Resolving LOGO! 8 'Download PC → LOGO! Failed' in Soft Comfort

Problem: LOGO! Soft Comfort Reports "Download PC → LOGO! Failed"

When a Siemens LOGO! 8 base module such as the 6ED1052-1MD00-0BA3 (LOGO! 8 12/24RCE, modular, no integrated display, USB + Ethernet) is connected to a PC over USB, the LOGO! Soft Comfort (LSC) programming software routinely completes the reverse direction (LOGO! → PC, "Download from LOGO!") yet refuses the forward direction with the line:

*** 12/30/22 3:19 PM Circuit Diagram3
*** PC --> LOGO!
Download PC --> LOGO! failed

The PC recognises the module, the project opens, and the on-board LED indicates link activity, but the transfer is aborted before any blocks are written. This document consolidates the field-proven root causes and the corrective procedure used by Siemens application engineers when the same unit previously programmed successfully and is now failing only in the upload direction.

Affected Devices and Software

Symptoms Reported in the Field

• LOGO! Soft Comfort shows the module in the "Online" dropdown, so USB enumeration and driver binding are working.
• "Download from LOGO! (LOGO! → PC)" completes and a circuit diagram is retrieved.
• "Download to LOGO! (PC → LOGO!)" terminates with Download PC --> LOGO! failed in the log pane.
• No diagnostic LED (LINK/ACT) abnormality is observed on the module.
• A second, identical LOGO! 8 module programs correctly on the same cable and PC, ruling out host environment.

Root Cause Matrix

Prerequisites

• Administrative rights on the PC to install / re-install the LOGO! USB driver.
• LOGO! Soft Comfort V8.x installed (matching or newer than the module's firmware generation).
• One known-good USB-A → USB mini-B data cable (≤ 3 m, shielded).
• Access to a second LOGO! 8 module of the same type for cross-testing (recommended).
• Stable 24 V DC supply (or 12 V DC for the 12/24RCE variant) capable of ≥ 1.5 A — undervoltage during flash writes can drop the USB stack.

Step 1 — Match the LSC Hardware Type to the Module

The single most common cause of the "Download PC --> LOGO! failed" error is a hardware-type mismatch. LSC embeds a target fingerprint inside every compiled project; when the fingerprint does not match the connected device, the transfer is aborted.

1. Read the full Siemens article number on the front of the module (e.g. 6ED1052-1MD00-0BA3).
2. Note the four-digit suffix: -0BA3 identifies the original LOGO! 8 (2014). -0BA8 is LOGO! 8.3.
3. In LSC, observe the hardware-type indicator in the bottom-right corner of the status bar.
4. Double-click the indicator (or use File → Properties → Hardware Type) to open the Device Selection dialog.
5. Pick the device whose article number ends in the same -0BAx suffix. The right-hand panel displays the model designation for confirmation.
6. Click OK, then File → Save before retrying the transfer.

Step 2 — Put the LOGO! Into Transfer Mode

For LOGO! 8 base modules equipped with a display, the unit must be in Transfer mode before LSC can write. For display-less modules such as the 1MD00-0BA3, transfer mode is engaged automatically when LSC initiates a download; manual intervention is only required if a LOGO! TDE is attached.

1. On the device, press ESC to enter the main menu.
2. Navigate to Network → Transfer (or, on older firmware, Setup → Transfer).
3. Select PC ↔ LOGO! and press OK; the display shows "PC <-> LOGO!".
4. Return to the process display (the LOGO! is now listening on both Ethernet and USB).
5. In LSC, click PC → LOGO! (Download). The log pane should now read Transfer started followed by block-by-block writes.

Step 3 — Validate the USB Cable and Driver

A surprising fraction of intermittent upload failures are caused by substandard cables that carry only the power pins. The data lines (D+, D−) are absent or broken.

Step 4 — Confirm Firmware Generation Compatibility

Cross-reference the LSC project version and the device's 0BAx suffix:

If the project was last saved in a newer LSC (for example, V8.3) and the target is a 0BA3 module, the transfer is rejected. Open the project on a PC with the matching LSC version, or "downgrade" the project to the older device by setting the correct hardware type and re-compiling — provided no V8.1+ blocks (such as the data log, astronomical timer, or Modbus TCP server blocks) are used.

Step 5 — Inspect Program Protection

1. In LSC, attempt File → Download from LOGO! to PC. If a password prompt appears, the unit has upload protection enabled.
2. Download the diagram, then check Tools → Protection. Disable program / copy protection and re-upload.
3. If the password is unknown, perform an online reset: in LSC choose LOGO! → Clear Program after putting the device in transfer mode. The new empty program overwrites protection.

Step 6 — Isolate a Hardware Fault on the USB Port

When the same PC, cable, and LSC instance program a second identical module successfully, the fault is on the original module. Common failure modes on the LOGO! 8 USB mini-B receptacle include:

• Cold-solder joint on the mini-B connector — reflow or replace the connector.
• ESD damage to the on-board USB PHY — module must be replaced; there is no field-replaceable USB controller.
• Contamination (flux, debris) on the connector pins — clean with isopropyl alcohol and a fiberglass brush.

Test procedure:

1. Reprogram the suspect unit on a second PC with a different LSC installation.
2. Try the Ethernet port (if present on the variant) — the 1MD00-0BA3 has an RJ45 port. Tools → Ethernet Interfaces in LSC lets you push the same project over TCP/IP. A successful Ethernet download alongside a failed USB download confirms the USB PHY is defective.
3. Run the module for ≥ 24 h in the application and verify the program does not spontaneously revert — flash corruption symptoms include mid-cycle resets and re-entry into stop mode.

Verification Procedure

1. Cycle power to the LOGO! (disconnect and re-apply the supply).
2. Open LSC and select PC → LOGO! (Download). Confirm the log pane shows block-by-block writes ending with Transfer OK.
3. Watch the device's display or the LINK/ACT LED for the green heartbeat, indicating RUN state.
4. In LSC, choose LOGO! → Online Test; force each input and verify the corresponding output or flag transition in the diagram view.
5. Power-cycle again and confirm the program survives — LOGO! 8 stores the program in non-volatile memory; a successful cold-start verifies the write was committed.

Preventive Practices

• Standardise on one LSC version per workstation; mixing V8.0 and V8.3 projects is the leading cause of the fingerprint mismatch.
• Maintain a USB cable inventory: tag every cable "data + power" with a coloured band. Charge-only cables must be physically segregated.
• Enable File → Settings → Project → Always show device selection so the hardware type is forced at every save.
• Document the 0BAx suffix in the project comment field (File → Properties → Comment) so future engineers know which generation the project targets.
• Always ground yourself (ESD wrist strap) before plugging into the USB mini-B port; the connector is a known ESD entry point on the LOGO! 8 PCB.

Field Diagnostic — Decision Flow

Frequently Asked Questions