Troubleshooting LOGO! 8.1 FS4 Program Deletion on Power Cycle

Problem Summary: Program Reverts After Power Cycle on LOGO! 8.1 FS4

When a Siemens LOGO! 8.1 FS4 (Firmware Status 4) is powered off and back on, a freshly transferred program disappears and an older program returns. The user observes the following sequence:

1. A new circuit diagram is created or modified in LOGO! Soft Comfort (LSC) 8.3.
2. The project is downloaded to the LOGO! 8.1 FS4 target selected in the software.
3. Power is removed from the LOGO! (24 V DC or 230 V AC depending on the variant).
4. When power is restored, the new program is gone and an older program reappears on the display.

Program loss in this configuration is not caused by a defective flash memory, nor is it a side effect of Windows update behavior or file-system corruption on the engineering PC. The behavior is the documented microSD card boot override feature of the LOGO! 8 platform. Once you understand the boot priority order, the symptom is fully explained and easily corrected.

LOGO! 8.1 FS4 Hardware Identification

The LOGO! 8.1 FS4 designation identifies a specific generation and firmware revision of the LOGO! 8 logic module family. Confirm the exact model before troubleshooting because the microSD handling and the part number are required when ordering replacements or accessing the correct manual.

The FS4 suffix on the module's splash screen or system information menu identifies the firmware status. FS4 introduced refinements in program loading and SD card handling compared with FS1–FS3. Always confirm the firmware status through the LOGO! on-board menu:

1. Navigate to System → FW Status on the LOGO! display.
2. Record the version string (for example, LOGO! 8.1 FS4: V1.81.01).
3. Cross-reference with the matching entry in the LOGO! system manual.

LOGO! Program Storage Architecture: Internal Flash vs. microSD

The LOGO! 8 platform supports two distinct non-volatile program locations:

Both locations are non-volatile. A program is never "deleted by a power failure." The internal flash is rated for at least 100 000 write cycles, far beyond what a normal commissioning produces. If the controller behaves as if a freshly loaded program vanished, the controller did not erase it — the controller simply loaded a different program on the next start.

Root Cause: microSD Card Boot Override Mechanism

The LOGO! 8 boot sequence is deterministic and follows a fixed priority order:

Whenever a microSD card is present at startup and a valid LOGO! program file (*.lsc or compiled *.lld) is detected in the card's \LOGO\ directory, the controller boots from the card regardless of the contents of internal flash. The card wins, every time.

This is the documented behavior described in the LOGO! system manual. The mechanism is designed for field service: an operator can carry a microSD with the new program, plug it in, and the machine restarts on the new logic. It is also a fast way to clone a controller. The drawback is that any stale file left on a card will silently override freshly commissioned internal memory.

Version Compatibility: LOGO! Soft Comfort 8.3 with LOGO! 8.1 FS4 Firmware

LOGO! Soft Comfort 8.3 is fully compatible with LOGO! 8.1 FS4 targets when the correct device is selected in the download dialog. Mixing versions in the field is normal: many panels are still on 8.1 FS4 hardware while the engineering team has standardized on LSC 8.3 for new features.

When downloading, the engineer must pick the correct target from the PC → LOGO! dialog. The label appears as something similar to LOGO! 8.1-8.2 FS4 for FS4 modules. Choosing a wrong target can lead to LSC refusing the transfer; it does not, however, cause the program loss symptom. Program loss is always a storage-location issue, never a version-translation issue.

Diagnostic Procedure: Confirm the microSD Override

Before changing anything, prove that the microSD card is the override source. The check takes less than one minute.

1. Power off the LOGO! 8.1 FS4 at the supply terminals. Wait for the display to go dark.
2. Open the front cover and locate the microSD slot above the display connector. Slide the card out. Do not use tools — finger pressure is sufficient.
3. Inspect the card. A standard LOGO! SD card shows files such as PROGRAM_001.lld, PROGRAM_001.lsc, PROJ_001.bin, and configuration *.bin files. Any *.lld or *.lsc in the root or in \LOGO\ is a boot candidate.
4. Power on the LOGO! without the card installed.
5. Read the program from the controller (LOGO! menu: Program → Card → No Program indicates flash is empty; otherwise the program hash matches the internal flash copy).

If the program on the controller's display matches the program you just downloaded when the card is removed, the diagnosis is confirmed: the card was the override source, not the flash.

Step-by-Step Solution

The fix has three independent actions. Apply all three to make the symptom go away permanently.

Action 1 — Stop the Override by Removing or Re-formatting the Card

1. Open the front cover of the LOGO! 8.1 FS4.
2. Press the microSD card in until it clicks, then release. The card pops out a few millimetres and can be pulled free.
3. Insert the card into a card reader attached to the engineering PC.
4. Open the card in Windows Explorer. Inspect the root and the \LOGO\ folder.
5. Decide between two paths:
   • Keep the card for cloning: back up the files to C:\LOGO_Backup\<date>\ and delete the program files from the card. The card can stay in the LOGO! as a data logger or recipe holder without affecting program boot.
   • Use the card as primary program storage: load the new program with LSC, choosing PC → SD Card instead of PC → LOGO! in the transfer dialog. The card will then host the new program and the controller will boot it on every restart.

Action 2 — Write the New Program to Internal Flash

1. Open the project in LOGO! Soft Comfort 8.3 (or whatever version is installed).
2. Select Tools → Transfer → PC → LOGO! (hotkey F9).
3. In the dialog, set Target to LOGO! 8.1-8.2 FS4 (the correct device string for an FS4 module).
4. Set Interface to Ethernet if the controller is networked, or to the USB LOGO! cable if connecting point-to-point.
5. Click OK. The progress bar shows write completion when the program is in internal flash.

Action 3 — Verify Flash and Card Are in Sync (or Card Is Absent)

1. Power off the LOGO!.
2. Reinsert the microSD card only if you intend to keep the program on the card. If the card is empty of *.lld and *.lsc files, it will not override the flash copy.
3. Power on the LOGO!.
4. Confirm the program on the display matches the new program.
5. Power cycle two or three more times to confirm deterministic behavior.

Program Transfer and Storage Best Practices

Always label the microSD card with the machine name and program revision. A card without labels is the most common cause of "mystery revert" reports on installed equipment.

Verification Procedure

After applying the fix, perform a controlled verification sequence:

1. Read the program from the LOGO! with LSC: Tools → Transfer → LOGO! → PC. Compare the block count, function block list, and parameter values against the project on the engineering PC.
2. Power off the LOGO! at the supply for at least 10 seconds (the internal super-capacitor-backed RTC requires a few seconds to discharge fully).
3. Power on the LOGO! and read the program on the on-board display. Navigate through the program blocks. Compare with the source project.
4. Repeat the power cycle five times. The program must remain identical each time.
5. With the controller running, monitor the SD card activity LED (if present on the variant) to confirm no boot-time read is happening when the card should be passive.

For networked installations, an additional check is to confirm the LOGO! is reachable on its configured IP address and that the Web server reports the correct program name and version on the home page.

Edge Cases and Related Issues

Edge Case 1 — The microSD Card Is Write-Protected

If a SD card with a write-protect switch is inserted, the LOGO! will still boot from it but cannot update the program on the card. Symptoms: the controller reads the card, the program appears to load, but any subsequent on-board edit is lost. Always use cards without write-protect or with the switch in the unlocked position for live systems.

Edge Case 2 — Card Contains Multiple Program Folders

LOGO! firmware looks for the file PROGRAM_001.lld in the card root or the \LOGO\ folder. If multiple PROGRAM_xxx.lld files exist, the lowest-numbered one is selected. Renaming the file to PROGRAM_999.lld hides it from the boot sequence without deleting it, useful for archiving several revisions on one card.

Edge Case 3 — LSC Says "Transfer Successful" but the Program Did Not Load

This happens when LSC writes to the card but the user expected flash. Re-run the transfer and verify the dialog target is LOGO!, not SD Card. The dialog labels are easy to misread.

Edge Case 4 — LOGO! 8.1 vs. 8.2 vs. 8.3 Firmware Differences

LOGO! 8.1, 8.2, and 8.3 modules all support the microSD override mechanism. The behavior is identical across generations. The differences are mostly in available function blocks, Modbus handling, and (in 8.3) the LOGO! Access Tool cloud connectivity. Program-loss symptoms should be treated identically on all three generations.

Edge Case 5 — Battery / RTC Considerations

The LOGO! uses a super-capacitor to back up the real-time clock and a small amount of retain data; it does not back up the program with a battery. A long power-off (months) does not affect the program at all. If the clock resets after a long outage, that is a separate RTC issue and does not explain program loss.

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