S7 F Systems Fail-Safe Block Library Architecture and Process Object Counting on CPU 410 SEC

In SIMATIC PCS 7 V9.0 SP3 with S7 F Systems, fail-safe function blocks (such as F_CH_DI and F_CH_AI) reside in the 'S7 F Systems Lib V1_3' library, which is installed directly by the S7 F Systems engineering software package without requiring a separate library download. Crucially, fail-safe channel driver blocks (F_CH_DI, F_CH_AI, F_CH_DO, F_CH_AO) and automated module diagnostic blocks do NOT consume AS Process Objects (POs) on the controller. In PCS 7 architecture, AS RT PO licenses—managed via the CPU 410-5H System Expansion Card (SEC)—are consumed exclusively by technological control blocks with operator control and monitoring capabilities (such as motor, valve, and dosing blocks) and SFC types. To audit actual PO consumption, select the S7 Program in SIMATIC Manager and execute 'Options > Process Objects > Calculate'.

Executive Summary

Designing safety instrumented systems (SIS) in SIMATIC PCS 7 utilizing S7 F Systems combines standard process automation with SIL 3 / Category 4 functional safety within a single controller (such as the CPU 410-5H). Safety engineers frequently seek guidance on locating the official fail-safe block library, navigating authoritative Siemens safety manuals, and planning hardware capacity licensing. A widespread licensing misconception is that every safety input and output channel driver consumes an AS Process Object from the CPU's System Expansion Card (SEC). In accordance with Siemens licensing rules, low-level signal channel drivers are zero-PO blocks. Realizing this prevents unnecessary, costly SEC hardware capacity upgrades during safety system engineering.

Architecture and Fundamental Mechanics

The architecture and licensing framework of S7 F Systems in SIMATIC PCS 7 is governed by strict functional safety standards (IEC 61508 / IEC 61511) and Siemens capacity licensing rules:

The S7 F Systems Library Structure (S7 F Systems Lib V1_3):
* When the S7 F Systems Engineering package is installed on the ES, it registers the official safety library: S7 F Systems Lib V1_3.
* The library includes:
- F-Channel Drivers: F_CH_DI, F_CH_AI, F_CH_DO, F_CH_AO (for ET 200 distributed fail-safe modules).
- F-Control Logic: F_AND4, F_OR4, F_XOR2, F_SR, F_RS.
- F-System Management: F_START, F_DIAG, F_SHUTDN.
- Fail-Safe Communication: F_SNDBO, F_RCVBO, F_SNDR, F_RCVR (for inter-CPU safety communication via Safety Data Diode / PROFIsafe).
The CPU 410-5H System Expansion Card (SEC) Licensing Architecture:
* Unlike legacy S7-400 CPUs with fixed hardware memory boundaries, the CPU 410-5H utilizes a physical hardware System Expansion Card (SEC) that defines the controller's runtime capacity in terms of AS Process Objects (AS RT PO) (e.g., 100, 500, 1000, 1600, or 2000+ POs).
* The SEC capacity is strictly enforced during compile and download.
PO Counting Rules for Safety vs. Technological Blocks:
* Zero-PO Blocks: Channel drivers (Pcs7AnIn, Pcs7DiIn, F_CH_DI, F_CH_AI), logic gates (AND, OR, F_AND4), and module diagnostic blocks (F_MOD_DI) consume 0 Process Objects.
* PO-Consuming Blocks: A block consumes 1 AS RT PO if it meets the criteria of an operable technological block with message capabilities and OS faceplate binding (e.g., MotL, VlvL, PIDConL, F_MOT_L, F_VALVE).
* Therefore, adding hundreds of fail-safe sensor inputs (F_CH_AI) to a safety matrix or shutdown routine incurs zero AS RT PO license consumption on the CPU 410 SEC.
Official Documentation Standard:
* The definitive, authoritative reference manual for configuring, programming, and safety certifying S7 F Systems is Siemens publication SIMATIC Industrial Software S7 F/FH Systems - Configuring and Programming (Manual ID: A5E00049068).

Step-by-Step Implementation Procedure

Follow these procedures to locate safety blocks, verify licensing, and audit PO consumption:

Locate the S7 F Systems Library in CFC:
* Open any CFC chart in SIMATIC Manager.
* In the left Libraries catalog pane, expand Libraries.
* Locate and expand S7 F Systems Lib V1_3.
* If missing, verify that the S7 F Systems Engineering V6.2/V6.3 license and software are installed on the ES via Automation License Manager.
Insert Fail-Safe Channel Blocks into Safety Charts:
* In the Plant Hierarchy, create a dedicated Safety chart folder.
* Insert fail-safe channel drivers: drag F_CH_DI for digital safety inputs or F_CH_AI for 4..20 mA analog safety transmitters (e.g., pressure transmitters, emergency stop buttons).
* Interconnect the VALUE input of the block to the symbolic tag defined for the fail-safe module in HW Config.
Audit Actual Process Object (PO) Consumption in SIMATIC Manager:
* In SIMATIC Manager Component View, select the target S7 Program under the CPU 410-5H.
* In the top menu, select Options > Process Objects > Calculate.
* The Process Object Statistics window opens, displaying:
- Configured AS RT POs: The exact number of technological blocks currently instantiated.
- Available AS RT POs on SEC: The hardware capacity licensed by the installed System Expansion Card.
- Remaining POs: Free license capacity.
- Verify that fail-safe channel drivers (F_CH_*) do not increment the Configured AS RT PO count.
Procuring Additional SEC Capacity (If Technological Blocks Expand):
* If plant expansion requires additional technological control blocks, do NOT order standard software ALM PO licenses (6ES7653-2BA...).
* CPU 410-5H requires a dedicated CPU 410-5H System Expansion Card (SEC) PO Upgrade package (6ES7653-2CC...).
* Submit the upgrade requirement to an authorized Siemens distributor for official quotation.

Verification

After compiling the safety program (Chart > Compile > Scope: Entire program with 'Generate module drivers'), open the safety compilation log; confirm that the F-Compiler generates the safety checksum (CRC) and signature without errors. Run Options > Process Objects > Calculate; confirm that the PO count exactly reflects only technological blocks, validating zero PO consumption by F-channel blocks.

Limitations and Operational Precautions

Safety programs must always be executed within dedicated safety runtime groups bounded by F_START and F_END blocks. Never mix standard logic blocks inside a fail-safe execution group. Modifications to safety logic require safety password entry and safety program compilation, which generates a new safety signature.

Engineer FAQ

Does S7 F Systems require a separate engineering license?

Yes. The S7 F Systems engineering software requires an engineering license key in Automation License Manager, and the runtime safety program execution requires a runtime license per F-CPU.