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Verify physical network cable connections at both the CNC control NCU and the remote PC. Check for loose or damaged RJ45 connectors and test cable continuity.

Test network connectivity using PING command from HMI or service laptop to verify bi-directional communication between CNC and remote PC IP addresses.

Check network switch and router status LEDs. Verify network infrastructure is operational and no IP conflicts exist on the subnet.

Review firewall settings on both CNC network and remote PC to ensure remote diagnostics ports (typically TCP 102, 8080, or custom ports) are not blocked or timing out.

Check Windows Event Viewer on the remote PC for application crashes or errors related to SINUMERIK Operate, PCU, or remote diagnostics software at the time of disconnection.

Verify the correct version of remote diagnostics software is installed and compatible with the 840D control software version. Update if necessary from Siemens support portal.

Temporarily disable antivirus and Windows Firewall on remote PC to test if security software is blocking or terminating the connection, then configure appropriate exceptions.

Temporarily disabling security software may expose the PC to threats. Re-enable immediately after testing.

Restart the remote diagnostics software and re-establish connection. Monitor for repeated disconnections and check PC system resources (CPU, memory) for performance issues.

Confirm with remote diagnostics user whether the connection was intentionally terminated. If unintentional, ask about any error messages displayed on their PC.

Check VPN connection stability if remote access is through corporate VPN. Verify VPN session timeout settings and ensure they allow sufficient time for diagnostics work.

Review remote diagnostics session timeout parameters in the CNC system settings under commissioning data. Increase timeout values if sessions are terminating prematurely.

Document the time and duration of the connection attempt. Re-establish the remote diagnostics connection and verify stable operation for at least 15 minutes to rule out timeout issues.