PROFINET can't find a device — device name, LLDP, topology

PROFINET (Siemens default) differs from plain Ethernet: an IO-Device is identified by its device name, not just IP. When a device won't come up and the CPU faults, it is almost always a wrong/missing device name, IP, GSDML or topology/LLDP — not a faulty device.

Common causes

1. Wrong/missing device name — a replaced device hasn't had the project name reassigned.
2. IP / subnet — wrong range, duplicate IP, DCP misassigned.
3. GSDML — missing the correct GSDML version in the project.
4. Topology / LLDP — topology declared (port-to-port) but wired to the wrong port.
5. Cable / port / speed — broken cable, bad port, forced 100Mbps full-duplex mismatch.

Step-by-step diagnosis

1. Check the device name Use TIA/Online & Diagnostics or PRONETA to view it; reassign the project name if lost.
2. Check IP/subnet Ensure the IP is in range, no duplicates; verify assignment (DCP/fixed).
3. Read diagnostics Check the CPU diagnostic buffer to see which module/slot faults.
4. GSDML Update the correct GSDML for the device + firmware; reconfigure on mismatch.
5. Topology / LLDP If topology is declared: wire the correct port per the diagram; LLDP flags mismatches.
6. Physical layer Try another cable/port; check speed/duplex; use PRONETA to scan the network for the big picture.

When to call an expert

PROFINET device-name/topology/GSDML faults get messy when replacing hardware. DeepDebug knows Siemens, uses PRONETA + capture to localize fast and support remotely.