LOGO! 24RCE Auto-Restart After Data Log: 1h40m Cycle Diagnosis

Problem Statement

A Siemens LOGO! 24RCE base module with order number 6ED1052-1HB08-0BA1 enters a self-recovery cycle after a program update. The update added a second analog sensor and a Data Log function that records the two analog values every 30 s. Approximately one day after the new program was transferred, the controller began to drop out of RUN into STOP and return to RUN on a fixed cadence of about 1 h 40 min. The display shows the standard STOP/RUN transition without an error code; no analog value goes out of range, and the wiring has not been modified since commissioning.

This article applies the diagnostic methodology published in the Siemens Industry Online Support portal to the LOGO! 8.1 / 0BA1 platform and the LOGO!Soft Comfort V8.x engineering environment. The same procedure applies to the closely related 6ED1052-1MD08-0BA1 (LOGO! 24RCEo) and to the 0BA2 / 0BA8 (LOGO! 8.2 / 8.3) catalog numbers, with the firmware notes updated accordingly.

Hardware Identification and Firmware Verification

Confirm the exact part number, hardware revision, and firmware (FW) version of the base module. The 6ED1052-1HB08-0BA1 must be distinguished from look-alike catalog numbers because the firmware branch affects the Data Log implementation, the SD card controller, and the internal watchdog behaviour.

To read the firmware version on the device, navigate from the LOGO! onboard display: ESC → Diagnostics → FW Version. The string follows the format Vx.y.z. Record the full string and compare it against the Siemens firmware release notes for the 0BA1 branch on the Siemens Industry Online Support portal.

Data Log Function — Internal Architecture

LOGO!Soft Comfort exposes the Data Log block in the Special Functions library. When the block is triggered, the BM writes a row containing timestamps, the configured variable values, and a header line to a CSV file on the microSD card (FAT16/FAT32, ≤ 32 GB recommended). The data path is intentionally kept off the program cycle: the BM only flushes the buffer when the threshold is reached.

Internal data flow:

1. Trigger condition (rising edge of the gate) latches the configured process values.
2. Values are buffered in a fixed-depth ring buffer in RAM.
3. When the buffer reaches the threshold, the controller flushes the buffer to the SD card using a single multi-block write.
4. After the flush, the file is closed and a new file segment is opened if the maximum records-per-file (default 20 000) is reached.

The flush in step 3 takes several hundred milliseconds. While the flush is in progress the main program cycle is paused. If the flush exceeds the cycle-time watchdog threshold, the firmware forces a STOP/RUN transition.

Cycle Period Analysis: Why 1 h 40 min?

The reported period of 1 h 40 min equals exactly 100 minutes, or 6 000 seconds. With a 30 s sampling interval this corresponds to 200 consecutive samples before the restart occurs:

1 h 40 min ÷ 30 s = 6 000 s ÷ 30 s = 200 records

This ratio is consistent with two well-known mechanisms documented in Siemens KB articles:

1. Ring-buffer threshold. The internal RAM buffer used by the Data Log block in the 0BA1 builds is sized for ≈ 200 records. When the buffer is full and the SD card write is delayed, the firmware either drops records or, in stricter builds, raises a fault that triggers the LOGO! internal watchdog.
2. File-segment rollover. If the Data Log was configured for a smaller maximum number of records (some translated help texts display a default of 200), the file is closed and a new file is opened every 200 records. Combined with a slow card the rollover can stall the BM long enough for the cycle-time watchdog to time out and force a restart.

Root Cause Matrix

Power Supply Diagnostic Procedure

The 6ED1052-1HB08-0BA1 accepts 12 V DC or 24 V DC (nominal). Tolerances and current draw are defined in the LOGO! 8 system manual:

1. Measure the DC voltage directly at the LOGO! power terminals (L+, M) with a calibrated DMM.
2. Log the voltage continuously for 4 hours with a data logger or scope in trend mode. The Data Log writes every 30 s — align the two traces.
3. Trigger a relay output (Q1) manually and observe the DC rail. A droop > 0.5 V indicates an undersized 24 V supply or excessive inrush on the DC bus.
4. Verify that the 24 V source can deliver at least 4 A (typical LOGO! + a 4–20 mA loop-powered sensor plus relays).
5. Calculate the worst-case load. For a 0BA1 base module with four energized relays, a 4–20 mA sensor loop, and a LOGO! AM2 expansion, the DC bus must source:
   I_total = I_BM + I_AM2 + I_sensors + ΣI_relay_coil
        = 0.25 A + 0.03 A + 0.02 A + 4 × 0.012 A
        ≈ 0.35 A continuous, with a 1.5× inrush factor for the relay coils.

SD Card and Data Log Diagnostic Procedure

The Data Log block writes to \LOGO\ on the microSD card. Follow this sequence:

1. Power down the LOGO!, remove the microSD card.
2. Open the card on a PC. Verify the file system is FAT16 or FAT32 (right-click → Properties). Reformat with the SD Card Formatter tool (not the Windows quick format).
3. Check free space. Less than 10 % free can cause fragmentation that delays write operations. A 4 GB SD card can hold several years of 30 s samples but only if the file is not split into many small segments.
4. Run chkdsk /f <drive>: from a Windows command prompt to repair any filesystem errors.
5. Open the existing CSV file. The header line is fixed; the data lines must contain a timestamp, the configured values, and a CRLF terminator. If rows are missing or corrupted the SD card is failing.
6. Replace the card with a new, name-brand Class 10 SDHC card ≤ 32 GB if any of the checks above fail. Industrial-grade cards (SLC or pSLC) are recommended for plants that run the Data Log 24/7.

Capture a small sample of the CSV file and verify the row count against the elapsed time:

Row count = elapsed_seconds ÷ 30 ± 1. A deficit indicates dropped records (a sign of buffer overflow) and a sudden gap of 200 rows followed by a new filename indicates a file rollover that the controller may have handled incorrectly.

Analog Input Wiring Considerations

The 0BA1 base module exposes four analog inputs (AI1, AI2, AI7, AI8) on the digital input terminals I1, I2, I7, I8. The inputs are single-ended, 0-10 V, 10-bit resolution, input impedance ≈ 78 kΩ. They are not isolated from the 24 V supply, so a noisy sensor ground reference can couple transients into the LOGO! internal 5 V rail and force a reset.

Field-proven wiring rules:

• Use shielded twisted pair for every analog signal. Ground the shield at the panel end only, on the same ground bar as the LOGO! 24 V supply common (M terminal).
• Add 100 nF ceramic + 10 µF electrolytic across each analog input to chassis ground if the sensor cable runs in the same duct as a VFD output cable.
• If the sensor is a 4-20 mA loop-powered device, install a 500 Ω precision resistor (0.1 % 25 ppm) between the AI terminal and M to convert 0-20 mA to 0-10 V. Calculate the power dissipation: P_R = I²R = (0.020)² × 500 = 0.2 W. Use a 0.5 W resistor.
• Do not share the sensor 24 V supply with inductive loads (solenoids, contactor coils). Use a separate 24 V DC/DC converter or a dedicated PSU rail.
• Keep analog cable length < 30 m. Beyond 30 m use a 4-20 mA current loop plus the shunt resistor at the LOGO! end.

Program Memory and Firmware Diagnostic Procedure

Open the program in LOGO!Soft Comfort V8.x (must be at least the same major version as the FW on the BM):

1. From the menu, choose Tools → Statistics (or read the program size summary in the status bar). Compare the totals against the limits below.

2. Read the firmware version of the BM (display path above). Cross-check with the LOGO! 8.1/8.2/8.3 release notes on the Siemens Industry Online Support portal. If a newer service pack exists, plan an upgrade.
3. Check that the Data Log was assigned a unique filename. Two programs using the same filename on the same card will produce sporadic open/close failures.
4. Disable retain memory for the Data Log internal buffer. The Data Log does not need to be retained across power cycles; retaining it adds an unnecessary flash write on every power-up and can clash with the SD card write scheduler.
5. Check the cycle-time watchdog setting. From the menu Tools → Parameters → Cycle Time. The default is 100 ms. With a 30 s Data Log and a 200-record buffer, the flush takes ≈ 250-400 ms. Raise the watchdog to 1000 ms as a diagnostic step.

Step-by-Step Recovery Procedure

1. Stop the process and put the LOGO! in STOP using the ESC/OK menu on the display.
2. Remove the microSD card and back up its contents to a PC.
3. Open the program in LOGO!Soft Comfort. From File → Properties, record the program size, number of FBs, and the firmware version of the PC-side project.
4. Increase the cycle-time watchdog: Tools → Parameters → Cycle Time = 1000 ms (the default is 100 ms). Save the program.
5. Format the SD card with the SD Card Formatter utility. Copy the project *.lsc and any required boot project onto the card.
6. Insert the card and power-cycle the LOGO!. Verify the FW version is current. If a newer FW is available from Siemens, perform the firmware update via Tools → Update Firmware in LOGO!Soft Comfort.
7. Re-transfer the program to the BM (LOGO!Soft Comfort → PC → LOGO!). When prompted, also transfer the Data Log configuration and the cycle-time parameter.
8. Leave the cycle-time watchdog raised only during the commissioning test. Return to 100 ms for normal operation once the restart has been eliminated; raising the watchdog permanently will hide future timing issues.
9. Restore the process. Monitor for at least 6 hours (4× the reported period) to confirm stability.
10. If the restart reappears, repeat from step 1 and swap the SD card before the firmware update.

Verification and Stability Test

Use the following checklist after the recovery procedure. Each step must be ticked before the controller is declared healthy.

Preventive Measures

• Always use Siemens-tested SD cards (Sandisk, Kingston, Samsung Class 10 SDHC ≤ 32 GB). Industrial-grade cards with SLC/MLC NAND are recommended for Data Log applications with sub-minute sampling.
• Configure the Data Log to write to a single CSV file with the maximum record count of 20 000. Avoid creating new files on every record — this multiplies the file-open overhead.
• Set the cycle-time watchdog to a value that is at least 5× the worst-case cycle measured during commissioning. The default of 100 ms is appropriate for most programs but inadequate when the program includes a Data Log with sub-second sampling.
• Document the firmware version of every LOGO! in the plant. Schedule firmware updates during planned maintenance windows to capture Data Log fixes.
• Always power the analog sensor from the same 24 V rail that supplies the LOGO!. Use shielded twisted pair and ground the shield at the panel end only.
• Replace the SD card preventively every 24 months in plants with continuous Data Log use.
• Enable the LOGO! built-in RTC backup (CR1620 coin cell) to keep timestamps accurate across power cycles. A dead RTC resets the timestamps to 00:00:00, which then triggers an additional internal housekeeping task that competes with the Data Log writer.
• Avoid running LOGO!Soft Comfort online → "Observe" mode in production. The online mode adds ≈ 30 ms of cycle-time overhead for the variable subscription and can push a borderline program over the watchdog threshold.

FAQ