Troubleshooting CP343-1 Lean TCP/IP Connection Failures on S7-300 with SCALANCE X208

The CP 343-1 Lean (Siemens order number 6GK7 343-1EX21-0XE0) is one of the most widely deployed Ethernet communication processors for the SIMATIC S7-300 family. When the module refuses TCP/IP traffic and the WinCC engineering station cannot reach it across a SCALANCE X208 switch, the symptom set is consistent: RUN LED on, SF LED red, BF PN LED blinking, no ping reply, and diagnostic buffers reporting "send/recv communication: not configured". This reference walks through every verified diagnostic and remediation step drawn from Siemens field practice, and it consolidates the configuration data the engineer actually needs on the bench.

1. Problem Statement

A SIMATIC S7-300 station is equipped with a CP 343-1 Lean communication processor and connected through a SCALANCE X208 managed switch to a WinCC PC station. Initial commissioning fails with the following observable symptoms:

• CP 343-1 Lean module is in RUN (green RUN LED).
• SF (Group Error) LED is solid red.
• BF PN (Bus Fault PROFINET) LED is blinking.
• Engineering PC cannot ping the CP at its configured IP address.
• NCM S7 diagnostics report:
  • Send/Recv communication: in stop — not configured
  • System connection: in stop — the task has not started
• STEP 7 Edit Ethernet Node / Primary Setup Tool (PST) cannot discover the CP MAC address.

2. Affected Hardware and Firmware Versions

The following Siemens part numbers are in scope for this article. Confirm the exact MLFB before applying any configuration change, as PROFINET capabilities differ between variants.

The reported discrepancy between HW catalog version V1.1 and diagnostics version V1.2.3 is normal. V1.1 is the HSP (Hardware Support Package) catalog entry; V1.2.x is the field-loaded firmware revision. Both are functionally compatible for PROFINET IO Device operation. Do not attempt to downgrade firmware to match the catalog entry.

3. LED Diagnostic State Analysis

Every LED on the CP 343-1 Lean front panel maps to a specific fault domain. Read the LEDs before opening any diagnostic tool.

With RUN=on, SF=on, BF PN=blink, LINK=off, the dominant root cause is almost always Layer 1 — either the wrong cable type (straight-through where crossover is required, or vice versa) or a dead link partner port.

4. Root Cause Identification

Two failure layers must be ruled out sequentially:

1. Layer 1 — Physical: cable type, port health, link LEDs at both ends.
2. Layer 3 — Configuration: IP address assignment, subnet mask, gateway, PROFINET IO Device role.

For the SCALANCE X208 + CP 343-1 Lean + WinCC PC topology the typical IP plan is:

All three addresses must reside in the same /24 subnet. If a single node uses a different mask, ARP resolution will fail and the CP will remain invisible to ping and arping.

4.1 Cable Topology Decision

The CP 343-1 Lean ships with a single RJ45 port and does not support Auto-MDI/MDIX on every firmware revision. SCALANCE X208 ports do support Auto-MDI/MDIX. Therefore:

• PC → SCALANCE X208: use a straight-through (patch) cable.
• PC → CP 343-1 Lean (direct, no switch): use a crossover cable if the CP does not negotiate MDIX on the given firmware. Carry one crossover cable in the commissioning kit.
• CP 343-1 Lean → SCALANCE X208: a straight-through cable is normally sufficient because the switch handles MDIX.

5. Step-by-Step Recovery Procedure

5.1 Prerequisites

• STEP 7 V5.5 with NCM S7 installed (part of the SIMATIC Manager DVD).
• Primary Setup Tool (PST) installed from the SIMATIC NET DVD or downloaded from Siemens Industry Online Support.
• PG/PC interface set to S7ONLINE → TCP/IP → (fixed Ethernet adapter). Do not use TCP/IP (Auto) when first provisioning — Auto-Negotiation issues are a documented source of discovery failures on CP 343-1 Lean.
• One crossover cable and one straight-through cable verified with a tester.

5.2 Procedure — Sequenced Actions

1. Power down the S7-300 station. Connect the CP 343-1 Lean directly to the engineering PC using a crossover cable. Confirm the CP LINK LED lights.
2. Set the PG/PC interface to S7ONLINE → TCP/IP → (your NIC, fixed). Disable any Wi-Fi NIC or virtual adapters.
3. Open SIMATIC Manager and choose PLC → Edit Ethernet Node. Click Browse. The MAC address of the CP should appear.
4. If Browse succeeds, assign IP 10.0.0.41, subnet 255.255.255.0, and confirm. The CP will store the IP in its non-volatile memory (or in the optional C-PLUG if fitted).
5. If Browse fails, repeat the procedure with the Primary Setup Tool. PST and STEP 7 cannot probe the network simultaneously on the same PC — close one before opening the other.
6. From a Windows command prompt on the engineering PC, execute ping 10.0.0.41 -t and confirm echo replies.
7. Power down again. Connect the CP 343-1 Lean to a free port on the SCALANCE X208 with a straight-through cable. Connect the engineering PC to another SCALANCE port, also straight-through. Restore power.
8. Open STEP 7 HW Config, add the SCALANCE X208 only if PROFINET IO functionality is required (see Section 7). Otherwise the switch may remain out of the project.
9. Compile and download the HW Config to the CPU. STEP 7 will write the system data blocks (SDB) for the CP.
10. Re-open NCM Diagnostics for the CP. Confirm:
    • SF LED → off
    • BF PN LED → off (unless a PROFINET IO controller is expected)
    • Send/Recv connection → configured
    • System connection → running

6. IP Assignment via STEP 7 "Edit Ethernet Node"

The Edit Ethernet Node function is the only out-of-the-box method to assign the first IP address to a CP 343-1 Lean without an existing project. It operates at Layer 2 by sending a Siemens-specific DCP (Discovery and Configuration Protocol) multicast.

6.1 Command Sequence

1. SIMATIC Manager → PLC → Edit Ethernet Node.
2. Click Browse. The tool will populate any reachable CP / SCALANCE / PN device in the broadcast domain.
3. Select the MAC of the CP 343-1 Lean. Click Assign IP Address.
4. Enter 10.0.0.41 and subnet mask 255.255.255.0. Leave the router field blank for a single-subnet plant.
5. Optionally tick Assign PROFINET device name if the CP is also configured as a PROFINET IO Device.
6. Confirm with Assign. The CP will reboot its communication stack and apply the new IP.

7. SCALANCE X208 Integration in HW Config

A common misunderstanding is that the SCALANCE X208 must appear in HW Config for every TCP-based application. In fact:

To insert the switch for PROFINET: in HW Catalog → PROFINET IO → Network Components → SCALANCE X-200. Drag the X208 onto the PROFINET IO System line and assign an IP (here 10.0.0.10) plus a PROFINET device name.

8. SDB Container, C-PLUG, and Persistent IP Storage

After an initial misconfiguration, the CP can retain stale parameters in two locations:

• SDB container on the CPU (system data blocks generated by HW Config).
• C-PLUG (removable configuration memory) inserted in the CP 343-1 Lean.

If a previous configuration existed and the current download produces persistent faults, the standard recovery sequence is:

1. Open SIMATIC Manager → PLC → Delete SDB Container (or use the Accessible Nodes → right-click → Delete SDBs).
2. Power cycle the station.
3. Open HW Config, recompile, and download again.
4. If a C-PLUG is fitted, remove it, power-cycle, reinsert, and re-download to force a fresh initialization.

9. PROFINET IO Device Mode Considerations

The CP 343-1 Lean (6GK7 343-1EX21-0XE0) supports PROFINET IO Device mode. If the property "Obtain IP address from PROFINET IO Controller" is enabled in HW Config and no PROFINET IO Controller is reachable, the CP will not acquire its IP and the BF PN LED will blink. Two remediation paths:

1. Disable "Obtain IP address from PROFINET IO Controller" and assign the IP via STEP 7 Edit Ethernet Node.
2. Connect the CP to an active PROFINET IO Controller that will assign the IP via DCP at startup.

For pure WinCC TCP/IP communication without a PROFINET controller, option 1 is the recommended approach.

10. Verification Checklist

11. Troubleshooting Matrix

12. Field-Proven Tips

• Always carry one crossover cable in the commissioning kit. CP 343-1 Lean firmware V1.x does not always negotiate MDIX reliably with consumer NICs.
• Use a static IP for the engineering PC — avoid DHCP on commissioning day.
• Set the SCALANCE X208 management IP before connecting the CP. This eliminates the switch as a fault candidate.
• When the BF PN LED keeps blinking after the link is up, check whether PROFINET IO Device mode was enabled by accident in HW Config. If only TCP/IP is required, leave the PROFINET checkbox off.
• After replacing the C-PLUG, power-cycle twice — the first boot re-initializes the C-PLUG, the second reads the freshly downloaded configuration.

FAQ