Do not use a standard MMC card reader or a PC to format or copy files onto an S7-300 SIMATIC MMC — the result will be a card that the CPU cannot read, even if the file structure appears correct in Windows Explorer. This Siemens S7-300 MMC memory card corruption recovery guide covers the complete format, backup, and replace procedure using the correct STEP 7 tools to restore a corrupted SIMATIC MMC. The S7-300 SIMATIC MMC (Multi Media Card) uses a proprietary Siemens format that is written only by STEP 7’s card writing tools or by the S7-300 CPU itself during a download. A card formatted by any other method — including Windows Format, diskpart, or standard MMC card reader software — produces a card that triggers the S7-300 DEFECT mode immediately on startup. This guide covers the complete MMC corruption recovery procedure using the correct STEP 7 tools.
- S7-300 Memory Architecture: RAM vs MMC vs Load Memory
- What the SIMATIC MMC Does and Why It Corrupts
- Symptoms of a Corrupted SIMATIC MMC
- Step 1: Remove the MMC and Verify CPU Behavior Without It
- Step 2: Format the MMC Using STEP 7
- Step 3: Restore the User Program to the Formatted MMC
- Step 4: Sourcing a Replacement SIMATIC MMC
- Preventing Future MMC Corruption
- Technical Validation
- Frequently Asked Questions
What the SIMATIC MMC Does and Why It Corrupts
The SIMATIC MMC in an S7-300 CPU serves two primary functions: 1. Non-volatile program storage: the user program, data block initial values, and configuration data stored on the MMC survive power loss and CPU MRES (memory reset) without requiring the CPU battery 2. Optional retain data storage: when configured, the CPU writes retain variable values to the MMC at specific intervals or on power-down, allowing retain data to survive even without the CPU battery
The MMC format contains a Siemens-specific file system structure that the CPU’s bootloader reads during startup. Corruption can occur when: – The mains supply is interrupted while the CPU is writing to the MMC (e.g., during a program download or an automatic retain data save) – The MMC is physically removed from the CPU while the CPU has an active write operation – The MMC card has reached its write cycle endurance limit (most SIMATIC MMCs are rated for 100,000 write cycles — sufficient for normal operation but potentially exceeded in high-cycle retain data save applications)
S7-300 Memory Architecture: RAM vs MMC vs Load Memory
Understanding the three memory areas prevents confusion during recovery. The S7-300 uses three distinct memory regions:
| Memory Area | Location | Contents | Survives Power Loss? | Survives MRES? |
|---|---|---|---|---|
| Work RAM | Inside CPU | Currently executing program + DB values | No (needs battery) | No — cleared |
| SIMATIC MMC | Removable card | Permanent copy of program + DB initial values | Yes (non-volatile) | Yes |
| Load Memory | Also on MMC | Compiled program blocks before linking | Yes (non-volatile) | Yes |
On power-up, the S7-300 CPU copies the program from the MMC (Load Memory) into Work RAM and begins execution. If the MMC is corrupted, the CPU cannot populate Work RAM and enters DEFECT mode. Critically, if the CPU was running for hours before the MMC failed and the battery is functional, Work RAM still contains a valid running copy of the program — the program may be recoverable from RAM even when the MMC is corrupted.
Backup without a STEP 7 project file: if the original STEP 7 project is lost but the CPU Work RAM is still intact, upload from CPU to STEP 7 before replacing the MMC:
STEP 7 SIMATIC Manager →
Online → Upload from module →
Select: Blocks → All blocks →
Save to a new STEP 7 project (File → Save)
This recovers the compiled program blocks from CPU RAM into a STEP 7 project. Once saved, download to the new/formatted MMC via the normal download procedure. This is the only method that allows MMC recovery when the STEP 7 project archive no longer exists.
Symptoms of a Corrupted SIMATIC MMC
A corrupted SIMATIC MMC produces one or more of these symptoms:
| Symptom | Likely MMC Condition |
|---|---|
| CPU enters DEFECT mode (RUN+STOP LEDs flashing) on startup | MMC content corrupted — bootloader cannot load program |
| STEP 7 diagnostic buffer shows “Memory card read error” | File system error on MMC |
| CPU starts normally but program blocks show wrong version | Partial write — some blocks updated, others not |
| CPU starts but alarm “Transfer error from MMC” appears | CRC mismatch — MMC content does not match expected checksum |
| STEP 7 “Memory Card” file view shows empty or garbled content | Complete file system corruption |
Step 1: Remove the MMC and Verify CPU Behavior Without It
The first diagnostic step is the same as for DEFECT mode: remove the MMC and power-cycle the CPU:
1. Set mode selector to STOP
2. Power off the CPU (remove 24VDC supply)
3. Remove the SIMATIC MMC from the faceplate slot
4. Power on the CPU
5. Observe: CPU should reach STOP state (solid red STOP LED)
— This confirms the CPU hardware is functional
6. STEP 7 Module Information → Diagnostic Buffer:
should show normal startup with "No program loaded" indication
If the CPU enters DEFECT mode WITHOUT the MMC, the problem is not the MMC — it is the CPU firmware or hardware. See Siemens S7-300 All Leds Flashing Defect Mode Recovery Procedure for the DEFECT mode recovery procedure.
If the CPU reaches normal STOP without the MMC, the MMC is confirmed as the fault source.
Step 2: Format the MMC Using STEP 7
To restore the SIMATIC MMC to a usable state, format it using the STEP 7 MMC format tool — not Windows format:
Method A: STEP 7 SIMATIC Manager — Card Format:
STEP 7 SIMATIC Manager →
PLC → Memory Card File → Format →
Select: S7 Memory Card (MMC) →
Confirm format operation
Method B: STEP 7 Memory Card Manager:
Windows Start → SIMATIC → STEP 7 → Memory Card Manager →
File → Format → Select the card → Confirm
The STEP 7 format tool requires a programming device with a MultiMediaCard slot (many older Siemens programming devices, PG M2, have this slot) or a SIMATIC Micro Memory Card Adapter (catalog number 6ES7954-8LC02-0AA0) that connects via USB to a standard workstation.
Note: standard MMC card readers in laptop slot bays are typically NOT compatible with the STEP 7 Memory Card Manager’s format operation — use only the Siemens MMC adapter or a Siemens programming device (PG) for MMC format operations.
After format, the MMC appears as empty in the STEP 7 Memory Card File view, with the correct Siemens file system structure written.
Step 3: Restore the User Program to the Formatted MMC
After formatting, the user program must be written to the MMC. There are two approaches:
Option A — Transfer from STEP 7 project (preferred if backup exists):
STEP 7 SIMATIC Manager →
Insert formatted MMC into CPU and power on CPU →
Online → Download to Module →
Select all blocks → Download
This writes the program from the STEP 7 project to both the CPU RAM and the MMC simultaneously.
Option B — Transfer from a backup MMC: If a backup MMC with the correct program exists, insert it in the CPU and use:
STEP 7 → Online → Copy RAM to ROM →
Copies current CPU RAM to the MMC
This preserves all program blocks and DB values currently in CPU RAM to the MMC, creating a new backup on the freshly formatted card.
Verification: after download, go offline and remove the MMC. Power-cycle the CPU with the MMC reinserted. The CPU should load the program and reach RUN (or STOP if the mode selector is in STOP). Verify the diagnostic buffer shows a clean startup sequence with no MMC read errors.
Step 4: Sourcing a Replacement SIMATIC MMC
If the MMC cannot be formatted (media failure — the card is physically damaged) or if the card’s write cycle count is exhausted, a replacement is needed.
Compatible SIMATIC MMC catalog numbers (capacity variants):
| Capacity | Catalog Number | Suitable For |
|---|---|---|
| 64 KB | 6ES7953-8LA11-0AA0 | Small programs (CPU 312C, CPU 313C) |
| 512 KB | 6ES7953-8LF11-0AA0 | Medium programs (CPU 315-2DP, CPU 317) |
| 2 MB | 6ES7953-8LG00-0AA0 | Large programs, data logging |
| 4 MB | 6ES7953-8LJ00-0AA0 | Very large programs, CPU 317/319 |
| 8 MB | 6ES7953-8LL00-0AA0 | Maximum size S7-300 MMC |
Select the capacity that matches or exceeds the size of the user program and stored data block content. The CPU will reject an MMC smaller than the program size during download.
For the relationship between MMC state and the BATF battery fault — including when MMC non-volatile storage can replace battery-backed retain data — see Siemens S7-300 Batf Battery Fault Replacement Procedure.
Preventing Future MMC Corruption
Configure UPS protection for the CPU rack: the most common cause of MMC corruption is a power interruption during a write cycle. A UPS (Uninterruptible Power Supply) on the 24VDC supply to the CPU rack provides a controlled power-down window — sufficient for the CPU to complete all pending write operations to the MMC before losing power.
Avoid unnecessary MMC write cycles: if the S7-300 is configured to save retain data to MMC after every power-down event (or at short intervals), the MMC accumulates write cycles rapidly. Configure retain data save intervals to match the actual process requirements — many applications need retain data saved only when it changes significantly, not every scan cycle.
Label the MMC with the program version: when the MMC contains the correct program version, label it with the version number and date. This simplifies recovery if the card needs replacement — the correct program version is known without searching STEP 7 archives.
For S7-1200 diagnostic and memory card compatibility reference, see: Siemens S7-1200 Error Codes Complete List
Technical Validation
S7-300 MMC fault code diagnostics and memory card behavior from the Siemens STEP 7 Error Code Reference Document 770453. SIMATIC Micro Memory Card specifications and S7-300 memory management from Siemens Industry Mobile Support Article 109478459. Industrial PLC memory management training from National Institute of Corrections — PLC Training Resources.
Frequently Asked Questions
Can the S7-300 SIMATIC MMC be read directly on a PC using a standard card reader to extract the program files if the CPU is not available?
No. The Siemens SIMATIC MMC uses a proprietary file structure that is not readable by standard Windows file manager, even with a compatible MMC slot. On a PC, the card may appear as an unformatted or unrecognized storage device, or it may appear readable but with filenames in a Siemens-internal format that cannot be opened by standard tools. To read the program from an MMC without the CPU: use the Siemens PG M2 programming device (which has an integrated MMC slot recognized by STEP 7) and the STEP 7 Memory Card Manager to copy the program blocks to a STEP 7 project.
The STEP 7 format tool shows an error “No card found” when the MMC is inserted in the Siemens USB MMC adapter. What prevents the adapter from recognizing the card?
“No card found” from the STEP 7 Memory Card Manager with a card physically inserted typically indicates: (1) the USB MMC adapter driver is not installed — install the Siemens SIMATIC Card Reader USB driver from the STEP 7 installation media; (2) the adapter is connected to a USB 3.0 port — try a USB 2.0 port, as some Siemens MMC adapters are not USB 3.0 compatible; (3) the card is physically damaged and not making proper electrical contact in the adapter slot — try reseating the card; (4) the MMC card is not a genuine SIMATIC MMC — some counterfeit or incompatible cards are not recognized by the Siemens driver.
After restoring the program to a new MMC, production reports that recipe setpoints are back to default values. Why do retain variables restore incorrectly after MMC replacement?
Retain variables in S7-300 are stored in CPU RAM backed by the battery. When the program is downloaded to a new MMC, the retain area on the new card is empty and the CPU initializes retain variables to the data block initial values (default values) — not to the last operational values. If the battery discharged during the repair process, the RAM content was also lost. Recovery procedure: before downloading to the new MMC, if the CPU still has battery-backed RAM intact and the old MMC is still installed, read the current retain DB values online in STEP 7 and document all setpoints. After the MMC replacement and program download, restore the documented setpoint values via the STEP 7 data block monitor or HMI. For future protection, implement periodic WRITE_DBL (SFC84) calls to save critical retain data to non-volatile MMC storage explicitly.