Troubleshooting

PROFINET Device Shows BF Red: Fixing "No Connection" Step by Step

The ET 200SP is powered, the cable is plugged in, and BF is red. Almost always it is the device name — here is how to prove it and fix it.

· 8 min read · By TryPLC Team

  • PROFINET
  • Siemens
  • Commissioning
  • Troubleshooting

The symptom

A remote IO station is powered and cabled, but its bus fault LED is red or flashing. On ET 200S and ET 200M interface modules that LED is labelled BF; on an ET 200SP with an IM 155-6 PN it is the ERROR LED. Below, BF means whichever of the two your module has. The S7-1500 diagnostic buffer reports IO device failure or device not found.

How a PROFINET controller finds its devices

A PROFINET controller does not find devices by IP address. It finds them by device name (for example et200sp-conveyor-1). At startup the controller asks the network, through DCP, "who is called et200sp-conveyor-1?", and then assigns the IP address itself. A brand-new or replaced device has no name, so nothing answers.

So on PROFINET, "the IP is correct" tells you nothing about whether the device will connect.

Step 1: read the LEDs precisely

  • BF flashing: the device has a link but no connection to the controller. The cause is usually a name or configuration mismatch.
  • BF steady red with no link LED: a physical problem. Check the cable, the port and the switch.
  • Link/activity LED off: there is no Ethernet link at all. Swap the patch cable and try another port.

    • Step 2: check the name online

    In TIA Portal, run Online access → Update accessible devices on the network adapter connected to the plant network. Each device is listed with its name, IP and MAC. Compare the listed name, character for character, with the name in the hardware configuration. The usual faults are a missing name, a typo, a hyphen where the project has an underscore, or a device that still carries the name from its previous machine.

    Step 3: assign the name

    Select the device in the network view and use Assign device name. Pick the correct physical device by its MAC address (printed on the module) or with Flash LED. Once the name matches, the controller assigns the IP and BF should go off within seconds.

    For replacements, consider enabling device replacement without exchangeable medium. With the topology configured, the controller then names a new device automatically from its port neighbour.

    Step 4: check that the configuration matches the hardware

    If BF stays red after naming, the controller is connecting but rejecting the configuration. Look for:

  • the wrong interface module article number or firmware version in the project;
  • modules plugged in a different order from the configuration;
  • an outdated GSDML file for a third-party device. Install the version that matches the device firmware.

    • Step 5: topology and duplicate names

    If topology is configured, moving a cable to a different port triggers a topology error; either put the cable back or update the topology. Two devices with the same name cause intermittent failures, so rename one of them.

    Order of checks

    On PROFINET, check the name first, the configuration second and the IP address last. The PROFINET lesson walks through the whole device lifecycle.

    Frequently asked questions

    What does a flashing BF LED mean on PROFINET?

    The device has an Ethernet link but no connection to its IO controller, most often because the device name is missing or does not match the project.

    Does PROFINET use the IP address to find devices?

    No. The controller identifies devices by device name via DCP and then assigns the configured IP address.