This guide explores the methods, tools, and best practices for resetting or formatting Siemens S7-300 MMCs, primarily focusing on using Step 7 software to ensure compatibility. What is a Siemens MMC Card?
Siemens Micro Memory Cards (MMCs) are proprietary storage devices used in SIMATIC S7-300 and ET200 PLC systems. Unlike standard commercial SD or MMC cards, Siemens cards use a specialized internal file system required by the PLC firmware. Formatting a Siemens MMC using standard Windows formatting tools will corrupt the hidden system file structure, rendering the card completely useless to the PLC.
: Reading or writing to these cards on a PC officially requires a Siemens Field PG (with built-in prommer) or a Siemens USB Prommer Industrial Monitor Direct 2. Official Formatting (Reset) Procedures Siemens officially refers to "formatting" an MMC as an Overall Reset
Commercial operating systems write FAT16, FAT32, or NTFS file systems over the entire storage media. Doing this wipes out the specific internal structure and block formatting that the Siemens SIMATIC S7 PLC expects. Once a standard Windows format is performed, the PLC will no longer recognize the card, and it will continuously blink its system fault (SF) or maintenance LEDs. Method 1: Formatting directly inside the S7-300 PLC CPU
This is normal behavior. Windows cannot parse the Siemens partition tree and may show a 64KB card as an unallocated or RAW 16MB drive. siemens mmc card format tool
💡 If the tool cannot read the card at all, try a different card reader. Some modern generic USB readers struggle with the proprietary format of Siemens industrial cards.
Insert the MMC card into the reader slot. Note the drive letter Windows assigns to it (e.g., Drive E:).
Siemens MMC Card Format Guide: Tools and Techniques Formatting a Siemens Micro Memory Card (MMC) is a delicate process because these cards use a proprietary filesystem. Using standard Windows formatting tools often renders the card unusable by the PLC because it destroys the hidden Siemens-specific header information. Industrial Monitor Direct Recommended Tools and Official Methods
Instead of a third-party "format tool," use the following built-in methods provided by Siemens software and hardware: 1. Hardware Reset via CPU ( Go to product viewer dialog for this item. This guide explores the methods, tools, and best
No. S7-300 CPUs require proprietary Siemens Micro Memory Cards (MMCs). Standard SD cards will not be recognized, and trying to use them may cause CPU errors. Why does my PLC keep asking for an MMC?
The definitive, industrial-grade external programmer designed to handle all S7 memory cards flawlessly via a USB interface. Summary Checklist for Troubleshooting Corrupted MMCs Root Cause Corrective Action SF/STOP LED Flashing on PLC Corrupted user program block or system conflict.
Best for: Control engineers, system integrators, and automation professionals.
: If you click "Format," Windows overwrites the proprietary hidden partition and registers (CID/CSD) that the Siemens CPU uses to identify the card. Once this happens, the card becomes a "dead" brick to the PLC, even though it works perfectly as a standard 16MB or 32MB card for a computer. Official Methods for "Formatting" (Resetting) Unlike standard commercial SD or MMC cards, Siemens
Unlike standard MMC cards, — doing so will corrupt the card and make it unusable in the PLC. You must use Siemens-specific software.
The most "official" way to format a card is through the Siemens programming software itself.
The card is full, and you need to clear all data blocks (DBs) and the program.
If your application uses the MMC for extensive data logging (writing .csv files or retentive data blocks every second), the Flash memory will wear out faster. If a machine is 15 years old and the card is failing, format it once. If it fails again shortly after, replace the hardware.
: Formatting a Siemens MMC using a standard Windows "Format" tool (FAT/NTFS) will destroy the proprietary header , rendering the card unusable in a PLC. Hardware Requirement