C135 PCS 7 (PCS7)
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Configuring Third-Party PROFIsafe Encoders and Resolving F_PS_12 Timeout Errors in SIMATIC PCS 7 S7 F-Systems

Configuring Third-Party PROFIsafe Encoders and Resolving F_PS_12 Timeout Errors in SIMATIC PCS 7 S7 F-Systems

When integrating third-party PROFIsafe absolute encoders (such as TR Electronic CDV 75M) into SIMATIC PCS 7 with S7 F-Systems and the generated module driver F_PS_12 reports communication error DIAG = DW#16#15 ('Timeout detected by F-I/O'), the failure occurs because of an F-parameter mismatch, an unacknowledged reintegration request, or an incorrect iParameter CRC (iPar_CRC). In PROFIsafe V2, the encoder requires a unique F_Dest_Add matching its hardware switches and an authentic iPar_CRC generated by the vendor tool entered into HW Config. To clear the timeout, verify the F_WD_Time, confirm the CRC checksums, recompile HW Config and Safety Program, and pulse input ACK_REI on the safety channel driver to execute operator acknowledgment.

Table of Contents

Executive Summary

Safety-related motion tracking in industrial processing (such as positioning hoists, centrifuges, or nuclear valves) relies on fail-safe absolute encoders communicating over PROFIsafe. In SIMATIC PCS 7 with the S7 F-Systems optional package, when third-party PROFIsafe field devices (e.g. TR Electronic CDV 75M) are added to HW Config on a CPU 410-5H, the safety compiler automatically generates fail-safe module driver blocks such as F_PS_12. However, upon downloading the program, engineers frequently observe that the encoder fails to deliver valid position values; the driver outputs fail-safe substitute values (SUBS_ON = 1), and parameter DIAG asserts DW#16#15 (Timeout detected by F-I/O). Even after extending watchdog times to extreme values, the fault persists. Resolving this issue requires aligning PROFIsafe source/destination addresses, verifying vendor iParameter CRCs, and executing operator reintegration.

PROFIsafe Protocol V2 Architecture in S7 F-Systems

PROFIsafe operates as a safety layer tunneled over standard PROFIBUS DP or PROFINET IO (Black Channel Principle):

  • Safety Telegram Structure: Encloses standard process data along with safety control/status bytes, a 24-bit consecutive sequence counter, and a 32-bit CRC signature (CRC2).

  • Addressing Rules: Safety frames are authenticated using two addressing parameters:

  • F_Source_Add: Unique safety identifier of the F-Host (CPU 410-5H). In PCS 7, this is managed automatically by S7 F-Systems (typically assigned to 1 or the central cluster).

  • F_Dest_Add: Unique address assigned to the individual F-Slave (encoder), which must match hardware switches or internal non-volatile memory.

Root Cause Analysis of F_PS_12 Diagnostic Error DW#16#15

In S7 F-Systems, the diagnostic word DIAG exposes specific fault conditions:

  • DW#16#00000015 indicates: 'Communication failure: Monitoring time exceeded (watchdog timeout) or safety frame rejected'.
  • While named 'Timeout', PROFIsafe drivers report this exact code whenever the slave rejects the parameterization telegram (Set_Prm) due to an invalid CRC or invalid safety address.
  • Because the slave rejects the frame, cyclic safety communication never establishes, causing the watchdog timer to expire.

F-Parameters and Vendor-Specific iParameter Checksums (iPar_CRC)

Standard Siemens F-I/O modules calculate CRCs automatically within STEP 7 HW Config. However, third-party intelligent encoders contain device-specific configuration parameters (measuring steps per revolution, total measuring range, rotational direction, and preset values) known as iParameters:

  • The encoder's manufacturer software tool calculates an iPar_CRC checksum based on these parameters.
  • This checksum must be entered into the corresponding hex parameter field in HW Config.
  • If the checksum in HW Config does not match the checksum calculated by the physical encoder's firmware, the encoder refuses the PROFIsafe handshake, triggering error DW#16#15.

Safety Module Driver Diagnostic and Troubleshooting Matrix

The following table summarizes diagnostic codes and corrective engineering responses for F_PS_12 / S7 F-Systems:

DIAG Code (Hex)Driver StateTechnical MeaningProbable Root CauseCorrective Engineering ActionDW#16#00000000HealthyNormal safety operationValid communicationReal-time position data valid (QBAD = 0).DW#16#00000015F-TimeoutWatchdog timeout / frame rejectedF_Dest_Add mismatch or invalid iPar_CRCCheck DIP switches; recalculate iPar_CRC.DW#16#0000000DCRC FaultTransmission CRC errorElectrical noise or termination failureInspect PROFIBUS shielding and baud rate.DW#16#00000040ReintegrationF-I/O awaits acknowledgmentFault cleared, requires operator pulsePulse input ACK_REI on channel driver.DW#16#00000002Module FaultInternal hardware sensor defectDefective optical disc or EEPROMReplace physical encoder hardware.

Step-by-Step Procedure to Integrate and Reintegrate PROFIsafe Encoders

Method 1: Hardware Parameterization and Checksum Verification

  1. Connect the encoder to the manufacturer's parameter tool (e.g. TR-Electronic software) via PC interface.
  2. Configure the operational resolution, gear ratio, and counting direction.
  3. Note the generated iPar_CRC hex checksum displayed in the tool.
  4. Open HW Config in SIMATIC Manager. Open the encoder's Profisafe parameter properties tab.
  5. Ensure F_Dest_Add precisely matches the physical DIP switch address on the encoder.
  6. Enter the verified iPar_CRC into the designated parameter field.
  7. Set F_WD_Time (Watchdog time) to a realistic margin (e.g. 200 ms for PROFIBUS DP at 1.5 Mbps).
  8. Save and compile HW Config.

Method 2: Compiling Safety Program and Executing Reintegration

  1. In SIMATIC Manager, open CFC and select Options > Edit Safety Program.
  2. Click Compile to re-generate safety drivers (F_PS_12) and recalculate the safety program collective CRC.
  3. Download the safety program to the CPU 410-5H in RUN.
  4. Power-cycle the encoder to ensure its internal ASIC initializes cleanly.
  5. In the CFC safety chart, locate the safety channel driver linked to the encoder.
  6. If output parameter ACK_REQ = 1 (Acknowledgment requested), apply a rising edge pulse to input ACK_REI.
  7. Verify that output QBAD switches to 0 and live position data updates.

Verification

To verify that the PROFIsafe encoder is operating safely:

  1. Driver Status Inspection: In CFC Test Mode, inspect F_PS_12. Confirm that DIAG = DW#16#00000000, QBAD = 0, and SUBS_ON = 0.
  2. Motion Validation: Physically rotate the encoder shaft. Confirm that the fail-safe position output tracks rotation continuously without data freeze or overflow.
  3. Trip and Reintegration Test: Disconnect the PROFIBUS cable. Verify that QBAD asserts immediately (substitute values active). Reconnect cable, pulse ACK_REI, and confirm bumpless return to process tracking.

Limitations and Operational Precautions

  • Safety Certification: Modifying any F-parameter or S7 F-Systems logic alters the safety collective signature; changes require formal validation and functional safety recertification according to IEC 61508 / IEC 62061.
  • Watchdog Settings: Setting F_WD_Time excessively high (e.g. 9000 ms) compromises safety response times; calculate proper watchdog limits using the Siemens Safety Response Time Table (SIMATIC S7 F-Systems Execution Times).
  • Power Supply Isolation: Ensure the encoder's 24V DC auxiliary power is sourced from a certified, regulated fail-safe power supply.

Engineer FAQ

Why can't I change the F-Source address cluster on a CPU 410-5H?

In CPU 410-5H with S7 F-Systems V6.1+, the F-Source address is managed automatically by the engineering system to ensure multiproject uniqueness.

Can ACK_REI be tied permanently to TRUE (1)?

No. S7 F-Systems strictly requires a 0-to-1 rising edge transition for reintegration acknowledgment to prevent automated un-safe restart.

Does S7 F-Systems require a dedicated hardware memory card on the CPU 410-5H?

Yes, CPU 410-5H requires a System Expansion Card (SEC) sized for the project's PO count and an active F-runtime license.

Technical References

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  • https://industrialautomation.net/search?query=simatic-pcs7
  • https://industrialautomation.net/search?query=s7-f-systems
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  • https://industrialautomation.net/search?query=safety
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