C135 PCS 7 (PCS7)
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Activating Simulation on Fail-Safe Analog Input Driver F_CH_AI in SIMATIC S7 F-Systems

Activating Simulation on Fail-Safe Analog Input Driver F_CH_AI in SIMATIC S7 F-Systems

To activate simulation and inject simulated values into the fail-safe analog input driver block 'F_CH_AI' (FB375) in SIMATIC S7 F-Systems on a CPU 414-4H or CPU 410-5H, you cannot directly connect standard data block (DB) variables to fail-safe inputs. Standard boolean and real signals must first be converted into fail-safe data types using 'F_BO_F' (Standard-to-F-BOOL) and 'F_R_F' (Standard-to-F-REAL) conversion blocks before wiring them to 'F_CH_AI.SIM_ON' and 'F_CH_AI.SIM_V'. Furthermore, the F-System safety runtime group must be compiled with safety administration rights, and simulation must be explicitly authorized.

Table of Contents

Executive Summary

In safety-instrumented systems (SIS) running SIMATIC S7 F-Systems, the F_CH_AI fail-safe driver block conditions analog signals from certified SIL-rated input modules (such as ET 200M or ET 200SP HA fail-safe analog inputs). During commissioning, loop checks, or functional safety testing, automation engineers frequently need to override field inputs by toggling SIM_ON and adjusting SIM_V from an operator test screen or standard commissioning data block. However, S7 F-Systems enforces strict boundaries between standard user logic and safety program code. Attempting to directly wire standard DB addresses to safety pins generates compilation aborts. This guide details fail-safe data boundary crossing, block wiring, and safety authorization.

Architectural and System Mechanics

Safety Program Architecture and Boundary Rules in S7 F-Systems:

  • F-Execution Groups: Safety blocks execute within dedicated F-Runtime groups (e.g., triggered by OB35) monitored by safety watchdogs and CRC checksum calculations.
  • Fail-Safe Data Types:
  • Safety inputs (F_BOOL, F_REAL) contain embedded parity and structural integrity validation.
  • Standard data block variables (BOOL, REAL) lack fail-safe signatures. Direct connection in CFC is strictly prohibited by the F-Compiler.
  • F-Conversion Blocks:
  • F_BO_F: Converts standard BOOL (from DB, marker, or WinCC) to F_BOOL.
  • F_R_F: Converts standard REAL to F_REAL.
  • Simulation Logic on F_CH_AI:
  • SIM_ON: When asserted, the block disconnects from the physical peripheral address (VALUE) and routes SIM_V to V.
  • Quality Output QUALITY: In simulation, output quality reflects simulated status (16#60), and the safety driver asserts maintenance flags.

Step-by-Step Engineering and Implementation Procedure

Step 1: Insert F-Boundary Conversion Blocks in CFC

  1. Open the fail-safe CFC chart containing the F_CH_AI instance.
  2. Open the S7 F-Systems library, navigate to 'F-Control Blocks'.
  3. Insert an F_BO_F block (Standard BOOL to F_BOOL) into the chart.
  4. Insert an F_R_F block (Standard REAL to F_REAL) into the chart.

Step 2: Wire Standard Simulation Variables to Conversion Blocks

  1. Create a standard global data block (e.g., DB_SimControl, non-safety) containing:
    - SimEnable (BOOL)
    - SimValue (REAL)
  2. Interconnect DB_SimControl.SimEnable to F_BO_F.IN.
  3. Interconnect DB_SimControl.SimValue to F_R_F.IN.

Step 3: Wire Conversion Outputs to F_CH_AI Driver Block

  1. Connect F_BO_F.OUT to F_CH_AI.SIM_ON.
  2. Connect F_R_F.OUT to F_CH_AI.SIM_V.

Step 4: Compile and Download Safety Program

  1. In SIMATIC Manager, select 'CFC' -> 'Compile' -> 'Charts as Program'.
  2. The F-Compiler verifies safety data flow, recalculates safety CRCs, and generates safety signatures.
  3. Download the program to the CPU 414-4H; enter the Safety Password when prompted.

Verification

To verify fail-safe simulation operation:
1. Open the CFC chart in test mode; verify F_CH_AI.SIM_ON = 0 and F_CH_AI.V reflects live field mA signals.
2. In your test DB or WinCC screen, write SimValue = 50.0 and set SimEnable = 1.
3. Verify that F_CH_AI.SIM_ON transitions to 1 and F_CH_AI.V outputs 50.0 immediately.
4. Verify that downstream safety interlocks evaluate the simulated value properly.
5. Reset SimEnable = 0; verify the driver instantly returns to tracking physical field sensors.

Limitations and Operational Precautions

Operational precautions:
- Activating simulation on fail-safe channels temporarily bypasses physical process trip protections. Never enable simulation on live operating chemical or nuclear processes.
- All safety simulations must be audited and reset to 0 before signing off safety validation documents.
- Modifying safety connections changes the Safety Collective Signature (CRC), requiring re-verification of safety documentation.

Engineer FAQ

Q: Why does CFC reject connecting standard DB bits to F_CH_AI?

A: S7 F-Systems prohibits direct connections between standard and fail-safe logic to prevent unvalidated standard memory corruption from compromising the safety runtime.

Q: What blocks convert standard types to safety types?

A: Block F_BO_F converts BOOL to F_BOOL, and F_R_F converts REAL to F_REAL.

Q: Can simulation be forced directly on the ET 200 module?

A: Yes, through HW Config or SIMATIC Manager variable tables, but software simulation via F_CH_AI is preferred for automated testing.

Q: Does simulation generate a safety alarm in WinCC?

A: Yes, APL and F-Systems drivers assert simulation status (QUALITY = 16#60), generating a maintenance warning on the operator station.

Q: What CPU models support S7 F-Systems?

A: S7 F-Systems is supported on fail-safe S7-400H controllers, including CPU 414-4H, CPU 414-5H, CPU 417-5H, and CPU 410-5H.

Technical References

Related Articles

Related Tags

F_CH_AI, S7 F-Systems, Fail-Safe Simulation, F_BO_F, CPU 414-4H

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