Reset SIMATIC MMC for S7-300 CPU: Avoid Format Damage

Overview

SIMATIC Micro Memory Cards (MMCs) used with S7-300 CPUs do not use a Windows-, FAT-, or ext-compatible filesystem. Siemens installs a proprietary image at the factory that the S7-300 CPU bootstrap loader expects on power-up. Treating the card like a consumer SD card and running a normal "Format" operation overwrites the Siemens image with an unrecognized file allocation table, leaving the card usable in card readers but unreadable inside any S7-300, S7-400 (with adapter), or ET 200S CPU. The supported recovery path is therefore a Siemens-defined Reset, not a generic Format.

This reference covers the 128 KB card 6ES7953-8LG20-0AA0, but the same procedure applies to every member of the 6ES7953-8L family that targets the S7-300 family (CPU 312, CPU 314, CPU 315, CPU 315-2 DP, CPU 316, CPU 317, CPU 318, and the integrated 31xC variants). When a card carrying program/HW-config data for one CPU type is slotted into a different CPU type, the destination CPU reports invalid system data and refuses to enter RUN; the MRES routine is the recovery action.

Why a Standard Format Damages the MMC

A SIMATIC MMC carries the following non-volatile regions when delivered from Siemens:

• Sector 0 / boot sector with the Siemens CPU bootstrap signature (S7MMCDAT family).
• Project blocks (OB, FB, FC, DB) written by STEP 7 / TIA Portal.
• System data blocks (SDB) including hardware configuration, MPI/PROFIBUS parameters, and CPU-to-CPU routing.
• Diagnostic buffer mirror, password table (CPU password + know-how protection), and optional force table.
• Operating system patch area used during firmware updates.

Microsoft's SD Card Formatter utility writes a clean FAT16/FAT32/exFAT header and zeroes the unused area, but it does not preserve the Siemens signature or the SDB layout. Once the card is formatted with consumer tooling and inserted into an S7-300 CPU, the CPU enters the STOP state, the STOP LED slow-blinks, and the diagnostic buffer logs Memory card not recognized. The card is not destroyed in the electrical sense - the flash is intact - but the missing Siemens signature makes it unusable without Siemens recovery tooling. Restore-by-format is not possible.

MMC Identification and Specifications

The card family is the 6ES7953-8L series, distinguished by memory capacity. The article number 6ES7953-8LG20-0AA0 decodes as a 128 KB card (suffix LG) at firmware/hardware revision 20 (suffix 20) in release 0 (suffix 0AA0). Refer to the Siemens catalog "SIMATIC S7-300 / ET 200S - Memory Cards" for the full order-number matrix.

Comparable cards in the same series with greater capacity use the same Siemens image; the reset procedure is identical. Higher densities include:

• 6ES7953-8LF20-0AA0 - 64 KB.
• 6ES7953-8LH20-0AA0 - 256 KB.
• 6ES7953-8LJ20-0AA0 - 512 KB.
• 6ES7953-8LK20-0AA0 - 1 MB.
• 6ES7953-8LL20-0AA0 - 2 MB.

Prerequisites

Before attempting a reset, verify the following items:

1. The MMC is physically clean and the gold-flash contacts show no oxidation, scratch, or contamination. Re-seat once to rule out contact failure; the CPU should recognize the card on power-up and slow-blink the STOP LED.
2. The S7-300 CPU is in STOP state with the mode selector in RUN or RUN-P position. The mode selector is a three-position key switch on the front of every S7-300 CPU: RUN, STOP, MRES.
3. The CPU firmware version is known. Check with STEP 7 / TIA Portal under Online > Diagnostics > Module Information or read the label on the front panel of the CPU. The reset path described in this article requires firmware V2.5 or higher; CPU 318 ignores firmware versioning and ships its own reset path.
4. The MMC has no write-protect (the 6ES7953-8L series does not feature a write-protect tab, but the CPU implements password-level protection that the reset will not clear).

MRES Reset Procedure (S7-300 CPU)

The MRES routine is implemented in S7-300 firmware V2.5 and higher for all CPUs except CPU 318. It is the supported path for clearing an MMC that the destination CPU reports as invalid system data - the condition created when a card programmed for one CPU type is moved into a different CPU type.

1. Insert the MMC into the bay at the front of the CPU. The card is keyed; the beveled corner must point downward toward the LED array.
2. Apply power to the CPU. The CPU enters STOP with a slow-blinking STOP LED because the firmware rejects the foreign data on the card. This is the expected state for an MMC containing data valid for a different CPU.
3. Move the mode selector to MRES and hold it there. The STOP LED changes from slow-blink to steady-on after approximately 9 seconds. Keep holding the selector.
   Watch the LED, not the wall clock. If the selector is released before the STOP LED is steady-on, the firmware interprets the gesture as a normal Stop transition and the reset is not triggered.
4. Within 3 seconds of the LED going steady-on, release the selector, then move it again to MRES. The two-step toggle is the second confirmation that the engineer explicitly requested the data-clear operation.
5. The STOP LED now flashes during the delete. The CPU erases every block on the MMC (OB, FB, FC, DB, SDB), clears the diagnostic buffer, and resets interface parameters (MPI/PROFIBUS address, baud rate, protocol selection, DP master/slave role) to default.
6. When the STOP LED returns to steady-on, the reset is complete. The MMC now contains only the Siemens-typical firmware image with no project data.

If the CPU resumes slow-blinking the STOP LED after step 6, the reset routine rejected the card. Pull the MMC, confirm it is a 6ES7953-8L card (not a generic SD card masquerading in an MMC adapter), confirm the gold contacts are clean, and retry. If the failure repeats, the MMC is defective and must be replaced.

Timing Reference

Firmware-Specific Behavior

The MRES routine's ability to clear an MMC directly from the CPU depends on the firmware version loaded on the CPU, not the card capacity. This matrix is taken from the Siemens Application Note "How do you delete the data on an MMC?" which references the operating manual S7-300 CPU 31xC and CPU 31x: Setting up.

Reset via Field PG or USB Prommer

When the in-CPU reset is unavailable - firmware V3.2, V3.3, or a defective-but-recognized card - the MMC must be cleared by an external tool. The Siemens FAQ How do you delete the data on an MMC (micro memory card)? documents two paths.

Path A - SIMATIC Field PG (or equivalent programming device)

1. Power down the Field PG. Insert the MMC into the internal MMC slot. The slot is identical to the bay on an S7-300 CPU.
2. Boot the Field PG into Windows. Start STEP 7 (or TIA Portal with S7-300 add-on).
3. In STEP 7: File > S7 Memory Card > Delete. In TIA Portal: Project > Memory Card > S7 Memory Card > Erase card.
4. Confirm the dialog. STEP 7 / TIA Portal writes a fresh Siemens image onto the card and reports Card erased successfully.
5. Verify by selecting File > S7 Memory Card > Show Properties. The card should report S7-300 as the target family and 0 blocks used.

Path B - SIMATIC USB Prommer

1. Connect the USB prommer to the Field PG via the supplied USB cable. Install the S7-Prommer driver from the SIMATIC Field PG DVD if necessary.
2. Open the S7-Prommer software. Select Erase as the operation. Choose the matching MMC device class (e.g., 6ES7953-8LG20).
3. Insert the MMC into the prommer socket and confirm the software recognizes the device (it reports part number, capacity, serial).
4. Click Erase. The software writes the Siemens boot signature and clears the SDB area.
5. After the erase completes, label the card with the date and re-insert into the Field PG's MMC slot for verification.

Writing or Deleting the MMC Outside an S7-300 CPU

External read/write of SIMATIC MMCs is documented in Entry ID 21829579 ("How do you write or delete (reset) a SIMATIC MMC outside an S7-300 CPU?"). The answer explicitly lists valid hosts: SIMATIC Field PG, SIMATIC Tablet PG, externally powered USB prommer with the S7-Prommer driver, and any laptop running STEP 7 with a SIMATIC PC adapter connected to a USB MMC reader. Hosts that are not supported include generic Windows Explorer drag-and-drop, SD-Card Formatter, and Android apps such as "Memory Format" from the Play Store - the card-protocol is Siemens-proprietary, so these tools simply see a non-standard removable drive and either ignore the content or overwrite the boot signature.

Verification

After the MRES reset or external erase, the following checks confirm the card is usable:

1. Re-seat the MMC in the CPU. Power-cycle the CPU. The slow-blink STOP LED should change to steady-on after roughly 5 s, indicating no project + valid MMC image.
2. Connect STEP 7 / TIA Portal to the CPU. Run Online > Diagnostics > Module Information. The diagnostic buffer should contain no error entries and the Memory tab should report 0 of N blocks loaded.
3. Open Accessible Nodes. The CPU must appear with its MPI/PROFIBUS parameters in their default state (MPI address 2, 187.5 kBd for the legacy default; PROFIBUS DP parameters unchanged since production at the user's specification, or default 1.5 Mbaud DP if never configured).
4. Test the slot-acceptance logic by attempting a small download: Online > Download to target device > Test and repair hardware. The CPU should accept the operation without complaining about an invalid card.

A more rigorous test is to swap the freshly reset MMC into a second S7-300 CPU of the same type and confirm that CPU also powers up cleanly. This verifies the Siemens image, not the project data.

Troubleshooting Matrix

Comparison: Reset Path vs. Format Path

Field Commissioning Notes

• Always power-cycle the CPU with the MMC pre-inserted. Hot-plugging an MMC on an S7-300 is allowed, but the bootstrap may finish with the previous state; a power-cycle clears this ambiguity.
• After a successful reset, the new MPI / PROFIBUS parameters default to whatever the firmware ships with, not necessarily the last project values. If the CPU is networked and unreachable, the engineering PC must scan with a direct MPI cable rather than expecting the previous DP address.
• If the project lives in a TIA Portal TIA Server archive, recovery is a single block import plus Download to device. If only a printed wiring list exists, the HW configuration must be rebuilt by hand or imported from a separate .s7p backup.
• A reset MMC accepts new project data identical to a brand-new card; no "format in CPU" prerequisite is required before downloading.
• Do not attempt the MRES routine on a card with a known-good project unless a backup exists. The routine is non-reversible and STEP 7's block-level "un-delete" is not available for MMC-resident data.
• For ET 200S IM151 stations, the same MMC and the same MRES sequence apply through the IM151's CPU mode. The CPU-side reset of the IM151 behaves identically to a standalone S7-300 CPU.

FAQ