Troubleshooting Siemens LOGO! 24RC 0BA3 Program Upload Failures to LOGO!Soft Comfort v8

The Siemens LOGO! 24RC is a compact logic module used widely in small automation tasks. Older 0BA3 generation units (order number 6ED1052-1HB00-0BA3) require a different upload workflow than the more common 0BA4 generation (6ED1052-1HB00-0BA4). Engineers upgrading tooling to LOGO!Soft Comfort v8 on Windows 7 frequently encounter a situation where the communication test passes, the upload dialog appears, the status bar prints Upload LOGO! --->PC completed followed by Upload LOGO! --->PC failed, and the resulting 0BA3.standard file is empty or corrupted. This article explains why the failure occurs and how to reliably retrieve a program from a 0BA3 module.

1. Problem Statement

Field-reported symptoms on a 0BA3 hardware under LOGO!Soft Comfort v8.1 over a USB LOGO! cable (6ED1057-1AA00-0BA0):

• The communication test in the PC <-> LOGO! connection dialog completes successfully.
• The transfer starts, and the status line reports:

  *** LOGO! -->PC
  0BA3.standard
  Upload LOGO! --->PC completed
  Upload LOGO! --->PC failed

• No on-device prompt to stop or continue is displayed on the LOGO! 0BA3, in contrast to the 0BA4 which reminds the operator to stop the program.
• The resulting file is not the running circuit diagram but an empty placeholder of the wrong device generation.

On the 0BA4 reference unit the same procedure finishes without error. The asymmetry is not a driver bug; it is an intentional difference in how the 0BA3 firmware negotiates the program-transfer state.

2. Affected Hardware and Order Numbers

The "C" suffix denotes relay outputs and integrated clock/calendar. Versions of the LOGO! 24/24o and 12/24RC/12/24RCo additionally expose I7 and I8 as analog inputs AI1/AI2 when configured, per the SIMATIC LOGO! Manual (PDF). This input-assignment detail is sometimes confused with the upload path but is unrelated to it.

3. Root Cause: Missing PC <-> LOGO! Mode on 0BA3

The 0BA4 firmware automatically enters the transfer state when the host requests a program read or write. The 0BA3 firmware does not. On a 0BA3, the module must first be placed into PC <-> LOGO! mode through its front-panel menu before the upload can succeed. If the module is left in RUN or STOP with the operator not having selected the PC-link menu entry, the transfer request is refused at the protocol layer and the host reports Upload LOGO! --->PC failed even though the cable is functional.

Two environmental factors compound the issue:

1. LOGO!Soft Comfort v8.x is a long-life release that supports all generations from 0BA0 to 0BA8, but the user interface does not visually distinguish 0BA3 upload behavior from 0BA4. The error message is identical for many failure modes.
2. Running LOGO!Soft Comfort inside a virtualized Windows 7 host (for example on a PG with limited native COM/USB support) routes the USB cable through a virtual COM port, which can be flaky for legacy protocol handshakes on the 0BA3.

4. Prerequisites

Verify the following before attempting the upload. Each item maps to a documented requirement in the SIMATIC LOGO! Manual or in the LOGO!Soft Comfort online help.

5. Step-by-Step Upload Procedure for LOGO! 0BA3

Follow this sequence exactly. The 0BA3 will refuse the upload if any step is missed.

1. Power the module. Apply 24 V DC. Wait for the display to finish the start-up self-test and show the date/time or the current I/O status screen.
2. Stop the running program (optional but recommended). Use the ESC key to reach the main menu, then navigate to Stop and press OK. The status LED should change from RUN to STOP. If the program is password-protected, enter the password at this point.
3. Enter PC <-> LOGO! mode on the device. From the main menu, navigate to:

   Menu → PC <-> LOGO!

   Select PC <-> LOGO! (LOGO! → PC) (also labeled Upload on some firmware versions) and press OK. The display confirms with PC <-> LOGO! and shows Connect to PC as a flashing status.
4. Connect the USB cable. Plug the 6ED1057-1AA00-0BA0 cable into the LOGO! interface port and into the host PC. The Windows device manager should show a new COM port such as COMx assigned to the LOGO! USB driver.
5. Launch LOGO!Soft Comfort v8.1. From the menu bar select Tools → PC <-> LOGO! (or click the PC <-> LOGO! toolbar button).
6. Set the COM port and baud. In the connection dialog, choose the COM port assigned to the USB cable. Auto-detect is supported but explicitly selecting COMx (LOGO! USB) at 19200 bit/s is more reliable on Windows 7.
7. Run the connection test. Click Test. A successful test returns OK and the module generation is reported (e.g. 0BA3).
8. Start the upload. Click Upload (LOGO! → PC). The status line shows:

   *** LOGO! -->PC
   0BA3.standard
   Upload LOGO! --->PC

   On a 0BA3 the line will be followed by the actual data transfer (no completed/failed double-line artifact). When the transfer finishes the file is opened in the LOGO!Soft Comfort project workspace.
9. Save the project. Use File → Save As to store the uploaded circuit diagram with a meaningful name; the 0BA3.standard file is the auto-generated scratch name.

6. Comparison: 0BA3 vs 0BA4 Upload Workflow

The most important engineering takeaway: the 0BA3 upload path is a two-sided handshake; both the host and the device must agree to enter the transfer state. The 0BA4 collapses that handshake into a one-sided request from the host.

7. LOGO!Soft Comfort v8 Configuration Reference

The connection dialog fields and their effect on the upload path are summarised below. Defaults shown are the values LOGO!Soft v8.1 uses on first launch.

8. Virtual Machine Considerations

LOGO!Soft Comfort v8 has not been formally validated against all hypervisor/USB-passthrough combinations. When the host OS is Windows 7 inside a virtual machine (for example on a Siemens Field PG that has been P2V'd, or a sandboxed engineering workstation), the following are known to cause intermittent or permanent upload failures on 0BA3 modules:

• USB passthrough latency. The 0BA3 protocol has a tighter timing window than the 0BA4. Passthrough stacks that add more than 50 ms of round-trip latency will cause the 0BA3 to abort the transfer.
• Virtual COM port mapping. Some hypervisors expose the USB device as a virtual COM port with a different device identifier than the host driver expects. The connection test may pass (because the COM port exists) while the protocol layer fails.
• Suspended host timer. When the VM resumes from suspend, the LOGO!Soft v8 background service can hold a stale COM handle. Restart LOGO!Soft Comfort after every resume.

9. Error Message and Status Code Reference

LOGO!Soft Comfort v8 does not surface a hex error code on the 0BA3 upload path; instead it reports textual status lines in the transfer log. The table below maps the field-observed status sequences to the underlying cause.

10. Verification Procedure

After a successful upload, validate the recovered project before relying on it as the source of truth:

1. Open the recovered .lsc project in LOGO!Soft Comfort v8.1. The status bar at the bottom should report 0BA3 as the target device, with no warning icons.
2. Check the simulation. Press F11 (or click the Simulate button) to start the on-screen simulation. The I/O behaviour should match the machine's documented operation. If a block is missing or shows "??" then the body transfer was partial and the file is unsafe.
3. Compare with the live device. Use Tools → PC <-> LOGO! → Compare. A clean compare should produce no differences. Any red entries indicate a divergent program that must not be downloaded back to the module without review.
4. Hash the file (optional but recommended for audit trails):

   sha256sum "recovered_project.lsc"

   Record the hash with the project version notes.

11. Alternative Upload Paths

If the front-panel PC-mode menu is not accessible (for example, the keypad is damaged or a password is lost), consider the following alternatives in order of preference:

1. LOGO! memory card / battery card. The 0BA3 supports the LOGO! memory card (e.g. 6ED1056-1BA00-0BA0). If one was used on the original installation, power down the module, insert the card, and use Card → LOGO! on the device menu to restore. This path is independent of the PC-link and not blocked by the upload-mode handshake issue.
2. Serial cable (legacy). The pre-USB PC cable (6ED1057-1AA00-0BA0 is the USB variant; the serial predecessor 6ED1057-1AA00-0BA0-era cable used DB9) is electrically compatible with 0BA3. On modern PCs use a USB-to-serial adapter, but be aware that some adapters cannot sustain the 19200 bit/s timing that 0BA3 requires.
3. Substitute a known-good 0BA3. If the production line must be restarted and no program is recoverable, a freshly programmed 0BA3 with a re-engineered equivalent circuit (built from I/O observation) is the most reliable fallback.

12. Prevention: Best Practices for Future 0BA3 Sites

• Always store a copy of the LOGO! project on a managed file server with version control. Treat the .lsc file as a controlled document.
• When commissioning, export the project and email it to the maintenance team immediately after download success.
• Label every LOGO! module with its MLFB and program revision (e.g. 6ED1052-1HB00-0BA3 / rev 2024-03-12).
• Keep a dedicated 0BA3-compatible LOGO!Soft Comfort license (v3.x through v6.x) in a known repository in case v8.x is no longer available on a particular service laptop. v8.x is preferred but a fallback path is cheap insurance.
• Document the upload-mode menu navigation in the site-specific recovery procedure so that on-call engineers do not have to relearn it during an outage.

Summary

The Upload LOGO! --->PC failed message on a Siemens LOGO! 24RC 0BA3 running against LOGO!Soft Comfort v8.1 is almost always caused by the device not being placed in PC <-> LOGO! mode before the upload is requested. The 0BA3 firmware does not auto-negotiate the transfer state the way the 0BA4 does. Navigate the device menu to PC <-> LOGO! → LOGO! → PC and confirm the on-device prompt before clicking Upload on the host. After a successful transfer, validate the recovered project with the simulator and the Compare function before treating it as the canonical source.

Frequently Asked Questions