Troubleshooting Siemens LOGO! 24RCo 6ED1052-2HB00-0BA6 STOP Mode Fault

The Siemens LOGO! 6ED1052-2HB00-0BA6 is a 24 V DC basic logic module (24RCo) from the LOGO! 7 generation (firmware/device suffix 0BA6). It is one of the most widely installed ultra-compact controllers in the field, commonly used for staircase lighting, facade control, blinds/shading, and small pump or ventilation logic. Years after commissioning, a 0BA6 base module can suddenly transition to STOP mode: the front status LED turns solid red, the program no longer executes, and all digital outputs drop to their de-energized state. Because the 24RCo has no integrated display, the on-device diagnostic menu is not available. This guide documents the field-proven recovery procedure, the underlying causes of STOP transitions, and the migration path to the current 0BA8 (LOGO! 8.3) successor module.

1. Problem Description

Field symptom (typical):

• LOGO! base module (24RCo) front LED is solid red (STOP). The controller is not in RUN.
• Power cycling (24 V DC OFF/ON) does not return the module to RUN. The LED reverts to red within 1-2 s after the boot screen.
• Connected DM8/DM16 digital expansion modules retain their green LED (module powered, backplane live) but their outputs are also off because the program master is in STOP.
• Inputs (push-buttons, sensors) are read but have no effect on outputs.

After 5-15 years of continuous service, this is a common, well-documented failure mode for the 0BA6 hardware revision. Siemens declared the 6ED1052-2HB00-0BA6 end-of-life around 2018. A repair is not technically possible — the program stored in the internal flash is no longer executable, and the diagnostic interface (no HMI, no display) prevents an interactive clear.

2. Affected Hardware Identification

Use the data matrix code or the printed label on the right side of the module to confirm the part number. The relevant variants and the current equivalents are listed in the table below.

Key clarifications on the part-number scheme:

• 2 after the 6ED1052- prefix: variant digit. 0BA6 uses the 7-series schematic; 0BA8 uses 8-series.
• HB: 24 V DC supply, relay outputs (8 relays, 10 A each, Q1-Q4 grouped, Q5-Q8 grouped).
• CC: 24 V DC supply, transistor outputs (sourcing).
• FB: 12/24 V DC supply, relay outputs.
• NB: 24 V AC/DC supply, relay outputs (no analog inputs).
• MR: 115/230 V AC supply, relay outputs.
• Suffix 08-0BA1 denotes the LOGO! 8.3 hardware revision (FS:03) with the updated Ethernet controller.

Confirm the exact MLFB before ordering: an 0BA6 will not be re-released, and a 0BA7 spare is not a direct drop-in for an 0BA8 expansion bus at the user program level (the migration path is non-trivial — see §7).

3. Root Cause Analysis: Why a 0BA6 Lands in STOP

The LOGO! transitions to STOP for one of the following reasons. For the 6ED1052-2HB00-0BA6 specifically, the first cause is the dominant one in field reports.

3.1 Retentive memory / internal flash wear

LOGO! stores the user program, the I/O configuration, and (optionally) a copy of the retentive markers (M7-M27) in the internal flash. After roughly 100,000 write cycles per sector — or after many years of slow bit-bleed — the flash block holding the program image can become unreadable. The boot loader detects the bad CRC and forces STOP. Power cycling does not help because the bad block is not auto-erased.

3.2 Corrupted program download

A power loss during a programming write via the LOGO! Soft Comfort download (LSC) will leave a half-written program image. The next boot will halt at STOP with a red LED. Power cycling is insufficient; a fresh download is required.

3.3 Watchdog / invalid function block

If the program contains a custom function block or UDF that references an I/O point that no longer exists (e.g. an analog threshold on a digital-only input), the runtime exits to STOP. This is rare for ladder-only programs but does occur in FBD programs with arithmetic blocks.

3.4 Expansion bus fault

A defective expansion module can hold the bus low. The base module detects no bus response within the timeout and switches to STOP to avoid undefined I/O. The diagnostic symptom is: expansion module LED still green (it has 5 V backplane power) but the base still red.

3.5 Supply undervoltage

24 V DC sag below ~19.2 V (per the LOGO! data sheet) will trigger a controlled shutdown. If the sag is borderline, the controller may boot, fail POST, and latch STOP. Always check the supply rail with a DMM or scope at the LOGO! terminals.

4. Diagnostic Procedure Without a Display

Because the 24RCo has no display, the only diagnostic surface is the LED and the communication interface. Proceed in the order shown.

1. Confirm the LED state. Solid red = STOP. Flashing red/green alternately = firmware update in progress (do not interrupt power). Solid green = RUN. Off = no 24 V supply.
2. Measure the supply. 24 V DC ±10% at the LOGO! terminals, ripple < 5%.
3. Disconnect the expansion bus. Pull the slide on the right side of the base (next to the green connector) and remove the right-hand expansion module. Power cycle. If the base recovers to RUN, the fault is in the expansion chain — re-seat modules one at a time from the base outward.
4. Connect LOGO! Soft Comfort (LSC). Use the LOGO! PC cable (USB, MLFB 6ED1 057-1AA00-0BA0) into the front socket of the 0BA6. Note: the 0BA6 has no Ethernet; only the 0BA7/0BA8 with the E suffix support IP connection. For the 0BA6, USB is mandatory.
5. Read the program in LSC. From the LSC menu: Tools → Connect → PC <-> LOGO!. If the program upload is empty or fails, the internal image is corrupted.
6. Try the LSC online "Start" command. The menu Tools → Start/Stop can force the LOGO! to RUN if the program is intact.

5. Program Recovery via LOGO! Soft Comfort

The standard recovery flow that resolves the symptom (STOP that does not clear on power cycle) is to write a fresh program image to the LOGO! using LOGO! Soft Comfort.

5.1 Prerequisites

• PC running Windows 10/11 (32 or 64-bit) with LOGO! Soft Comfort V8.x installed. For 0BA6 hardware the project is opened in LSC V8.0 or later as a legacy project (0BA6 is supported as a target). The newer LSC V8.3 SP1 (released 2022) remains compatible with the older 0BA6 targets.
• Siemens LOGO! USB PC cable, MLFB 6ED1 057-1AA00-0BA0 (LOGO! PC cable, ~3 m, USB-A to LOGO! 8-pin RJ12). Earlier USB cables with the older 6ED1 057-1AA00-0BA0 (no dash) are also acceptable.
• Administrator rights on the PC (the LSC driver installs a virtual COM port).

5.2 Step-by-step recovery

1. Install LSC V8.x from the Siemens Industry Online Support portal. Apply the latest hotfix.
2. Launch LSC, open the File menu and select New. Choose target device LOGO! 0BA6 (24RCo variant).
3. Recreate the application. For lighting-only programs (push-button -> output, group toggles, staircase timing), the typical program is 8-20 FBD blocks or its LDR equivalent.
4. Save the project locally (project_name.lsc or LSC zip).
5. Power the LOGO!. Plug the USB cable into both the PC and the front socket of the 24RCo.
6. In LSC, click Tools → Connect (or the toolbar PC↔LOGO! icon). The status bar should show Connected (PC ↔ LOGO!).
7. Click Tools → Download → to LOGO! (or the keyboard shortcut Ctrl+D). Accept the "stop the LOGO! during download" prompt.
8. When the transfer completes, LSC will offer to start the LOGO! in RUN. Click Yes.
9. The LED on the 24RCo turns green. The outputs activate per the program. The expansion modules return to green as their I/O backplane scans resume.
10. From the LSC menu, choose Tools → Upload from LOGO! (PC ← LOGO!) to save the verified program. This is the master backup; store a copy on the network share.

If the download is interrupted (power loss, USB unplug), repeat from step 7. The LOGO! will boot to STOP again until a complete image is written. There is no brick risk at this point — the bootloader always accepts a new image.

5.3 Ladder example for a 2-way staircase (0BA6, 24RCo)

For documentation purposes, the canonical 2-way staircase circuit as a LOGO! FBD program:

+-------+      +---------+      +-------+
|  I1   +-->---+ B01 Latch+--+---+ Q1   |
+-------+      |  S=R   Q |  |   +-------+
               +---------+  |
+-------+                   |
|  I2   +-------------------+
+-------+

In LSC, this is implemented as one self-latching (set/reset) function block: S = push-button I1, R = push-button I2, output Q1 drives the lighting contactor. The same block scales to N light groups; each group needs one B01 block. A staircase auto-off is built with a B05 (off-delay) on the Q output, retriggered by an OR of the two push-buttons.

6. When the Base Module Must Be Replaced

If the 0BA6 does not respond to LSC, the front socket is dead, or the controller is physically damaged, the entire base module must be swapped. Three options exist, listed in order of preference.

6.1 Recommended: 6ED1052-2HB08-0BA1 (LOGO! 8.3 basic)

The Siemens-published current equivalent is 6ED1052-2HB08-0BA1. Spec parity with the 6ED1052-2HB00-0BA6:

• Supply: 24 V DC, same 24 V/24 RCo I/O pinout (I1-I8, Q1-Q4, Q5-Q8, no analog inputs on the HB variant; analog I7/I8 are not exposed on the relay-output 24RCo).
• Outputs: 4 × 10 A relays + 4 × 5 A relays, identical to 0BA6.
• Digital inputs: 8, with the same I1-I8 mapping; I7/I8 are usable as digital inputs at 24 V.
• Form factor: same 71.5 mm wide (4 SU), same depth, same mounting rail footprint.

To program it, install LOGO! Soft Comfort V8.3 (current). The basic 0BA8 module is supported from LSC V8.0 upward; the migration tools in V8.3 will upgrade the 0BA6 program automatically when you re-open it.

6.2 Spare part from Siemens

The 6ED1052-2HB00-0BA6 is still listed in some regional spare-parts catalogues. The price is typically higher than a new 0BA8 (LOGO! 8.3) base. Use this only when a like-for-like swap is required and a program migration is unacceptable (e.g. regulatory re-qualification of a certified assembly). Verify availability with Siemens spare parts before ordering.

6.3 Surplus / refurb

Surplus dealers and refurbishment suppliers still stock tested 0BA6 modules. Inspect for: clear Siemens label, intact slide mechanism, no green corrosion on the green expansion bus connector, and verify the LED sequence (red → green within 2 s of power on) on a bench supply before installing.

7. Compatibility and Migration Considerations

Migrating an installed LOGO! 0BA6 system to 0BA8 (LOGO! 8.3) is straightforward in principle, with three caveats.

1. Expansion modules are NOT compatible. The DM8/DM16/AQ/AM2/AM2 RTD expansion modules sold for the 0BA6 are mechanically and electrically incompatible with the 0BA8 base. The slide connector keying is different. Replace all expansion modules with their 0BA8 equivalents (e.g. 6ED1055-1MB00-0BA2 DM8 24R, 6ED1055-1MA00-0BA2 DM8 24).
2. The program migrates cleanly. Open the 0BA6 .lsc file in LSC V8.3. The compatibility checker will flag deprecated function blocks; the 0BA6/0BA7 block set is a subset of 0BA8, so simple ladder/FBD programs need no edits. Save under a new project name (e.g. lights_0BA8.lsc) and download.
3. Log retention and clock. The 0BA8 base retains the real-time clock for 480 hours on a fresh battery-free backup (super-cap). If the 0BA6 was used with astronomical timers or weekly schedules, verify the clock after migration.

8. Wiring Reuse: Same Pinout, Different Bus

The 0BA6 to 0BA8 base swap is pin-for-pin compatible on the green power and I/O connector for the HB variant:

Replacement procedure:

1. Power down. Lock out.
2. Label every wire on the green connector. Note any jumpers between terminals (e.g. neutral link from terminal 2 to 10, common on the relay output groups).
3. Depress the orange lever on the green connector with a 3 mm flat-blade screwdriver and pull the connector off the base.
4. Slide the expansion module(s) off to the right, marking their order (left-to-right = base, slot 1, slot 2, …). On a 0BA6/0BA8 system, the LOGO! supports up to 24 digital inputs, 20 digital outputs, 8 analog inputs, 8 analog outputs when fully expanded.
5. Mount the 0BA8 base on the same DIN rail.
6. Re-attach the green connector.
7. Re-attach the 0BA8 expansion modules (if you are reusing 0BA6 expansion modules, they will not lock — see §7).
8. Apply 24 V DC, verify the LED goes from red (boot) to green (RUN) within 2 s, or solid red (STOP — upload program).

9. Verification Procedure

After recovery or replacement, run the following checks before handing the panel back to the user.

1. LED state: solid green on base and on every expansion module.
2. Input test: with a multimeter or test lamp on the output terminals, force each input I1-I8 in turn. Verify the program responds (LSC online mode can be used to view I-state and Q-state in real time: Tools → Online).
3. Output test: drive each output Q1-Q8 from LSC: Tools → Start → Online Test → force Qx. Verify the contactor pulls in and the lamp lights.
4. Retentive value test: if the program uses M7-M27 retentive markers, toggle the controller to STOP and back to RUN. Verify the marker values are restored.
5. Clock test: if the program uses a weekly or astronomical timer, set the clock via Tools → Set Clock. Force a known switching time and verify the output pattern.
6. Backup: perform Tools → Upload from LOGO! and store the resulting .lsc on the site engineering share, version-controlled by date and a hash (e.g. SHA-256).
7. Power-loss test: pull the 24 V DC for 5 s and re-apply. Verify the controller returns to RUN within 3 s and the outputs resume per program.

10. Preventive Recommendations

For fleet of 0BA6/0BA7 base modules in service, mitigate the STOP risk with the following actions.

• Annual backup: cycle the door of every LOGO! cabinet and perform an LSC upload. Save the file in a versioned repository.
• Watch the power supply: the cheap 24 V DC switch-mode supplies are a common cause of strangeness. Specify a Siemens SITOP or Phoenix Contact QUINT power supply. Log the rail voltage with a 24 V UPS in critical plants.
• Pre-stage spares: keep one 6ED1052-2HB08-0BA1 in the maintenance store, plus the LOGO! PC cable. A 5-min swap with a pre-loaded SD card (LOGO! 0BA8 supports SD) gets a plant back on air without LSC being on site.
• Replace at 8 years. the 0BA6 has a 20-year design life in office environments and 10-15 years in industrial environments. A scheduled replacement at 8 years (well before the typical failure point) is the most economical field practice.

11. Frequently Asked Questions