS driversFully optimized for multi-client and Web Server deployment

While existing facilities can continue running FB51 to avoid re-engineering faceplates, all new technological engineering should deploy MonAnL (FB1845).

Installation and Integration of the Legacy Classic Library

To restore FB51 in a modern PCS 7 engineering environment, follow this procedure:

Locate Installation Media: Open the official SIMATIC PCS 7 installation media (DVD 2 or Software Package). Navigate to the folder Additional_Products\PCS7_Classic_Library.
Alternative SIOS Download: If DVD media is unavailable, download the legacy library package directly from Siemens Industry Online Support under Entry ID 109480136 (titled 'SIMATIC PCS 7 Libraries - Previous versions').
Execute Installation: Run Setup.exe for the classic library. Ensure the target directory points to your STEP 7 installation path (C:\Program Files (x86)\Siemens\Step7\s7libs).
Copy to Master Data Library: Open SIMATIC Manager. Open the newly installed PCS 7 Library V7.1 SP3 Upd4. Locate MEAS_MON (FB51) in the block folder and copy it into the Multiproject's Master Data Library (MDL).
Update Block Types: In SIMATIC Manager, select the chart folder and execute Options -> Update Block Types to register FB51 into the CFC block catalog.

Verification

Verify that FB51 is fully accessible and compilable using these checks:

CFC Catalog Inspection: Open any CFC chart. Open the block catalog (Ctrl+K), select the Libraries tab, and expand the Master Data Library. Verify that MEAS_MON appears with FB51.
Chart Placement and Compilation: Drag FB51 onto a CFC sheet, connect a test analog input, and execute a compilation. Verify that the compiler finishes with 0 Errors.
Installed Software Verification: In SIMATIC Manager, select Help -> About -> Installed Software -> Display. Confirm that PCS 7 Library V7.1 is listed alongside modern PCS 7 components.

Limitations and Operational Precautions

When deploying legacy classic library blocks in modern PCS 7, observe these critical rules:

No Mixing on Same Screen: Never mix classic @pg_meas_mon.pdl faceplates with modern APL @pg_monanl.pdl faceplates on the same plant display screen without uniform operator training; the visual controls and acknowledgment buttons differ significantly.
Support Lifecycles and Legacy System Services (LSS): As noted in Siemens Product Note Entry ID 109749392, free technical support applies to PCS 7 versions V8.0 and higher. Legacy software components prior to V8.0 may require a commercial Legacy System Services contract for complex technical escalations.
Commercial Procurement and Licensing Disclaimer: Although the classic library software media is downloadable or provided as an additional product, runtime usage consumes standard AS Runtime Process Objects (PO). Verify license counts and software maintenance agreements with an authorized Siemens sales representative.

Engineer FAQ

Can FB51 run on a modern AS 410-5H controller?

Yes. FB51 consists of standard S7 machine code and executes perfectly on AS 410-5H controllers, provided its block number and interface DBs do not conflict with active APL blocks.

Why didn't Siemens delete the classic library completely?

Siemens protects customer capital investment. Millions of process loops worldwide run on classic library blocks; keeping the library available allows plants to upgrade engineering stations without rewriting entire legacy applications.

Can I convert an existing FB51 block instance directly to MonAnL?

Not automatically. Because MonAnL uses structured inputs (Val.Value and Val.ST) rather than simple REAL pins, converting an instance requires manual reconnection in CFC or utilizing automated Control Module Type (CMT) replacement scripts.