Solving LOGO! Soft Comfort 'Unable to Access Remote Device' Error

Problem Statement

Engineers deploying the Siemens LOGO! 12/24RCE logic module with order number 6ED1052-1MD08-0BA2 encounter the dialog error "Unable to access the remote device" when attempting an Ethernet download, online test, or program transfer from LOGO! Soft Comfort. The error fires from the LOGO! Soft Comfort project tree even though the physical layer is healthy: the LAN LED on the front of the LOGO! base module is lit solid green, the link partner (PC or switch) reports the port up, ICMP ping to the configured IP succeeds, and the host PC sits on the same subnet as the module. Despite all green lights at L1/L2 of the OSI stack, the LOGO! Soft Comfort session manager cannot negotiate an application-layer session, leaving the program locked inside the editor.

This failure is the most common first-day engineering hurdle on LOGO! 8 (BM) hardware, and almost all field occurrences resolve to one of three causes:

1. LOGO! Soft Comfort version is older than the firmware generation of the target LOGO! base module.
2. Windows firewall, antivirus, or VPN client is silently blocking TCP port 10005 / UDP 10005 traffic.
3. The LOGO! base module is locked into a S7/Modbus pre-existing connection that the engineering workstation cannot preempt.

Each cause produces the same symptom string, so the diagnostic sequence below is mandatory before re-architecting the network.

Affected Hardware Identification

The order number 6ED1052-1MD08-0BA2 decodes as follows and uniquely identifies the failure window:

The 0BA2 sub-revision shipped with a base firmware that rejects online connections from LOGO! Soft Comfort versions prior to V8.4. The engineering tool performs an interlock handshake during session negotiation that compares the LOGO! Soft Comfort major.minor release against the firmware build signature. If the comparison fails, the LOGO! base module terminates the TCP session before any project data is exchanged, and LOGO! Soft Comfort surfaces the generic "Unable to access the remote device" string.

Root Cause Analysis

The "Unable to access the remote device" error is generated by the LOGOCom.dll component inside the LOGO! Soft Comfort session manager when one of the following conditions is detected by the application-layer handshake:

Version Mismatch Mechanism

The LOGO! base module transmits a 16-byte device descriptor during the discovery phase of the LOGO! Soft Comfort connection. The descriptor encodes the firmware build, hardware revision, and feature flag bitmap. LOGOCom.dll ships with a catalog of known descriptors. If the descriptor hash is not found, LOGO! Soft Comfort rejects the session as a safety measure to prevent mismatched firmware/program blocks being written to a module that cannot interpret them. The error string returned to the operator is generic and does not distinguish a missing descriptor from a blocked socket.

Diagnostic Workflow

Perform the following checks in order before applying the upgrade. Each check has a deterministic pass/fail result and should be documented in the commissioning log.

1. Verify physical layer: Confirm the LAN LED on the front of the LOGO! base module is solid green. A blinking LED indicates link negotiation failure (check the cable, the switch port, and the auto-negotiation settings).
2. Verify IP reachability: From the engineering workstation, execute ping <LOGO_IP>. A response with TTL < 64 confirms a successful ICMP round trip. Set the LOGO! default IP to 192.168.0.1 if the address has been lost.
3. Verify subnet alignment: Run ipconfig /all on the engineering workstation. Confirm the subnet mask matches the LOGO! IP subnet exactly. A common field error is a workstation on 255.255.255.0 while the LOGO! is on 255.255.0.0 — pings still pass within the narrower subnet, but the LOGO! Soft Comfort session manager may route differently when DNS or NetBIOS name resolution fails.
4. Capture the TCP handshake: Use Wireshark with display filter tcp.port == 10005. If no SYN packet originates from the workstation within 3 seconds of pressing the transfer button, the LOGO! Soft Comfort session manager has rejected the local descriptor catalog lookup and never opened the socket. This is the diagnostic signature of the version-mismatch cause.
5. Verify the LOGO! firmware build: Connect a web browser to http://<LOGO_IP>. If the Web-Based Access login page renders, navigate to Diagnostics → Module Information and read the firmware build string. Confirm it matches the catalog of LOGO! Soft Comfort V8.4 or higher.
6. Confirm version of LOGO! Soft Comfort installed: Open Help → About. Note the exact build (e.g. V8.3.1 Service Pack 0).

Solution Path A: Upgrade LOGO! Soft Comfort to V8.4

The primary and most common resolution is to upgrade LOGO! Soft Comfort to V8.4 or higher so the descriptor catalog in LOGOCom.dll matches the firmware build running on the 6ED1052-1MD08-0BA2.

Prerequisites

• Administrative rights on the engineering workstation.
• Approximately 1.2 GB free disk space for the V8.4 installation package.
• A Siemens Industry Online Support account registered to the company Siemens support contract.
• Project archive (.lsc file) backups of every LOGO! program that must be migrated. V8.4 is downward-compatible, but archived programs from V8.3.1 should be exported and archived before the upgrade in case of rollback.

Step-by-Step Upgrade Procedure

1. Close all running instances of LOGO! Soft Comfort.
2. Open the Siemens Industry Online Support download portal. Navigate to Automation → LOGO! Logic Module → LOGO! Soft Comfort.
3. Download the LOGO! Soft Comfort V8.4 Upgrade installer (an upgrade installer, not a full installation, since the existing V8.3.1 installation shares user preferences and the project archive database).
4. Run the installer with administrative privileges. The installer detects the existing V8.3.1 installation and replaces only the binaries, the device catalog, and the LOGOCom.dll library.
5. Restart the engineering workstation to clear any cached sockets held by the previous LOGOCom.dll.
6. Open the upgraded LOGO! Soft Comfort and verify Help → About reports V8.4 or higher.
7. Power-cycle the LOGO! base module (interrupt the 12/24 V supply for 5 seconds) to flush any half-open TCP sessions.
8. Re-establish the connection: Tools → PC ↔ LOGO! or Transfer → Connect. The session manager should now complete the handshake within 2 seconds and present the project upload/download dialog.

Verification

Confirm a successful transfer with the following checks:

• LOGO! Soft Comfort status bar reads "Online" with the LOGO! base module IP address.
• The LOGO! module RUN/STOP LED transitions to steady green.
• The program download completes without error; the diagnostic log records Transfer OK.
• A subsequent ping -t <LOGO_IP> shows consistent replies with no timeouts.

Solution Path B: Network and Host Remediation

If upgrading LOGO! Soft Comfort is not immediately possible (legacy project compatibility, validation freeze, offline factory), the connection can still succeed by remediating the network and host stack. This path is appropriate when the diagnostic step 4 above shows a SYN packet is sent and a TCP RST is returned, indicating the LOGO! base module has accepted the handshake but the session manager cannot negotiate.

Windows Firewall Configuration

1. Open Windows Defender Firewall with Advanced Security.
2. Create an inbound rule allowing TCP port 10005 from the local subnet.
3. Create an outbound rule allowing TCP port 10005 to the LOGO! base module IP.
4. Repeat for UDP port 10005 if the connection uses the LOGO! Soft Comfort UDP discovery fallback.

Antivirus / EDR Exclusions

Add the following paths to the antivirus exclusion list to prevent real-time scanning from intercepting the LOGO! Soft Comfort IPC pipes:

VPN and Multi-Homed Hosts

On multi-homed engineering workstations (for example, a docked laptop with wired Ethernet to the LOGO! subnet and Wi-Fi to the corporate VPN), the routing table may force return traffic from the LOGO! base module through the VPN tunnel adapter, where it is dropped. Use the following remediation:

1. Open an elevated command prompt.
2. Execute route print and identify the interface metric for the LOGO! subnet gateway.
3. Add a persistent route that pins the LOGO! subnet to the wired interface: route -p add <LOGO_SUBNET> mask 255.255.255.0 <WIRED_GATEWAY> metric 5 if <INTERFACE_IDX>
4. Disconnect and reconnect the LOGO! Soft Comfort session.

Switch and VLAN Configuration

Industrial managed switches frequently enable IGMP snooping, broadcast storm control, or DHCP Option 82 that interferes with the LOGO! discovery protocol. Confirm the following switch settings:

• IGMP snooping: disabled or with a static multicast entry for 224.0.0.0/4.
• Storm control threshold: above 5 % to allow broadcast discovery frames.
• VLAN trunking: if the LOGO! is on a tagged VLAN, the engineering workstation NIC must be a member of that VLAN or a 802.1Q-aware driver must be loaded.

Solution Path C: Firmware-Level Recovery

If the LOGO! base module firmware is corrupted or below the version supported by LOGO! Soft Comfort V8.4, perform a firmware refresh using the SD card slot on the LOGO! base module. The procedure is:

1. Download the latest LOGO! 8 (BM) firmware from the Siemens Industry Online Support portal. Verify the firmware file name follows the pattern LOGO_BM_.bin.
2. Copy the firmware file to a micro SD card (FAT32, < 32 GB, MBR partition table).
3. Insert the micro SD card into the LOGO! base module with the supply power removed.
4. Reapply the supply power. The LOGO! base module detects the firmware file on the SD card and automatically flashes it. The RUN/STOP LED pattern alternates orange/green during the flash process.
5. When the LED returns to steady green, remove the SD card and cycle the supply power once more to clear the bootloader state.
6. Reconnect with the upgraded LOGO! Soft Comfort.

LOGO! Soft Comfort ↔ Firmware Compatibility Matrix

Inline Network Topology Diagram

The recommended commissioning topology isolates the engineering workstation, the LOGO! base module, and any other plant devices on a single L2 broadcast domain. The diagram below shows the canonical star wiring used during online session troubleshooting.

Troubleshooting Matrix

Version Validation Procedure

After applying the primary solution, validate the entire upgrade pipeline:

1. Open LOGO! Soft Comfort V8.4 and confirm Help → About reports the expected build (e.g. V8.4.0 Build R1.0.0.0).
2. Open the LOGO! Web-Based Access page and confirm the firmware build is recognized in the LOGO! Soft Comfort Online → Module Information dialog.
3. Perform a program download, program upload, and online test in sequence to validate bidirectional communication.
4. Document the final configuration in the commissioning report: LOGO! Soft Comfort version, LOGO! firmware build, IP address, subnet mask, gateway, DNS, and any switch port settings.

FAQ