Resolving Siemens CP 342-5 V5.2 SF and BUSF LED Faults

Problem Overview

During commissioning of a SIMATIC S7-300 station that combines a CPU 314-IFM (6ES7 314-5AE03-0AB0), a CP 342-5 (6GK7 342-5DA02-0XE0) and an ET 200S distributed I/O station (IM 151-1 Standard), the CP's red SF (Group Fault) and BUSF (Bus Fault) indicators may light simultaneously. Communication with the DP slave never reaches cyclic exchange, and online diagnostics from STEP 7 V5.1 SP4 show the CP module in error state.

This fault pattern typically appears in one of three configurations:

• The hardware catalog in STEP 7 lists a CP 342-5 firmware revision that does not match the actual module (for example, the catalog shows V5.0 while the physical module is labeled V5.2).
• The DP master system was assembled incorrectly in HW Config, or the ET 200S slave was inserted from the wrong catalog path.
• The CP 342-5 internal firmware requires an update, but the selected loading path is not supported by the connected PC adapter.

The diagnostic value of the SF and BUSF indicators, the appropriate STEP 7 hardware configuration, the official Siemens firmware delivery release, and the safe loading of replacement firmware are covered in the sections that follow.

Affected Hardware Components

Verify the article numbers (MLFB) of every component in the rack before applying any firmware or diagnostic procedure. The following components are referenced throughout this document.

CP 342-5 LED Status Reference

The CP 342-5 front panel provides three primary indicators. Use the table below to interpret the LED combination that is observed at power-up and during operation.

When both SF and BUSF are lit while RUN remains dark, the CP has detected a fault that prevents it from entering DP-V0/V1 cyclic exchange. The combination is independent of the firmware catalog version installed in HW Config.

Root Cause Analysis

The pattern described in the source case has three independent root-cause layers. Resolve them in the order shown, because the lower items can mask the upper items in the CP's diagnostic buffer.

1. Configuration Mismatch in HW Config

STEP 7 V5.1 SP4 catalog entries for the CP 342-5 typically reach revision V5.0. A physically-installed module labeled V5.2 still operates correctly when the slot is configured as V5.0 because Siemens ships each catalog version with backward-compatible firmware. The mismatch is recorded only as an informational diagnostic entry and does not, by itself, drive the SF/BUSF LEDs.

However, if the slot is left completely unconfigured, or the wrong module type is dragged from the catalog (for example a CP 343-1 Ethernet instead of CP 342-5 PROFIBUS), the CP powers up with no valid configuration and reports a parameterization error. This raises SF.

2. DP Slave Not Found on the Bus

The most common cause of BUSF after SF is a DP slave that is either physically absent, has the wrong PROFIBUS address set on its DIL switch, or has been inserted from the wrong catalog node. Confirm that the ET 200S IM 151-1 is dragged from PROFIBUS-DP > ET 200S in HW Config, that the station address matches the DIL switch on the IM 151-1, and that the bus terminator is present at both ends of the segment.

3. Firmware Incompatibility

If the CP has been flashed with a corrupted image or a beta firmware that prevents STOP-to-RUN transition, the SF LED remains on even after the DP master has been correctly configured. In this case the only recovery path is a clean firmware reload. The official Siemens delivery release for V5.2 of the CP 342-5 / CP 342-5 FO contains only error corrections and introduces no new functions; subsequent revisions such as V5.7 are also released for the same hardware.

Firmware V5.2 Specifications

Firmware V5.2 for the CP 342-5 (6GK7 342-5DA02-0XE0) and CP 342-5 FO (6GK7 342-5DF00-0XE0) was published as an error-correction release. The official Siemens support page states explicitly that V5.2 contains no new functions compared to its predecessor, only bug fixes.

Because V5.2 introduces no functional change, the visible behavior of the CP on the bus is identical to V5.0 when the module is correctly configured. The release exists to address specific known bugs that may surface as intermittent DP failures or DP watchdog timeouts.

Diagnostic Procedures

Before changing firmware or hardware, capture the CP's diagnostic buffer and the bus topology. Three diagnostic entry points are available from STEP 7 with the PG connected to the MPI/DP port of the CPU 314-IFM.

2.1 CP Diagnostic Buffer

1. In HW Config, right-click the CP 342-5 slot and select PLC > Module Information.
2. Open the Diagnostic Buffer tab and scroll to the most recent entries. The text field at the bottom of the dialog decodes each entry; double-click an entry for the full OB1 / OB82 / OB86 context.
3. Note any entry containing the strings "Parameter error", "Station failure", "DP slave not found", or "Firmware".

2.2 DP Slave Diagnostic Status

1. Right-click the DP master system line in HW Config and choose PROFIBUS > DP Slave Diagnostics.
2. Read the station status byte 1-3 of each slave. A non-zero status byte 3 indicates a slave-side diagnostic pending.
3. Open the IM 151-1 standard slots and inspect module status. A greyed-out slot means the configured module type is not present in the physical rack.

2.3 DP_SEND / DP_RECV Inspection

If your PLC program uses FC1 DP_SEND and FC2 DP_RECV on the CP 342-5, evaluate the status word STATUS returned by each call. The status word maps the CP's bus and parameter state to a 16-bit code. Typical patterns include:

The exact set of codes returned by the CP 342-5 is documented in the CP 342-5 manual, "Function Blocks for the CP 342-5" section. Use the table above as a triage; the manual is the authoritative reference for production code.

STEP 7 Hardware Configuration

Regardless of the firmware label printed on the CP, the configuration in HW Config determines the parameter set the CP will accept. The following sequence eliminates configuration errors as a source of SF.

3.1 Rack Assembly

1. Open the SIMATIC Manager and the S7 station that contains the CPU 314-IFM.
2. Open HW Config. Slot 1 must contain a PS 305/307 (or PS 300 Standard), slot 2 the CPU 314-IFM (6ES7 314-5AE03-0AB0), and slot 4 the CP 342-5 (6GK7 342-5DA02-0XE0) from SIMATIC 300 > CP 300 > PROFIBUS.
3. Right-click the DP port of the CP 342-5 and choose Add Master System if no master system exists. This creates the PROFIBUS(1) subnet.

3.2 DP Slave Insertion

1. From the hardware catalog expand PROFIBUS-DP > ET 200S.
2. Select the IM 151-1 Standard that matches the physical module. Confirm the order number on the catalog entry matches the label on the IM 151-1.
3. Drag the IM 151-1 onto the DP master system line. Set the PROFIBUS address to the value set on the DIL switch of the physical IM 151-1 (typically 3 for the first slave).
4. Insert the required power and electronic modules into the ET 200S slots in the same order as the physical rack.

3.3 Compile and Download

1. Save and compile the HW Config. The compiler will warn if a slot is configured but empty, or if a module exists in the rack that has no slot in HW Config.
2. Download the HW Config to the CPU. The CP 342-5 will receive its parameter set during the next STOP-to-RUN transition of the CPU.

If the SF and BUSF LEDs clear after this step, the original cause was a configuration or addressing error rather than a firmware problem. If both LEDs remain on, proceed with the firmware diagnostic below.

Firmware Update Procedure

The CP 342-5 firmware can be updated in the field. The Siemens delivery package contains the firmware file (typically *.UPD) and a wizard that automates the download. The CP must be reachable from the PG over a path that supports firmware loading.

4.1 Selecting the Correct Loading Hardware

The CP 342-5 firmware is loaded through the CP's PROFIBUS interface. The PG side of the link must therefore be a PROFIBUS master that exposes the firmware-load (FWL) service. The following Siemens devices support the FWL service on PROFIBUS:

4.2 Wizard Operation

1. Launch the firmware update wizard from the STEP 7 Start menu (Siemens Automation > Firmware Update) or from the support package provided by Siemens.
2. Select the CP 342-5 from the discovered modules list. The wizard identifies the module by its PROFIBUS address and current firmware revision.
3. Choose the firmware file matching the target revision (e.g., CP3425_V5_7.UPD for V5.7).
4. Confirm the operation. The CP enters firmware-load mode (RUN and STOP LEDs blink alternately), erases the old image, and programs the new image.
5. Wait for the wizard to report Update successful. Power-cycle the rack only if instructed by the wizard.

PC Adapter and PROFIBUS Pin 7 Clarification

A second question that often arises during CP 342-5 commissioning is whether pin 7 of the PROFIBUS DP connector must supply 24 V DC. The answer depends entirely on which PROFIBUS interface is being used.

The CP 342-5 PROFIBUS port follows the standard 9-pin D-sub pinout defined by IEC 61158 / EN 50170 for PROFIBUS DP. In this pinout, pin 7 is reserved and is not connected to the internal 24 V DC rail. Measuring 0 V DC on pin 7 of the CP 342-5 DP socket is therefore normal and does not indicate damage.

If the PC Adapter is connected directly to the CP 342-5 PROFIBUS port rather than to the CPU MPI/DP port, the absence of 24 V on pin 7 means that any termination device that depends on this supply (for example, an active terminator with a 24 V power LED) will not light. Use the CPU MPI/DP port for PC Adapter connection unless the segment has its own 24 V termination.

Verification and Commissioning

Once the configuration is corrected and any firmware update is complete, run the following verification sequence.

1. Power up the rack and observe the CP 342-5 LEDs. Expected steady-state: RUN on, SF off, BUSF off, STOP off.
2. In SIMATIC Manager, right-click the CP 342-5 and open Module Information > Diagnostic Buffer. The last entry should be Module startup OK with no parameterization or bus entries following it.
3. Open the PROFIBUS > DP Slave Diagnostics view and confirm the ET 200S IM 151-1 reports station status byte 1 = 0x00, status byte 2 = 0x00, status byte 3 = 0x00.
4. Force a single bit in the DP output area from the CPU and read it back through the DP input area. Use the Monitor/Modify view in HW Config for a fast check.
5. Cycle the CPU power and confirm the CP 342-5 returns to RUN within 5 seconds.

If the SF/BUSF condition reappears after cycling, the most likely remaining causes are a loose DP connector, an incorrect PROFIBUS address on the IM 151-1, or a corrupted GSD file in STEP 7. Reinstall the GSD file from the ET 200S support package and re-compile HW Config.

Preventive Recommendations

• Document the CP 342-5 firmware revision and the STEP 7 catalog version on the cabinet label. Future service technicians will know which firmware is installed without opening the rack.
• Keep the CP 5611 / CP 5621 / CP 5622 PROFIBUS master board in the maintenance kit. A PC Adapter alone is not sufficient for firmware service on PROFIBUS CPs.
• When upgrading STEP 7, also refresh the CP 342-5 GSD / HSP entries before going online. Newer HSP packages add bug fixes that can eliminate intermittent SF events.
• Always power the CP 342-5 from the same PS 305/307 that feeds the CPU. Undervoltage at the backplane connector can trigger spurious SF entries.
• Mark the firmware-load-capable PG ports in the maintenance documentation. Mistakenly using a PC Adapter port for firmware update is the single most common cause of a failed firmware attempt.

Frequently Asked Questions