Troubleshooting S7-1200 MMC Error 02:4175 and 02:3956 After TIA Portal V13 Operations

1. Problem Overview

A SIMATIC S7-1200 CPU is operated in Program card mode with a SIMATIC Memory Card (SMC) inserted. After several successful days of operation and minor program edits, the CPU begins reporting two simultaneous diagnostic events the moment the SMC is inserted:

• 16# 02:4175 — Memory card evaluation error: Unknown or incompatible version of CPU configuration description. The CPU's startup routine rejects the on-card SDB/SDB-container.
• 16# 02:3956 — Hardware component not available due to a parameter assignment error — Parameter assignment in progress. The hardware configuration is flagged as inconsistent with the inserted card.

Removing the card returns the CPU to normal RUN. Inserting the card again reproduces both events and forces a new startup inhibit. TIA Portal V13 (or V13 SP1) cannot push a new project to the CPU; the online dialog returns "Loading the hardware configuration failed (0050 79 2 2458)".

This document consolidates the four field-confirmed recovery paths for this class of fault on a CPU 1214 DC/DC/DC (6ES7 214-1AG40-0XB0) with SMC 6ES7 954-8LC02-0AA0 (12 MB) and explains when to use each one.

2. Affected Hardware, Firmware, and Software

The fault has been reproduced specifically on the following component combination. Other members of the S7-1200 family exhibit identical behavior because they share the same SMC boot logic.

Refer to the SIMATIC S7-1200 Programmable Controller System Manual (entry ID 109741275) for the full SMC chapter and the S7-1200 CPU 1214C DC/DC/DC device manual (entry ID 109744417) for the AG40-specific operating modes.

3. Diagnostic Event Reference

The S7-1200 raises these two events as a coupled pair. The first is the root cause; the second is the cascading symptom emitted while the CPU attempts to bring up the hardware configuration referenced by the card.

4. Root Cause Analysis

The SMC is a FAT-formatted SD card that contains a Siemens-specific header structure with a CPU configuration description block. At every power-on the CPU performs the following sequence:

1. Read the SMC's master boot record and Siemens container header.
2. Validate the embedded CPU configuration version against the firmware's expected schema.
3. If valid, transfer the configuration to internal work memory and start RUN.
4. If invalid, raise 02:4175, raise 02:3956, and set the startup inhibit.

The most frequent observed triggers for a previously-good card to be rejected are:

1. TIA Portal V13 / V13 SP1 partial-write interrupt. A "Download to device" operation that is interrupted (USB hub removed, screen-lock, power-loss to the PC, TIA crash) leaves the on-card configuration block half-updated. Subsequent CPU boots cannot reconcile the on-card header with the in-memory image.
2. Project edits without recompile. Touching HMI tags, adding a DB, or modifying the hardware catalog without a full "Software (rebuild all)" can produce a configuration block the V4.x firmware tags as "incompatible version."
3. Card inserted while CPU is in RUN and TIA Portal attempts a partial block download directly to the card.
4. Cross-firmware boot (less common): card written by V13 against a FW V4.0 CPU later inserted into a FW V4.2 CPU with a different SDB schema.

The card itself is electrically healthy; the file system is logically inconsistent from the CPU's perspective.

5. Symptom Matrix and Triage

6. Resolution Path A — MRES Memory Reset Recovery

Use when: the card content is suspect but the on-CPU project image is intact, and the CPU can be taken out of service briefly. This is the fastest non-destructive recovery and is field-confirmed to clear the 02:4175 / 02:3956 pair.

1. Power down the S7-1200 (PS OFF).
2. Insert the suspect SMC into the CPU slot.
3. Power up the CPU. Wait for the diagnostic events to appear in the buffer.
4. Set the mode selector to MRES. Hold for approximately 3 seconds until the STOP LED flashes slowly.
5. Release, then rotate the selector to MRES again within 3 seconds. Hold until the STOP LED flashes rapidly (this is the second MRES acknowledgment — the CPU erases internal work memory).
6. Wait for the CPU to remain in STOP with a steady STOP LED (no flashing).
7. Toggle the selector to RUN. The CPU re-evaluates the SMC and re-loads the project into work memory.

Expected outcome: the CPU returns to RUN with no 02:4175 entry in the diagnostic buffer. The events that were already in the buffer remain for forensic review; only the new power-up must be clean.

7. Resolution Path B — Manual Block Restore via Card Reader

Use when: MRES does not clear the events, the card is readable in a PC, and you have a valid TIA project on the engineering station. This is the most reliable non-formatting recovery path on the S7-1200 (it does not require a CPU 1500-era SD formatter).

1. Remove the SMC from the CPU. Power down the PLC first if it is currently powered.
2. Insert the SMC into a Windows-readable SD card reader (USB or built-in).
3. In TIA Portal, open the project that matches the on-CPU configuration (or the most recent known-good version).
4. In the Project tree, expand Card Reader/USB memory in the bottom-left area.
5. Drill down to the SD card → Program blocks folder.
6. Select all program blocks from the project tree (OB, FB, FC, DB) and drag-and-drop them onto the Program blocks folder of the SD card. Confirm the overwrite prompt.
7. Repeat for the PLC data types, Watch and force tables, Recipes, and any other folders under the card that the project uses (e.g. Sources, System data).
8. Safely eject the SD card from Windows.
9. Re-insert the card into the CPU with the PLC powered down, then power up.

Expected outcome: the CPU re-evaluates the card, transfers the project, and enters RUN. The 02:4175 / 02:3956 events do not recur. Online download from TIA is restored.

8. Resolution Path C — Firmware-Update Trick

Use when: the SMC is not mountable in a normal PC reader, the card has been used as a firmware-update card, or the boot descriptor is so corrupted that Path B fails to commit blocks. This is the documented field workaround for "access to memory card denied" plus 02:4175.

1. Insert the SMC into the S7-1200 and power up. The CPU reports 02:4175 / 02:3956.
2. From the engineering station, in TIA Portal, open Online → Accessible nodes and confirm the CPU is online.
3. Navigate to Online & diagnostics → Functions → Firmware update.
4. Set the SMC operating mode to Firmware update via the "Configure card" dialog. Confirm the prompt to reformat the card for firmware-update use. The CPU will write a new Siemens container header appropriate for firmware update.
5. Power down the CPU, remove the SMC, insert into a PC, copy the desired firmware update file (e.g. S7_1200_CPU_1214C_V4_2_3.upd) to the card root.
6. Re-insert the SMC into the CPU and power up. The CPU recognizes the firmware-update card and applies the update (or, if the firmware version is already current, completes the cycle without changes).
7. Power down, remove the card, re-insert into the PC. Delete the firmware update file from the card root.
8. Use the TIA Portal "Configure card" dialog again to set the card mode back to Program transfer (not firmware update).
9. Insert the card into the CPU, power up. The card now boots in program mode with a freshly written header.

Expected outcome: the card boots cleanly in program mode; the 02:4175 event no longer fires; download from TIA succeeds. This path effectively performs a logical reformat of the card header without a full low-level format, and is the only method available on the S7-1200 line that does not require a CPU 1500 or an external SD formatter.

9. Resolution Path D — TIA Portal Version Upgrade

Use when: the project must be re-downloaded from the engineering station, the SMC is readable, and the engineering PC has access to TIA Portal V14 SP1 or later. V14 SP1+ writes the SMC with a configuration block that the V4.x firmware accepts without ambiguity.

1. On the engineering PC, upgrade TIA Portal to V14 SP1 (or later) and the matching S7-1200 CPU support package. Siemens has published the upgrade package in entry ID 109740537.
2. Open the existing project; allow TIA to migrate the project to the new TIA version.
3. Place the SMC in the PC reader. TIA Portal will detect it and surface the option to write the project to the card.
4. Use the card reader's context menu to select Write to memory card. Choose Program card.
5. If TIA reports "access to memory card denied," apply Recovery Path C first to reset the card header, then retry.
6. Insert the card into the CPU and power up.

Expected outcome: clean boot, no diagnostic events, full online connectivity. This path is appropriate as a permanent engineering-side fix because the V13 SMC write path is no longer maintained by Siemens.

10. Verification Procedure

After applying any of the recovery paths, perform the following checks before returning the PLC to production:

1. Diagnostic buffer. In TIA Portal, Online → Online & diagnostics → Diagnostic buffer. Confirm the most recent power-up entry shows no 02:4175 or 02:3956. Older events may remain for forensic purposes; only the latest cycle must be clean.
2. CPU operating mode. Confirm the RUN LED is solid green and the STOP LED is off. Verify the mode selector position.
3. Project version. In Online → Accessible nodes, compare the on-CPU project timestamp with the expected version. Confirm the OB1 cycle counter is incrementing (online monitor on a counter tag).
4. Online download test. Attempt a small online edit (e.g. flip a bit in a DB used only for diagnostics). Confirm TIA Portal completes the download without 0050-79-2 2458.
5. Power-cycle repeat. Power down the PLC for at least 10 seconds, restore power, and confirm the CPU returns to RUN with the project intact. This validates that the SMC boots cleanly, not just that the in-CPU image was reloaded.
6. Retentive behavior check. If your process uses retain variables, verify the retain area was not lost during the MRES (if Path A was used). Loss of retain is expected and acceptable for an emergency recovery; the process must be re-initialized.

11. Preventive Measures and Best Practices

To prevent recurrence, apply the following on every S7-1200 site that uses TIA Portal V13 with V4.x firmware:

• Standardize on TIA V14 SP1 or later for any CPU with firmware V4.0+. V13 SP1 is the latest version that reliably writes the SMC for these CPUs; V14 SP1 is the first version with the corrected header write.
• Always perform a full "Software (rebuild all)" before every "Download to device" that involves a hardware change. Partial recompile leaves the SDB partially in sync with the program blocks.
• Never interrupt a download. Do not use sleep/hibernate on the engineering PC, do not disconnect the USB/PROFINET cable mid-download, and do not pull the SMC mid-download. TIA Portal V13 has no atomic write for the SMC header.
• Disable Windows indexing on the SD card reader. Windows can re-write access timestamps on files on the SMC, which can interfere with the Siemens container's consistency checks. Use the registry or the drive's properties dialog to disable indexing on the reader drive letter.
• Keep a project-archive copy of every SMC's content. In TIA Portal, right-click the card reader and select Read from memory card to snapshot the card after every successful download. Store the snapshot in a versioned project archive.
• Avoid using the SMC in devices other than the PLC between commissioning steps. SD cards formatted by Windows cameras or phones are not compatible and will trigger 02:4175 immediately.
• Use the S7-1200 internal load memory where possible. The 1214 DC/DC/DC with firmware V4.x has sufficient internal load memory for many small-to-medium projects. The SMC is only required when the project exceeds internal load memory or when the card is used as a service/transfer tool. Operating without an SMC eliminates this entire fault class.

12. Related Issues and Edge Cases

The same 02:4175 / 02:3956 pair can appear in the following adjacent scenarios; the recovery differs and must be selected correctly:

Frequently Asked Questions