LOGO! 8 Stays in STOP After Power Loss 6ED1052-1MD08-0BA1

1. Problem Statement

A Siemens LOGO! 8 base module configured as the master in a stand-alone installation is observed to leave RUN mode and enter STOP after a short (sub-second) interruption of the 12/24 V DC supply. Once the supply recovers, the controller does not automatically resume execution of the user program; an operator must clear the program-password lockout and re-issue a RUN command from the front-panel keypad. The field symptom is repeatable and occurs even on interruptions too brief to cause any other piece of equipment in the cabinet to reset.

The article in front of you applies specifically to SIPLUS LOGO! 8.1 with display, order code 6ED1052-1MD08-0BA1, but the diagnostic logic, supply-stability test, and corrective actions also apply to the successor 6ED1052-1MD08-0BA2 and 6ED1052-1MD08-0BA3 generations and to the 230 V AC variants (e.g. 6ED1052-1FB08-0BA1) with the supply rails renamed.

2. Product Identification and Firmware Matrix

Before any corrective action, capture the full order code, hardware release (FS:), and firmware version. Read them from the LOGO! 8 on-board display: navigate ESC > Setup > Diagnostics > Device Info, or read them remotely through LOGO!Soft Comfort V8.x under Tools → Transfer → Diagnostic.

The MLFB suffix -0BA1 identifies the 8.1 hardware generation; -0BA2 identifies 8.2; -0BA3 identifies 8.3. There is no functional difference in the power-on behavior we are discussing, but firmware bug lists differ between generations. Always log both FS and FW when you open a service ticket.

3. Documented Power-Up Behavior

The LOGO! 8 firmware implements four possible post-power-up operating states, selectable on the device or in the program properties dialog of LOGO!Soft Comfort:

1. RUN — the program starts automatically; this is the factory default.
2. STOP — the program is loaded but does not execute; useful for commissioning.
3. RUN/STOP conditional — starts in RUN only if a defined digital input is HIGH at power-up; otherwise enters STOP.
4. Manual / PC — control is locked to the front-panel cursor keys or to LOGO!Soft Comfort over Ethernet.

None of these documented modes cause the program to leave a previously running state and adopt STOP after a power interruption. Per the LOGO!Soft Comfort online help, the only path that results in STOP after a power-on is the case where the device was already in STOP when the supply fell. A controller in RUN that experiences a brown-out should, on the rising edge of a healthy supply, re-execute the user program from the top of the OB1-equivalent scan.

Internal hold-up time (the time the firmware continues to run from the reservoir capacitor after the supply collapses) is rated at typically 5 ms at full load for the 24 V versions. Anything beyond ≈5 ms produces either a clean restart in RUN or, if the retention flag is set incorrectly, an apparent “freeze” with no execution. It does not, by itself, produce a deterministic drop to STOP.

4. Root-Cause Matrix

Repeatable STOP-on-restart on a LOGO! 8 base module is almost always explained by one of the following five mechanisms. Use the matrix as a triage sheet before pulling wires or replacing hardware.

5. Diagnostic Procedure (Step by Step)

1. Capture device info. Note MLFB, FS, FW, and current date/time after a clean 60 s power-on. If the date resets to 01.01.00 and the time to 0:00, the controller performed a cold restart, not a warm wake-up — this confirms the supply dropped below the reset threshold.
2. Connect a memory-recorder oscilloscope across the 24 V terminals L+ and M. Trigger on falling edge below 21 V. Sample at ≥1 MS/s and capture at least 1 s pre-trigger and 5 s post-trigger. A clean power supply should show no event; a defective one will show a <100 ms dip coincident with the STOP.
3. Inspect the front-panel menu ESC > Setup > Program > Power-on. The screen reads either RUN, STOP, or RUN with condition on I1..I8. If anything other than RUN is selected, that is your root cause.
4. Check the Settings tab in LOGO!Soft Comfort (project → properties → device). Confirm Startup behavior is set to RUN. Also check the Block Protection bits; a corrupted protection block will sometimes manifest as a STOP state on the next warm boot.
5. Force a controlled brown-out: with the oscilloscope still attached, briefly open the DC feed via a switch or relay for 200 ms, then close it. If the controller enters STOP while identical hardware in the same cabinet continues to RUN, the issue is local to the LOGO! 8 supply conditioning, not to the upstream PSU.
6. Compare master vs. slave behavior in a network. If the same project, run on a second identical MLFB, does not drop to STOP, swap the base modules. If the symptom follows the module, the module is suspect.

6. Primary Fix — Power-Supply Stabilization

The most common cause in field reports is the upstream DC supply. The 6ED1052-1MD08-0BA1 is rated for a continuous 12 V DC (10.8–28.8 V) or 24 V DC (20.4–28.8 V) input, but the integrated step-down for the logic rail will produce a brown-out reset whenever the input dips below 9.6 V (on the 12 V range) or 19.2 V (on the 24 V range) for more than the ≈5 ms hold-up window.

Apply the following three-layer defense:

1. Use a Siemens LOGO! Power module (e.g. 6EP3330-6SB00-0AY0 24 V/3 A or 6EP3331-6SB00-0AY0 12 V/3 A) with a published mains dip immunity of 30 ms at full load. These supplies are explicitly designed to ride through the same kind of micro-cut that destabilizes a generic Chinese DIN-rail PSU.
2. Add a buffer module in parallel with the LOGO! 8 input, e.g. 6EP1931-2DC21 (capacitor-based, 4 s @ 1 A) or

   6EP1931-2EC21 (electrolytic, 16 s @ 1 A). The buffer module clamps the DC bus during the PSU ride-through, so the LOGO! 8 never sees a sub-threshold event.

3. Add a 1000 µF / 35 V electrolytic across L+ and M at the LOGO! 8 terminal strip, observing polarity. This local reservoir extends the integrated hold-up from 5 ms to ≈120 ms, comfortably absorbing the typical 50 ms mains dip of a heavy industrial load.

For full hold-up sizing, apply the capacitor energy equation:

where V₁ is the nominal 24 V rail, V₂ is the 19.2 V brown-out threshold, C is the buffer capacitance, and E is the energy delivered to the load (in joules) over the hold-up window. Solving for a 0.5 A load and a desired 200 ms hold-up yields a minimum capacitance of:

Round up to a standard 68 000 µF / 35 V aluminum-polymer bank, or simply specify the Siemens 6EP1931-2EC21 module and skip the math.

7. Secondary Fix — Restore the Power-on Mode

If the supply test in Section 6 shows no dip, the next-highest probability is that the Power-on setting was changed during commissioning and never restored. The recovery is a three-key sequence on the front panel:

1. Press ESC to return to the main menu.
2. Use the cursor ↑/↓ to highlight Setup, press OK.
3. Select Program, then Power-on. The radio button must be on RUN.
4. Press OK to confirm. The change is written to non-volatile memory immediately.

Repeat the same change in the project file: open the program in LOGO!Soft Comfort, choose File → Properties → Device Selection, click Settings, and set Startup behavior to RUN (no condition). Re-compile and re-transfer so the on-disk project matches the device.

If the program is password-protected, the operator must enter the password before the menu allows the change. A service-level password recovery requires either the original project file or a 30-minute "super-cap discharge" reset that erases the user program — never the right answer for a production machine.

8. Hard Reset Procedure (Last Resort)

Use this only when the controller is locked in STOP and the operator password is lost. The hard reset erases the user program but does not touch the firmware or the power-on default.

1. Power off the LOGO! 8 for at least 30 s.
2. Hold the four front-panel keys ↑, ↓, OK, and ESC simultaneously.
3. Re-apply power while holding the keys. The display shows RESET?.
4. Press OK to confirm, ESC to abort. A confirmed reset wipes program, password, retentive flags, and the running-state selection.

After the reset, the device boots to the factory default — Power-on = RUN, no program, no password. Re-transfer the original project from LOGO!Soft Comfort. The on-board supply monitor is unchanged; this procedure does not fix a defective upstream PSU.

9. Firmware Considerations

Two firmware-related behaviors deserve mention:

• V1.84.01 (the version referenced in the field report): this build includes a correction to the power-on watchdog that, in earlier 8.1 builds, occasionally left the scan engine halted after a <50 ms brown-out. If the affected module is still on V1.84.01 or older, upgrade to the latest service pack available in the Siemens Online Support portal. Open LOGO!Soft Comfort, choose Tools → Firmware Update, and select the base module. The updater handles both master and slave modules on a network, one at a time.
• Cross-master / slave boot propagation: when a master and a slave share an Ethernet bus, the slave may go to STOP for a few seconds while re-establishing the master handshake, even if the supply is clean. This is a different symptom from the field report (a true STOP state on the master, requiring operator action) but worth ruling out: temporarily disconnect the slave and repeat the brown-out test.

Always read the current firmware release notes from the official Siemens Industry Online Support portal before flashing. Do not interrupt the supply during a firmware update; a corrupted flash can permanently brick the module, requiring RMA.

10. Verification Checklist

After applying any combination of the fixes above, run the following validation sequence before returning the cabinet to production:

1. Power the LOGO! 8 from a clean laboratory PSU, 24.00 V ±0.05 V, current limit 1.0 A.
2. Transfer the original program; confirm Power-on = RUN on the device and in the project.
3. Force the program into RUN from the front panel; observe that the green RUN LED is steady and that the date/time retain their values.
4. Use a calibrated relay to interrupt the 24 V feed for 100 ms, 250 ms, 500 ms, and 1000 ms, in that order, with a 30 s rest between each test.
5. After each interruption, the controller must return to RUN without operator intervention. The display must show the current date and a time within ±1 s of the pre-test value (the RTC crystal is rated ±5 s per day).
6. Repeat step 4 with the laboratory PSU replaced by the actual cabinet PSU. If a brown-out is captured by the oscilloscope, address Section 6 before the controller is released.
7. Re-install the program-password protection and set the front-panel Setup > Program > Password to the original value. Document the value in the maintenance log.

11. Quick Field Triage (5-minute version)

12. Frequently Asked Questions