Troubleshooting LOGO! 0BA2 No-Display Run Mode Switch Failure

Overview: The 0BA2 Generation and the Run Mode Problem

The Siemens LOGO! 6ED1052-2MD00-0BA2 is a member of the LOGO! 0BA2 hardware generation, an early series of the LOGO! logic module family that predates the current 0BA8 platform. The 0BA2 generation shipped from the early 2000s and is characterized by a limited program memory footprint, a fixed set of digital and analog I/O, and support for the original LOGO! Soft Comfort programming environment. Spares documentation and entry-level support for the 0BA2 line is hosted on the Siemens Industry Online Support portal.

The 0BA2 hardware is produced in two principal form factors: with a built-in 4-line text display and integrated cursor keys, or as a "pure" (no-display) variant intended for cabinet installation where configuration is performed entirely from a PC running LOGO! Soft Comfort. The 6ED1052-2MD00-0BA2 part number identifies the no-display, mixed-I/O, 24 V DC powered variant.

When a valid circuit program is loaded into a 0BA2 no-display module, the module does not always transition to RUN mode immediately. Engineers commonly observe the module's status LED illuminating solid red, indicating STOP mode with a valid program present. The transfer reports successful, the program is in non-volatile memory, yet the module refuses to execute. This article documents the root cause, the toolbar-driven solution, and the power-cycle fallback that recovers normal operation.

Hardware Identification and LED Diagnostics

Part Number Decoding: 6ED1052-2MD00-0BA2

The Siemens MLFB (Maschinenlesbare Fabrikate-Bezeichnung) breaks down as follows:

This resolves to a LOGO! 24 logic module, pure variant, with 4 digital inputs (two of which double as 0–10 V analog inputs I1 and I2), 4 relay outputs rated 10 A resistive, and 24 V DC supply. The module is programmed exclusively through LOGO! Soft Comfort via the LOGO! programming cable, typically part number 6ED1057-1AA00-0BA0 for the 0BA2 era.

LED Status Codes on the 0BA2

The 0BA2 no-display variant communicates its operating state through a single tri-color status LED on the front face. The following table summarizes the state encodings per the LOGO! 0BA2 system manual:

A solid red LED after a successful transfer indicates the module received the program correctly and computed a valid checksum, but has not been commanded to enter RUN. The transfer concluded in STOP mode because the host PC did not issue the mode-change command before the connection was closed. The first diagnostic question is therefore "did the user press the Mode button or simply click Transfer?"

Root Cause: Why the 0BA2 Stays in STOP After Download

The 0BA2 logic module maintains two independent state machines: the program memory write process and the operating-mode command. When LOGO! Soft Comfort transfers a program, it performs three discrete operations in sequence:

1. Erase the internal program EEPROM.
2. Stream the compiled Ladder / FBD bytecode into RAM and verify the transfer.
3. Issue a "store to non-volatile memory" command and disconnect.

Step 3 includes an optional "switch to RUN" subcommand. On later generations (0BA4 onward, including the current 0BA8 line) the default behavior is to issue the RUN command at the end of every transfer. On the 0BA2 generation, the default behavior is to leave the module in STOP after the transfer completes. The operator must explicitly issue a mode change, or rely on the auto-start-on-power-up behavior described in the 0BA2 system manual.

The relevant text quoted from the 0BA2 manual is the controlling specification:

"If there is a valid circuit program in LOGO! memory, the LOGO! automatically switches from STOP to RUN when power is switched on."

This auto-start behavior is triggered by a power transition (OFF → ON), not by the end-of-transfer command. Consequently, an engineer who powers the module from the programming cable's auxiliary port or leaves the supply energized during the entire transfer will not see the module enter RUN.

A second contributor is the LOGO! Soft Comfort toolbar behavior. The "PC → LOGO!" download dialog (the Transfer menu item, or the toolbar transfer icon) includes a checkbox for "Switch LOGO! to RUN mode." On 0BA2-era builds of LOGO! Soft Comfort, this checkbox is sometimes disabled and the toolbar Mode button is the only path to the mode change. If the user is unaware of the dedicated Mode button, they may conclude that the module is faulty when in fact the controller is behaving to specification.

Operating Mode State Diagram

Solution 1: Power Cycle Auto-Start Procedure

The cleanest field fix uses the manual's documented power-cycle path. Perform the following steps in order:

1. Confirm the program download completed without error and the LOGO! Soft Comfort editor reports "Transfer successful."
2. Disconnect the LOGO! programming cable from the module's serial port. Leaving the cable connected is not strictly required but eliminates a potential ground-loop path during the cycle.
3. Switch the 24 V DC supply OFF. Verify with a multimeter that the supply terminals read 0 V DC.
4. Wait a minimum of 20 seconds. The 0BA2's power-on-reset circuit requires this dwell time to fully discharge the bulk capacitor and reset the mode latch. Shorter intervals (for example 2–3 seconds) can leave the module in a partially reset state and the red LED will persist.
5. Re-apply the 24 V DC supply. The status LED should transition from solid red to solid green within 1–2 seconds, indicating RUN mode with the loaded program.

If the LED remains red after a clean 20-second power cycle, the program is not being recognized as valid. Move to Solution 2 to explicitly issue a mode change from the PC, and then verify the program integrity in the LOGO! Soft Comfort editor.

Solution 2: LOGO! Soft Comfort Toolbar "Change Mode" Command

When a power cycle is not practical – for example, when the module is wired into a live control panel with shared 24 V distribution – the operator can issue an explicit mode change from the PC. This is the path that resolves the reported fault when the module cannot be de-energized on demand.

The procedure varies slightly between LOGO! Soft Comfort V6 and V7, but the toolbar control is the same:

1. Connect the LOGO! programming cable to the 0BA2 module and to the PC. Open the LOGO! Soft Comfort project that contains the running program.
2. Click the menu bar item Tools → Transfer → PC → LOGO! (or simply press F11 if a custom hotkey is configured).
3. Wait for the transfer-complete dialog.
4. Click the toolbar button labeled Mode (in some localizations "Mode of LOGO! Change," in German "Betriebsart wechseln"). The icon typically depicts the module outline with a small green arrow. In LOGO! Soft Comfort V6 the Mode button is on the Standard toolbar at the far right. In LOGO! Soft Comfort V7 (including V7.0 and V7.0.9) the same button is on the "LOGO!" toolbar; if the toolbar is not visible, enable it via View → Toolbars → LOGO!.
5. A dialog box opens asking for the target state. Select RUN and confirm.
6. The module's status LED should switch from red to green within 1 second. The dialog closes automatically.

If the Mode button is greyed out, the PC is not in active communication with the module. Check the cable seating, verify the COM port assignment under Options → Interface, and confirm that no other instance of LOGO! Soft Comfort is running on the PC.

Software Version Compatibility

The 0BA2 generation predates LOGO! Soft Comfort V8 and is fully supported by V6 and V7. Compatibility specifics:

The Mode toolbar button was present in every V6 and V7 release. If a user has installed V8 in error, the 0BA2 device entry will not appear in the device catalog and no program transfer is possible. The user must install V6.1 or V7.0.9 in parallel for legacy 0BA2 work. Both are still available through the Siemens Industry Online Support portal.

Programming Cable and Serial Communication

The 0BA2 module uses the legacy LOGO! programming cable, Siemens part number 6ED1057-1AA00-0BA0. This cable terminates in an RS-232 DB-9 female connector on the PC side. Modern PCs without a native RS-232 port require a USB-to-serial adapter. Verified adapters are based on the FTDI FT232 or the Prolific PL2303 chipset with the older (pre-1.5) driver. Newer Prolific drivers intentionally block non-Prolific hardware and can refuse to enumerate the LOGO! cable.

The LOGO! serial protocol uses fixed parameters. Configure the COM port with these values in Device Manager → Ports (COM & LPT) → Properties → Port Settings:

Baud rate: 9600
Data bits: 8
Parity: None
Stop bits: 1
Flow control: None

In LOGO! Soft Comfort, open Options → Interface and select the matching COM port. Click Test to verify round-trip communication. A green "Test OK" message confirms the link layer. From this point the toolbar Mode button should activate.

Verification

After either solution, confirm correct operation with the following checks:

1. LED state: The status LED is solid green. Solid green confirms RUN mode with the loaded program executing. A blinking red LED would indicate the program slot is empty or has a checksum error – re-transfer the program and verify with the V6 / V7 upload function.
2. Output behavior: The digital output mapped to the program (for example Q1) is at the expected energization state. Measure across the output relay contacts with a multimeter on the resistance range. A closed contact reads < 1 Ω; an open contact reads > 1 MΩ or OL.
3. Input recognition: Apply a control signal to the input (for example I3) and observe the input LED on the module illuminating. The 0BA2 input LEDs are located on the front face directly above each input terminal and illuminate when the input reads logic 1.
4. PC upload test: Click PC ← LOGO! (upload). The project should appear in the editor and match the program on disk. A checksum mismatch suggests memory corruption – erase and re-load the program from the source file.
5. Power-loss retention: Remove and re-apply the 24 V supply once more. The module should return to RUN mode and resume execution of the program. This confirms the program is in non-volatile memory, not in the transfer buffer.

Failure Mode Matrix and Replacement Guidance

When to Replace the Module

The 0BA2 generation is fully mature and out of active Siemens production, having been superseded by 0BA3 through 0BA8. If a 0BA2 module fails to accept a known-good program across two different PCs, two different LOGO! Soft Comfort versions, and a known-good programming cable, the internal EEPROM is most likely end-of-life. The EEPROM used on 0BA2 has a rated endurance of approximately 100,000 write cycles. A module that has been in field service for 15+ years with frequent program changes can reach this limit.

Replacement candidates in the current Siemens catalog include:

• 6ED1052-1MD00-0BA8 (LOGO! 24 CE, 0BA8, mixed I/O with display, 24 V DC)
• 6ED1052-2MD00-0BA8 (LOGO! 24 CE, 0BA8, mixed I/O pure, 24 V DC)
• 6ED1052-1FB00-0BA8 (LOGO! 230 RCE, 0BA8, mixed I/O with display, 115/230 V AC)

Note that 0BA8 programs are not directly backward-compatible with 0BA2 – the program must be re-authored in LOGO! Soft Comfort V8.x with the new device selected. The ladder / FBD constructs are largely portable, but the device-specific I/O mapping must be re-validated. The LOGO! product page lists all current variants and their MLFBs.

Field Commissioning Checklist

For new installations using a 0BA2 no-display module, perform the following before energizing the controlled load:

1. Pre-load: Connect the programming cable and confirm round-trip communication. Set the COM port and verify the test dialog.
2. Transfer: Download the program. Confirm the transfer-complete dialog.
3. Run command: Click the toolbar Mode button. Verify the green LED.
4. I/O check: Force each input and observe the corresponding input LED. Force each output from the LOGO! Soft Comfort online monitor and measure the contact state.
5. Power-loss test: Remove 24 V supply for at least 20 seconds. Re-apply. Verify the module auto-runs (green LED).
6. Cable removal: Disconnect the programming cable. Verify the module continues to run.
7. Long-term retention: Leave the module powered for one hour, then re-verify the program and operating mode.

Items 1, 5, and 6 are the three steps that catch the most field issues with the 0BA2 generation. Skipping item 5 is the most common cause of field reports that the module "won't stay in RUN" – the user assumed the post-transfer Mode button click was sufficient and never tested the auto-start path.

FAQ