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EtherCAT & PROFINET DISCUSSION
We replaced a failed PROFINET remote I/O station with an identical new one, set the same IP address on it with a vendor tool, and the PLC still reports the station as missing with a bus fault LED. The old station had worked for years with the same hardware configuration.
Why is the IP address not enough, and what does a PROFINET controller actually use to identify its devices?
A PROFINET controller identifies devices by their device name (the name of station), not by IP address or MAC address. At start-up the controller sends a DCP Identify request on the network asking which device is called, say, conveyor-io-1. The device with that name answers, the controller assigns it the IP address stored in the project, and only then is the connection (the application relation) established and the configuration checked.
A factory-new device has an empty name, so it never answers the Identify request. Setting the IP address by hand does not help, because the controller is not looking for an address.
The fix is to assign the name. In the engineering tool there is an online function usually called "assign device name": it lists the devices found on the subnet by DCP, with MAC address, type and current name, and lets you write the configured name to the right one. The name must match the project exactly. It is stored in non-volatile memory in the device, or on a removable memory card on some product lines, which is why moving the card from the old unit also works on those.
DCP operates at Ethernet layer 2, so the programming PC must be in the same subnet or VLAN as the device; it does not pass through a router.
Replacement without a laptop is possible if the project was set up for it. With the topology configured (which port of which device connects to which neighbour) and the option for device replacement without removable medium enabled, the controller uses LLDP neighbour information: it sees an unnamed device of the right type on the expected port of the neighbouring device and assigns the name automatically.
Two conditions apply. The replacement must be in the factory-reset state, since by default a device that already carries a different name is not overwritten, and the neighbouring devices must support LLDP. It is worth enabling on any machine where maintenance staff swap devices at night.
If the name is correct and the device still shows a fault, the next check is the module configuration. After the connection is set up, the controller compares the expected modules and submodules from the project with what is actually plugged in, based on the identification numbers from the GSDML file. A different module in a slot, or a newer hardware generation that needs a newer GSDML file, produces a module mismatch diagnosis rather than a missing station.
The diagnostic buffer of the PLC distinguishes these cases: a device that is not reachable points to name, cabling or a duplicate name, while a module difference points to the configuration.
