CP 343-1 BF LED: Diagnosing IP Conflicts and PROFINET Name Faults

Overview

The SIMATIC NET CP 343-1 (order number 6GK7-343-1EX30-0XE0, firmware V2.2) is a single-port Industrial Ethernet communications processor for the S7-300 family. It supports PROFINET IO Controller/Device, open TCP/IP, ISO-on-TCP, UDP, S7 communication, and Modbus TCP (with the optional AddOn). A persistently lit or flashing BF (Bus Fault) LED on the front panel indicates a layer-1, layer-2, or layer-3 Ethernet anomaly that prevents the CP from participating in normal communication. A critical point missed in the field: a lit BF LED does not force the CP into STOP. According to the device manual, If a double IP address is detected, the CP also reports this via the LED (BF) and generates an entry in the diagnostics buffer. The CP remains in RUN mode. (Source: SIMATIC NET S7-300 CP 343-1 manual, GH_CP343-1-EX30_76.)

This reference consolidates the four most frequently documented root causes of the BF LED on the EX30 module: duplicate IPv4 address, PROFINET device name conflict, misuse of local TCP port 502 for non-Modbus applications, and a missing networked flag in the STEP 7 HW Config. The procedural flow concludes with a firmware upgrade option that suppresses the BF LED for a name-conflict condition when no PROFINET IO is configured on the CP.

CP 343-1 Hardware Identification and LED Meanings

The 6GK7-343-1EX30-0XE0 is the EX30 revision of the CP 343-1 family and ships with a single RJ45 port. The front panel contains five visible status indicators (SF, BF, RUN, STOP, plus a per-port Link LED inside the RJ45 jack):

BF LED Root Cause Matrix

Root Cause 1: Duplicate IP Address

The CP performs a gratuitous ARP at power-up and periodically thereafter. If a second node on the same subnet responds with its MAC for the configured IPv4 address, the CP raises the BF LED, writes a diagnostic buffer entry, and continues to RUN. The CP's IP is held in a tentative state and is not advertised in normal traffic, so application traffic to the CP is silently dropped until the conflict is resolved.

Resolution steps:

1. From a maintenance PG connected to the same subnet, run arp -a to list the cached MAC/IP pairings.
2. From the STEP 7 project, identify the IP configured on the CP (Properties → PROFINET Interface → Ethernet addresses).
3. Scan the subnet with ping -n 1 <CP-IP> followed by arp -a. A successful ping followed by a MAC different from the one listed in HW Config indicates a duplicate.
4. Re-IP the conflicting device or the CP. Avoid 192.168.0.0/24 default addresses on brownfield networks with mixed vendors.

Root Cause 2: PROFINET Device Name Conflict

PROFINET identifies stations by a textual device name stored inside the CP. The name is broadcast in DCP (Discovery and Configuration Protocol) identify requests. If two devices share a name, the IO Controller fails to start the AR (Application Relationship) with either, and the affected CP raises BF. The name must be unique across the entire PROFINET subnet — not just unique to the local station.

Inspection sequence:

1. In SIMATIC Manager or TIA Portal, open HW Config for the S7-300 station.
2. Select the CP 343-1 → Properties → PROFINET Interface → PROFINET tab.
3. Note the configured PROFINET device name.
4. From the same dialog, use "Special Diagnostics" or run PLC → Ethernet → Edit Ethernet Node → Browse to list every device and its name.

Resolution:

1. Rename the CP to a unique value (e.g., cp343-1-station01).
2. Assign the name via PLC → Ethernet → Assign PROFINET Device Name. The CP must be reachable on layer 2 (DCP) for this wizard to function.
3. Power-cycle the CP or trigger an ARP refresh so the new name is announced.

Root Cause 3: Local Port 502 Mis-configuration

TCP port 502 is the IANA-registered port for Modbus TCP. The CP 343-1 can be configured to use any local port for open TCP connections, but binding the CP to local port 502 when the licensed ModbusTCP CP AddOn is not installed leaves the socket in an undefined state. The behavior typically manifests as a solid BF LED that returns immediately after each restart, combined with TCP socket errors in the diagnostic buffer.

Corrective action:

1. Open the CP properties → Ethernet → ISO-on-TCP / TCP connections.
2. Edit each connection and change the local port from 502 to a free value ≥ 2000 (e.g., 2000, 2001, 2002).
3. Mirror the change on the partner PLC / HMI / panel.
4. Recompile HW Config and download to the CPU.

Root Cause 4: Networked Flag Not Set in HW Config

The Networked checkbox in HW Config attaches the CP's PROFINET interface to a configured Ethernet subnet. If the flag is cleared (a frequent oversight in brownfield retrofits), the CP will not respond to DCP identify requests and shows BF even with a healthy cable. The Link LED stays green because the physical layer is intact; the failure is on layer 2 because no subnet is logically assigned.

Set the flag as follows:

1. Open HW Config and double-click the CP.
2. Open the Properties dialog for the PROFINET interface.
3. Enable the Networked check box and select the correct Ethernet subnet.
4. Recompile and download the configuration.

Diagnostic Procedure Using NCM Diagnostics

From STEP 7, online diagnostics deliver the fastest path to a root cause. The Special Diagnostics button in the Accessible Nodes window launches NCM S7 Diagnostics without requiring a STEP 7 project to be open.

1. Open SIMATIC Manager → Options → Set PG/PC Interface → select the Ethernet NIC bound to the plant network.
2. PLC → Ethernet → Edit Ethernet Node → Browse.
3. The Online Accessible Nodes table lists every PROFINET device, its IP, its MAC, and its PROFINET name.
4. Right-click the CP 343-1 and choose Special Diagnostics to launch NCM S7 Diagnostics.
5. Open the Diagnostic Buffer. Look for entries with the ID bytes consistent with IP conflict, name conflict, or TCP binding failure.

Firmware V2.2 to V2.3 Upgrade Path

Siemens shipped firmware V2.3 for the 6GK7-343-1EX30-0XE0 with a behavioral change: when no PROFINET IO is configured on the CP, the firmware no longer raises a BF LED for a duplicate device name. If your CP only uses open communication (TCP, ISO-on-TCP, S7) and PROFINET IO is disabled, upgrading to V2.3 is the cleanest mitigation for sporadic name-conflict BF events caused by partner devices generating name collisions.

Release-note excerpt (Siemens entry ID 42123315): If no PN IO functionality is used, no check is made for duplicate device names. Therefore, no error LED is activated.

Upgrade procedure:

1. Download the latest firmware package from Siemens support, entry ID 42123315.
2. Connect the PG directly to the CP via Ethernet (direct or via switch) on the same subnet.
3. Open SIMATIC Manager → Options → Firmware Update (or the standalone FWUpdate tool).
4. Select the CP, browse to the extracted FW file (.upd), and execute the update.
5. Wait for the CP to reboot. The BF LED may blink during the update; this is normal.
6. After the upgrade, perform a CPU STOP → RUN transition to clear stale diagnostic buffer entries.

Diagnostic Flow Diagram

Step-by-Step Recovery Procedure

The following sequence resolves the BF LED in the majority of field service calls. Work the steps in order; stop when the LED clears.

1. Verify the physical layer. Confirm the Link LED is lit; if it is off, swap the patch cable and the switch port.
2. Open NCM Diagnostics via the Special Diagnostics button. Read the diagnostic buffer and identify the most recent fault.
3. If the buffer reports a duplicate IP, scan the subnet with arp -a and remove or re-IP the conflicting station.
4. If the buffer reports a name conflict, list the PROFINET names with the Browse tool and rename the CP to a unique value.
5. If the fault is TCP-socket related, change the local port from 502 to ≥ 2000 in every open TCP connection on the CP.
6. If the buffer is empty, open HW Config, set the PROFINET interface Networked flag, recompile, and download.
7. Re-check the BF LED. If still lit, upgrade the firmware to V2.3 (entry ID 42123315) and repeat the configuration check.

Verification and Commissioning Checks

Use the following checks to confirm the fault is cleared and to prevent recurrence:

• BF LED is OFF and RUN LED is solid green.
• Diagnostic buffer shows no new entries after 5 minutes of network traffic.
• From the PG, run ping -t <CP-IP> for 1 minute; all replies should return in < 10 ms on a healthy plant network.
• Open a test S7 connection from the PG and read the CP's system state list (SSL) with ID W#16#0132.
• Capture a Wireshark trace of the CP's MAC and confirm that DCP identify responses contain the new unique PROFINET name.

FAQ