Resolving CP 343-1 Lean SF LED Fault in TIA Portal V13

1. Problem Overview

A recurring commissioning fault involves a Siemens S7-300 station consisting of a CPU 315-2DP, a CP 343-1 Lean communications processor, and a TP 1900 Comfort panel. When the project is built and downloaded using TIA Portal V13, the CP 343-1 Lean stops with a continuously illuminated SF (System Fault) LED. The same hardware and identical application logic operate correctly when configured in STEP 7 V5.5, which narrows the fault to project generation, port assignment, or PROFINET interface configuration differences introduced by TIA Portal.

This article walks through the diagnostic procedure, identifies the most probable root causes (PROFINET subnet binding, missing error OBs, and HMI connection parameters), and provides a verified remediation path. It is written for commissioning engineers who need to bring an existing STEP 7 V5.5 S7-300 project up in TIA Portal V13 without losing runtime behavior.

2. Affected System Configuration

The CP 343-1 Lean provides an industrial Ethernet interface for programming, S7 communication, S7 routing, and PROFINET IO device or PROFINET IO controller operation depending on the variant. The Lean variant (6GK7343-1CX10-0XE0) supports PROFINET IO Device mode and basic open IE communication; it does not replace the full CP 343-1 (6GK7343-1EX30-0XE0) featureset.

For official device documentation, refer to the Siemens Configure CP 343-1 as a PROFINET I device in STEP 7 (TIA Portal) entry ID 109744406, which contains canonical configuration sequences and verification screenshots that match the fix paths described in this article.

3. CP 343-1 Lean LED State Interpretation

Before assuming a hardware fault, decode the CP front-panel LED pattern. The CP 343-1 Lean ships with the following status indicators:

4. Read the S7-300 Diagnostic Buffer

The PLC diagnostic buffer is the fastest way to translate the SF LED into an actionable error code. Open the online connection in TIA Portal and navigate to:

1. Project tree → Devices & networks → select the S7-300 station.
2. Right-click the CPU 315-2DP → Online & Diagnostics.
3. Open the Diagnostics buffer tab.
4. Sort by time (most recent first) and read every entry whose event ID maps to a CP-related cause.

Typical diagnostic buffer entries for the symptom reported in this article:

5. Root Cause Analysis

When a STEP 7 V5.5 configuration works but the equivalent TIA Portal V13 project fails with SF on the CP 343-1 Lean, the failure is almost always one of three causes, listed in order of frequency:

5.1 PROFINET Interface Not Networked

In STEP 7 V5.5, the CP's Ethernet port was implicitly associated with the project's Ethernet subnet through the NetPro editor. In TIA Portal V13, the equivalent binding is explicit: the CP 343-1 Lean PROFINET interface must be assigned to a PN/IE subnet object in the Devices & networks view. If the assignment is missing, the CP downloads an empty interface configuration, fails to come up, and asserts SF.

Remediation: open Devices & networks, select the CP 343-1 Lean, and on the Properties → PROFINET interface tab, set Interface networked with to the project's PN/IE subnet. Recompile and download the station.

5.2 Missing Error OBs (OB 82, OB 83, OB 85, OB 86, OB 100, OB 121, OB 122)

When the CP raises a diagnostic interrupt and no corresponding error OB exists in the S7-300 program, the CPU transitions to STOP on certain events and the CP remains in startup with SF lit. The classic STEP 7 V5.5 project often contained only OB 1 plus a small number of user OBs. The TIA Portal V13 generated project may inherit the same OB set, leaving a gap on startup.

Remediation: add at minimum OB 82 (diagnostic interrupt), OB 85 (program execution error), OB 86 (rack/IO failure), and OB 122 (I/O access error) to the S7 program. Download the blocks to the CPU.

5.3 Connection Configuration Mismatch on the HMI Side

The TP 1900 Comfort HMI connection target was set to the CPU 315-2DP's MPI/PROFIBUS interface in the original STEP 7 V5.5 project. After migration to TIA Portal V13, the connection may default to the CP 343-1 Lean's PROFINET interface IP. If the CP is not yet operational (because of cause 5.1), the HMI cannot establish the connection, the CP logs additional communication faults, and the SF LED stays on.

Remediation: in the TP 1900 Comfort connection list, set the access point to the correct PLC interface (either the CPU's integrated PN interface if present, or the CP 343-1 Lean) and verify the IP/Subnet/TSAP parameters.

6. Solution: Re-bind the PROFINET Subnet

This is the single most common fix for the reported symptom. Follow the procedure below in TIA Portal V13.

6.1 Prerequisites

• TIA Portal V13 SP1 (or later SP compatible with the firmware of CP 343-1 Lean 6GK7343-1CX10-0XE0).
• Online connection to the S7-300 station (Ethernet via the CP 343-1 Lean, or MPI/PROFIBUS via the CPU 315-2DP X2 interface).
• Project file (.ap13) open in TIA Portal.

6.2 Procedure

1. Open the project and switch to Devices & networks.
2. Click the CP 343-1 Lean in the device view.
3. Open the Properties inspector → PROFINET interface [X1].
4. In the dropdown Interface networked with, select the existing PN/IE subnet. If no subnet exists, click Add new subnet and assign a name and IP range consistent with the rest of the plant (for example 192.168.0.0/24, CP IP 192.168.0.10).
5. Confirm that IP address, Subnet mask, and (if used) Router address are populated.
6. If the CP is operated as a PROFINET IO Device, expand Operating mode and select IO device. Set the PROFINET device name to a value that matches the name assigned by the IO Controller (the controller's PROFINET name table).
7. Compile the project (Project tree → Station → Compile).
8. Download the hardware configuration only to the station (Online → Download to device → Hardware configuration).
9. Monitor the CP LEDs: SF should clear within 5 to 10 seconds of RUN.

7. Solution: Add Error OBs to the S7 Program

Even after the subnet is bound, residual diagnostic interrupts from the CP during startup will place the CPU in STOP if no error OB is present. Insert the following blocks into the S7-300 program:

7.1 OB 82 — Diagnostic Interrupt

// OB82 - Diagnostic Interrupt
// Triggered when the CP 343-1 Lean raises or clears a diagnostic interrupt.
// Empty body is sufficient; presence prevents CPU STOP.

7.2 OB 85 — Program Execution Error

// OB85 - Priority class error (e.g. OB not loaded)
// Must exist before the CP startup sequence to avoid CPU STOP
// when the CP signals a transient error during startup.

7.3 OB 86 — Rack Failure

// OB86 - Failure of a rack, DP master system, or PROFINET IO system
// Critical when CP 343-1 Lean is configured as a PROFINET IO Device
// and the controller is briefly absent during bring-up.

7.4 OB 122 — I/O Access Error

// OB122 - Error when accessing an I/O address that is faulty or absent
// Prevents CPU STOP when the CP process image is updated during a fault.

7.5 OB 100 / OB 101 — Warm/Cold Restart

Insert OB 100 (warm restart) or OB 101 (cold restart) if the project requires controlled startup actions. Without these OBs the CPU executes the default restart path.

7.6 Download and Verify

1. Compile the S7 program.
2. Download all blocks to the CPU.
3. Switch the CPU from STOP to RUN.
4. Confirm that the CP 343-1 Lean RUN LED is on and SF is off.

8. Solution: Verify HMI Connection Parameters

The TP 1900 Comfort must reach the correct PLC interface. Open the TP 1900 Comfort connection editor in TIA Portal:

1. Project tree → TP 1900 Comfort → Connections.
2. Select the S7 connection to the S7-300 station.
3. Confirm the access point is set to S7ONLINE and the partner is the CPU 315-2DP (not the CP 343-1 Lean).
4. Set Interface to PROFINET for IP-based communication, or PROFIBUS/MPI if wiring is via the X2 connector.
5. Enter the partner IP address (CPU or CP depending on routing), subnet mask, and verify TSAPs.
6. Recompile the HMI image and download.

9. TIA Portal V13 Project Migration Notes

When migrating a STEP 7 V5.5 project to TIA Portal V13 using the Migrate project wizard, the following CP 343-1 Lean settings must be re-checked manually:

• Subnet binding: V5.5 NetPro assignments do not always survive migration. Re-bind every CP Ethernet port.
• Connection list: V5.5 "S7 connection" properties may lose the partner TSAP. Re-validate TSAPs after migration.
• CP firmware descriptor: V13 SP1 ships with newer device descriptors; the CP may report a firmware update prompt. Defer unless the firmware on the physical CP matches the new descriptor.
• Error OBs: Migration sometimes leaves OBs in the Sources folder but not compiled into the Blocks container. Re-compile the S7 program explicitly.

10. Verification Procedure

After applying the fixes, validate end-to-end using the following checklist.

11. Diagnostic Decision Matrix

12. Prevention and Best Practices

• Always bind PROFINET subnets explicitly in TIA Portal. Do not assume migration preserves NetPro bindings.
• Use a project template that pre-includes OB 82, OB 85, OB 86, OB 100, OB 121, OB 122 for every S7-300 station. This avoids the "missing OB during CP startup" failure mode entirely.
• Pin the TIA Portal version across the engineering team. TIA Portal V13, V13 SP1, and V14 generate different system data blocks for the CP 343-1 Lean; mixed versions can produce SF on download.
• Read the diagnostic buffer before every code change. The buffer contains event IDs that map directly to the cause table in section 4 above.
• Document the PROFINET device name assigned to the CP 343-1 Lean if it acts as a PROFINET IO Device. The name must match the controller's PROFINET name table exactly, including case.
• Keep firmware and project descriptor in sync. When the CP firmware is upgraded, update the TIA Portal hardware catalog to the matching HSP or install the latest Service Pack.

13. Frequently Asked Questions