Resolving Siemens IM151-8 F-CPU Error 0xD242 SDB99 F-Block

Resolving Siemens IM151-8 F-CPU Error 0xD242: SDB99 F-Block Protection Violation

The Siemens IM 151-8 F-CPU is the fail-safe interface module / CPU of the ET 200S distributed I/O system, designed for safety-oriented applications up to SIL 3 (IEC 61508) / PL e (EN ISO 13849-1). Error 0xD242 ("Protection violation while processing F blocks. F blocks can be processed only after a password has been entered") is one of the most common stop-cause diagnostics encountered during commissioning, program download, or maintenance of an F-CPU that still holds a safety program protected by an active safety password. Because memory-card formatting and standard MRES do not always clear the F-region, the error persists after those actions, and an additional procedure is required.

1. Hardware Context: IM 151-8 PN/DP F-CPU

The IM 151-8 F-CPU (article numbers 6ES7151-8FA00-0AB0 and 6ES7151-8FB00-0AB0 for the PROFINET variant) integrates the CPU function into the ET 200S head module. Key characteristics relevant to error 0xD242:

The F-CPU stores the safety program in a logically separated region. This region is protected by:

1. The standard CPU password configured under PLC > Protection (know-how protection / write protection).
2. The F-CPU safety password configured inside the safety program editor, stored within SDB99 and verified against the F-blocks at every download / upload / online operation.

2. Error Code Definition: 0xD242

Diagnostic buffer entry (STOP cause / OB121 / OB80 / OB82-style event) reads:

(D242) Protection violation while processing F blocks.
F blocks can be processed only after a password has been entered.
F block SDB99 cannot be deleted.

Hexadecimal 0xD242 in the Siemens S7 diagnostic notation maps to the diagnostic event class "Protection violation" (high byte 0xD2) with the secondary identifier 0x42 ("F block processing blocked, safety password required"). The CPU enters STOP mode and refuses any online operation against the F-region until the safety password is presented in clear text to the CPU.

3. LED Diagnostics on the IM 151-8 F-CPU

The front of the IM 151-8 F-CPU carries six diagnostic LEDs. Use them to confirm STOP reason and force status before any reset attempt. Reference: Status and error displays of the IM 151-8 PN/DP CPU.

When 0xD242 is active, expect a steady yellow STOP LED with no F-STOP / F-MAINT alternation, which differentiates it from hardware-side safety faults.

4. Root Cause Analysis

Error 0xD242 has five reproducible root causes observed in commissioning and service work:

4.1 Safety password not entered before online operation

The PLC password was removed under PLC > Properties > Protection, but the safety password (a separate password inside the F-program editor) was never explicitly entered when the engineer went online. Removing the PLC password does not lift F-block protection.

4.2 Inconsistent safety password between project and CPU

The project on the engineering station contains one safety password; the CPU was flashed earlier with a different safety password (e.g., project migrated from another station, password changed by a previous user and not communicated). STEP 7 / TIA Portal cannot decrypt SDB99 with the project password, so every online operation against the F-region is rejected.

4.3 SDB99 protected against deletion by CPU safety mechanism

The IM 151-8 F-CPU marks SDB99 with a "lock against delete" attribute the moment the safety program is downloaded with a non-empty password. Standard Delete user program operations via STEP 7 ignore this attribute and fail with 0xD242. A formatted memory card repopulates SDB99 from internal Flash because the CPU retains the F-shared DB in its load memory image.

4.4 F-library version mismatch after firmware or STEP 7 upgrade

After a firmware update of the IM 151-8 F-CPU or an upgrade of the F-library (e.g., F-LB / F-FB / F-FC / F-DB blocks from the Distributed Safety / STEP 7 Safety library), SDB99 stored on the CPU has a different internal signature than what the current project expects. The CPU refuses online operations because the F-runtime group signature differs.

4.5 Corrupted safety program after interrupted download or voltage loss

If the supply voltage was removed mid-download of the F-program (or the memory card was pulled), the CPU's internal F-shared DB may be partially written. The CPU detects an inconsistent SDB99 and refuses all F-block operations.

5. Pre-Resolution Checklist

1. Confirm supply voltage stable; record CPU order number (MLFB) and firmware version (read via STEP 7 > PLC > Operating Mode > Module Information > "Identification" tab). IM 151-8 F-CPU firmware must be compatible with the installed STEP 7 version (TIA Portal V17+ requires CPU FW V3.x for full F-function support).
2. Verify the original project archive (zap / tiar / pcap) is available, including its safety password.
3. If the safety password is unknown, gather proof of CPU ownership / F-acceptance certificate; Siemens technical support cannot bypass the safety password without it.
4. Stop any active force jobs; the FRCE LED must be off before any reset.
5. Document the current diagnostic buffer (PG > Accessible Nodes > F-CPU > Module Information > Diagnostic Buffer). Export as text for audit trail before proceeding.

6. Step-by-Step Resolution

6.1 Attempt 1 — Standard password-protected online delete

1. Open STEP 7 / TIA Portal with the matching project.
2. Go online (Online > Go Online). The engineering tool prompts for the standard CPU password — enter it.
3. When prompted for the safety password, enter the project safety password.
4. Once online, right-click the S7 Program / Safety Program block and choose Delete safety program (or in TIA Portal: CPU > Safety Administration > Delete safety program).
5. Confirm the prompt that all F-blocks and SDB99 will be erased.
6. The CPU performs a safety-program delete followed by an automatic restart. The STOP LED clears, RUN returns, and 0xD242 disappears from the diagnostic buffer.

6.2 Attempt 2 — MRES after password entry

If 6.1 fails (rare, but observed when the safety password in the project differs from the one in the CPU):

1. Go online, enter both passwords successfully (you must know the CPU-side password).
2. Toggle the mode selector to STOP.
3. Hold MRES until STOP flashes at ~2 Hz (about 3 s), release, then within 3 s press MRES again for 3 s.
4. STOP returns to solid; RUN goes off. The CPU has now cleared work memory and reloads the load memory image, which includes the (already empty) F-region.
5. Power-cycle, then perform a full download of the standard program and the F-program from the project.

6.3 Attempt 3 — Delete F-program with empty safety password

For projects originally compiled with an empty safety password:

1. Go online.
2. Select PLC > Safety > Delete F-program.
3. When prompted for the safety password, leave the field blank and confirm.
4. The F-shared DB SDB99 and all F-FB / F-FC / F-DB instances are erased.

6.4 Attempt 4 — Load memory wipe (last resort)

Use when the safety password is lost and the project is rebuilt from scratch. This is destructive — the F-program, the standard program, and any non-volatile process data are lost.

1. Power off the IM 151-8 F-CPU.
2. Remove the SIMATIC memory card (MMC).
3. Insert the MMC into a PC card reader and format it with the SIMATIC Manager / TIA Portal card-format tool (do not use Windows format — it leaves a non-Siemens FAT image and the CPU will reject the card).
4. Alternatively, use the PG function PLC > Operating Mode > Format Memory Card with the empty project.
5. Reinsert the MMC into the powered-down CPU.
6. Hold MRES on power-up until STOP flashes at 2 Hz (~6 s on F-CPU); release; the CPU performs a complete reset, including the F-region.
7. The CPU now boots in STOP with no F-program loaded. 0xD242 cannot occur because SDB99 is empty.
8. Download the rebuilt project (standard + safety) into the cleared CPU.

7. Verification

After any of the procedures above, validate the fix with the following sequence:

1. CPU is in RUN, STOP LED off, SF LED off, FRCE LED off.
2. Open Online > Module Information > Diagnostic Buffer. The last entry should be "Restart" or "Mode transition from STOP to RUN"; no 0xD242 entry should remain.
3. In the F-program editor, perform an Online > Compare safety program. The result must be "Identical".
4. Read the F-runtime group signature from Safety Administration > F-signature and compare with the printed acceptance report. Signatures must match exactly.
5. Execute a test stop / start cycle by toggling the mode selector to STOP then back to RUN. The CPU must restart without entering 0xD242.
6. Trigger a PROFIsafe passivation (disconnect one safety sensor channel). The CPU must passivate the channel cleanly and re-integrate it once the fault is cleared; no 0xD242 may be logged.
7. Re-run the F-acceptance test (per EN ISO 13849-2 or IEC 62061) and re-sign the safety acceptance certificate.

8. Field-Engineering Notes and Common Pitfalls

• The IM 151-8 F-CPU does not support hot-swap of the safety program during RUN; any modification requires STOP. Plan a maintenance window.
• If you swap the MMC between an F-CPU and a non-F CPU of the ET 200S family, the F-CPU will refuse the card because the F-runtime group signature stored on the MMC does not match the target hardware.
• TIA Portal V18+ added an "Force F-signature update" option that intentionally invalidates the stored SDB99 signature. This is useful when migrating F-libraries but produces a temporary 0xD243 (signature mismatch) — confirm whether the operator requires a re-acceptance test before using this option.
• Password aging: F-passwords configured in Distributed Safety V5.4 are stored as DES-hash; TIA Portal V14+ uses SHA-256. After migration, the password is reset to empty on first download. Document this in your change log.
• Diagnostic buffer entries are volatile on power-down — always export the buffer before a reset.

9. Cross-Reference of IM 151-8 F-CPU Articles

All variants share the same diagnostic event codes (0xD242 included). Refer to the manual IM 151-8 PN/DP CPU manual for the complete LED and diagnostic buffer catalog.

10. Troubleshooting Matrix