Resolving the Siemens LOGO! 8 microSD Card "Unknown / Read Error" Fault

The Siemens LOGO! 8 base module (order number 6ED1052-1MD08-0BA0, firmware 1.82.01) reports "Card unknown" or a microSD read error in the on-board diagnostics menu whenever the inserted memory card is rejected by the file system driver. The fault is not a hardware defect in the LOGO! in the majority of cases - it is a filesystem, card-class, or card-orientation incompatibility. This article documents the root cause, the supported card matrix, the formatting procedure that produces a working card on Windows, and the verification steps required to confirm a stable fix.

1. Affected Hardware and Software

2. Root Cause Analysis

The LOGO! 8 firmware mounts a microSD card with a minimal FAT implementation that supports a single, narrow set of on-disk parameters. Anything outside that envelope is reported as unknown. The four most common root causes observed in the field are listed below in order of frequency.

2.1 Filesystem Type Mismatch (most common)

The LOGO! 8 firmware only mounts FAT32 volumes. It does not mount FAT12, FAT16, exFAT, NTFS, ext3, ext4, HFS+, or APFS. A card that is "formatted out of the box" by an SDXC-grade manufacturer is formatted as exFAT, not FAT32, and is therefore rejected on first insertion.

2.2 Card Class Outside the Supported Range

Siemens documentation restricts the supported microSD card capacity to 2 GB up to 32 GB. Cards above 32 GB (microSDXC, 64 GB to 1 TB) ship pre-formatted with exFAT and cannot be reformatted to FAT32 by the native Windows format dialog (Windows refuses to create FAT32 above 32 GB by default).

2.3 Cluster Size / Sector Geometry

Even within a FAT32 volume, the LOGO! file system driver expects a specific set of cluster sizes that the card-formatter tools on Windows produce. Cards that are re-partitioned by third-party Linux tools, by macOS Disk Utility, or by camera-internal formatters may have non-standard bytes-per-sector or 64-KB clusters that the LOGO! bootloader does not read. The FAT32 specification defines a fixed geometry table that the LOGO! matches against - any deviation triggers the unknown-card fault.

2.4 Card Insertion Orientation or Damaged Holder

The card holder is a push-push microSD socket on the front of the module. The card must be inserted with the contact pins facing upward when the LOGO! display is upright. If the holder has been pushed in too aggressively the internal contacts can be deformed and the card will read intermittently as detached.

3. Card Class and Filesystem Compatibility Matrix

4. Pre-flight Checks Before Reformatting

1. Power the LOGO! to STOP mode before inserting or removing the card. Hot-plugging in RUN is permitted by the firmware, but a card swap during program execution can leave a corrupt file in the FAT directory and reproduce the unknown-card fault on the next boot.
2. Verify the card holder is flush with the housing bezel. The slot must sit level; if the bezel is recessed the internal contact springs are bent and no amount of formatting will recover the slot.
3. Verify the card orientation: the metal contact pads must face upward when the LOGO! display is upright. Pads facing down will be electrically invisible and the diagnostic will show Card detached rather than Card unknown.
4. Test the same card in a Windows PC. If Windows cannot read the card either, the card is faulty - replace it before proceeding.

5. Solution: Step-by-Step FAT32 Format on Windows

The procedure below produces a FAT32 volume with the cluster geometry expected by the LOGO! 8 bootloader. Use a Windows 7 / Windows 10 / Windows 11 PC for best results - macOS and Linux do not reliably reproduce the same cluster table.

5.1 Using Windows Disk Management (preferred for end users)

1. Insert the microSD card into a USB card reader attached to the PC.
2. Open Start > diskmgmt.msc to launch Disk Management.
3. Locate the microSD card by its size. Right-click each existing partition on the card and choose Delete Volume. Confirm until the card shows as Unallocated.
4. Right-click the unallocated space and choose New Simple Volume.
5. In the wizard, set:
   • Volume size: maximum (the full card capacity).
   • Drive letter: any free letter.
   • File system: FAT32.
   • Allocation unit size: Default (this is the cluster size matched to the FAT32 spec).
   • Volume label: optional (for example, LOGO8).
   • Uncheck "Perform a quick format" - the full format is required to write a clean boot sector that the LOGO! driver can parse.
6. Click Finish. The full format will take 2-15 minutes depending on card size.

5.2 Using the SD Card Formatter (preferred for OEM / service use)

The SD Card Formatter tool distributed by the SD Association writes the official SD specification boot sector and is the safest formatter for SD / SDHC / SDXC cards. Settings:

• Card: select the microSD device from the drop-down (not a single partition).
• Format Type: Full (Overwrite) - not Quick.
• Volume Label: optional.

The Full (Overwrite) option writes zeros to every sector before re-creating the FAT32 file system. This erases all bad-block remapping entries that the manufacturer wrote during factory pre-format and is the recommended procedure whenever a card has been used in a non-PC device such as a camera or industrial sensor.

6. Solution: Program Transfer Procedure

Once the card is FAT32-formatted, transfer the LOGO! program to the card using one of the two methods below. The on-card file must be named exactly LOGO_U_P.BIN - the LOGO! firmware scans the root directory for that file name on every boot and on every card insertion event.

6.1 Method A - Transfer from LOGO! Soft Comfort

1. Start LOGO! Soft Comfort V8.2 and open the project.
2. Select File > Save As.
3. In the Save As dialog, change the Save as type to *.bin (LOGO! binary program).
4. Navigate to the drive letter of the microSD card and save the file. The filename MUST be LOGO_U_P.BIN.
5. Safely eject the card from the PC.
6. Power the LOGO! to STOP, insert the card (pins up), and apply power.
7. Select Card > Copy from Card on the LOGO! display, or let the LOGO! auto-load LOGO_U_P.BIN on the next power-up.

6.2 Method B - Transfer from LOGO! to Card

1. Connect the LOGO! to the PC and transfer the program into the LOGO! using LSC V8.2.
2. Disconnect from LSC and place the LOGO! in STOP mode.
3. Insert a FAT32-formatted microSD card.
4. On the LOGO! display, navigate to Card > Copy LOGO → Card. The firmware writes the current program to the card as LOGO_U_P.BIN.

7. Verification Procedure

1. After insertion, on the LOGO! display navigate to Options > Diagnostics > Micro SD Card.
2. Expected reading: card size, free space, and the program name (for example, LOGO_U_P.BIN). If this page returns numeric values, the file system mounted successfully.
3. From the same menu, select Card > Copy from Card to confirm the program loads. The display will return to the program view and the program should start in RUN mode.
4. Cycle power to the LOGO! and confirm the program is auto-loaded from the card on the next power-up - this proves the card survived a hot-swap and that the FAT32 file system is clean.
5. Optional PC verification: re-insert the card into the Windows PC and confirm the file LOGO_U_P.BIN is present, the filesystem is FAT32, and the cluster size is 32 KB (for cards 4-16 GB) or 64 KB (for 32 GB). Cluster sizes outside this range are not supported.

8. Advanced Troubleshooting Matrix

9. Specific Findings from Field Reports

Several user reports across multiple installations confirm the following consistent behaviour on firmware V1.82.01:

• Branded SDHC cards in the 2 GB to 16 GB range (SanDisk, Emtec, Transcend) work reliably when formatted as FAT32 on a Windows 7 PC with Quick Format disabled.
• Unbranded "bulk" 16 GB cards from online marketplaces have been reported to fail even after a clean FAT32 reformat. The failure mode is not a filesystem error - the LOGO! reports Card unknown - and points to counterfeit or factory-rejected NAND chips that misrepresent their capacity. Replace with a branded card from a reputable distributor.
• macOS Disk Utility formats cards above 32 GB as exFAT by default and refuses to produce a FAT32 volume above 32 GB. macOS users must use a Windows PC or a Linux tool with explicit FAT32 sector-size selection.
• Linux mkfs.vfat with default options produces a valid FAT32 file system, but with a 64 KB cluster size on cards 4-16 GB, which the LOGO! bootloader does not always mount. Use -s 1 -S 512 to force 512-byte sectors and -s 32 to force 32 KB clusters on cards up to 16 GB.
• Some card readers present the card as a removable disk with a hidden second partition used by the manufacturer diagnostic tool. Delete all partitions in Disk Management and create a single primary FAT32 partition spanning the whole card.

10. Best-Practice Card Preparation Procedure

1. Buy a SanDisk or Transcend microSDHC card from a reputable distributor (capacity 4 GB to 16 GB recommended for the largest compatibility margin).
2. Use a dedicated USB microSD reader - not the card slot in a laptop, which often presents a SATA-bridged device that masks the underlying geometry.
3. Run SD Card Formatter with Full (Overwrite) format type. This writes a clean SD-specification boot sector.
4. Verify the result in Windows Explorer: file system FAT32, allocation unit size 32 KB or 64 KB.
5. Save the LOGO! program as LOGO_U_P.BIN to the card root directory using LSC V8.2.
6. Eject safely, insert into the LOGO! in STOP mode (pins up), and verify in Diagnostics > Micro SD Card.
7. Label the card with the project name and revision - this prevents accidentally swapping cards between machines.

11. LWE / Project Files vs. Program Files

LOGO! Soft Comfort stores two kinds of file on the card:

• *.bin - the compiled program that the LOGO! executes.
• *.lsc / *.lwe - the editable LOGO! project (LWE / LSC). These are not executable on the LOGO! and are used to restore the source into LSC on a PC.

If the user opens the project in LSC and then saves it to the card, only the LWE / LSC project is written. The LOGO! will then see a card without LOGO_U_P.BIN and report no program on card - this is often confused with a card-formatting fault. The remedy is to save the file as *.bin in LSC explicitly, or to use the LOGO! itself to write the BIN to the card.

12. Quick Diagnostic Decision Tree

Insert the card. From the LOGO! display, navigate to Diagnostics > Micro SD Card.

• Card detached → re-insert with pins up; check holder seating.
• Card unknown → reformat FAT32 on Windows with Full Format.
• Read Error → reformat FAT32; if it persists, replace card.
• Card visible with size but no program → check that LOGO_U_P.BIN exists in the root and is not stored in a subfolder.
• Card visible and BIN present but not auto-loading → verify filename is exactly LOGO_U_P.BIN; check free space > 0.

13. Prevention Checklist

• Use only microSDHC cards from 4 GB to 16 GB, branded, formatted FAT32 on Windows with Full (non-quick) format.
• Keep one spare formatted card on site - the most common cause of a service callout is a mis-formatted replacement card.
• Maintain a project library: one card per project, with the BIN and the LWE / LSC side by side in a labelled plastic case.
• Avoid removing the card while the LOGO! is in RUN. Always transition to STOP first.
• Avoid placing the LOGO! in environments with sustained vibration > 5 g - the push-push holder can unseat the card.

14. Frequently Asked Questions