Resolving CP342-5 BF LED with S7-400H Y-Link DP Slave

Resolving CP342-5 BF LED with S7-400H Y-Link DP Slave Configuration

A Siemens CP 342-5 (firmware V5.7.3) configured as a PROFIBUS-DP slave behind a Y-Link coupler that is in turn connected to a redundant S7-400H DP master will run user data exchange without complaint while still showing a permanent red BF (Bus Fault) LED on the CP front panel. Intermittent EX fault and bus-fault flags appear on the Y-Link slave address (typically PROFIBUS station 17). User program blocks DP_SEND and DP_RECV return no error and the S7-300 CPU continues to operate, which masks the actual cause. The root cause is a single STEP 7 hardware-configuration checkbox on the slave side: Test, Commissioning, Repeating. Leaving this option enabled in the DP-slave properties of the CP 342-5 forces the module to interpret every cyclic frame from the Y-Link's lower DP segment as a token/test sequence, generating spurious collision diagnostics and holding the BF LED set. Deselecting the option clears the fault, drops the BF LED, and stabilizes the segment. This reference details the exact topology, symptom matrix, the underlying mechanism, the resolution procedure, the verification checks, and the related pitfalls you should avoid on similar H-system/Y-Link/CPU-312C installations.

Affected System Topology

The configuration in question is a three-tier PROFIBUS-DP tree with a redundant master at the top, a Y-Link in the middle, and a CP 342-5 as a non-redundant slave at the bottom. The Y-Link is mandatory because the CP 342-5 is not H-system capable and cannot be plugged into the redundant PROFIBUS DP master system of the S7-400H directly. The Y-Link terminates the H-system's lower PROFIBUS segment and exposes it as two physically separate PROFIBUS-DP segments (DP2 and DP3) on its lower side. Each of these segments is itself a master system for the lower-tier slaves.

The figure above shows the canonical topology. The S7-400H hosts a PROFIBUS-DP master system. The Y-Link appears on that master system as a standard DP slave at station address 17 (the Y-Link's own address). On its lower side, the Y-Link exposes a non-redundant PROFIBUS-DP segment (DP2) which acts as a separate master system for the CP 342-5. The CP 342-5 is configured as a DP slave on DP2.

The relevant Siemens components in this configuration are:

• CPU 417-4H (or any H-CPU with PROFIBUS-DP master capability), configured in HW Config as PROFIBUS-DP master on segment 1.
• Y-Link coupler, e.g. 6ES7 157-1AA00-0AB0 (IM 157) or successor 6ES7 157-1AA01-0AB0. Configured as DP slave on the H-system's master system (default station address 17).
• CP 342-5 communications processor, MLFB 6GK7 342-5DA02-0XE0 or 6GK7 342-5DA03-0XE0, firmware V5.7.3, plugged into the S7-300 rack and configured as DP slave on the Y-Link's lower segment.
• S7-300 CPU (e.g. CPU 312, 312C, 314, 315-2 DP, or 317-2) acting as the host CPU that calls DP_SEND / DP_RECV.

The Y-Link is required for two reasons: first, the S7-400H master system is a redundant PROFIBUS segment with H-CPU failover, and not all DP slaves are H-capable. The Y-Link acts as the bridge and terminates the redundancy on the slave side. Second, the Y-Link provides diagnostic and isolation functions: if a lower-tier slave fails, the Y-Link reports it back to the H-system without disrupting the redundant master segment.

Fault Symptoms and Indicators

When the Test, Commissioning, Repeating checkbox is enabled, the configuration exhibits a very specific symptom pattern that confuses even experienced commissioning engineers:

The most deceptive element of this fault is that user data exchange is functioning. Inputs are read, outputs are written, and the S7-300 program in the CPU 312 sees valid data. Therefore the bug is often dismissed as a cosmetic LED problem or as a Y-Link tuning issue, and engineers chase non-existent electrical, termination, or shielding problems on the PROFIBUS cable. None of those are the cause. The actual cause is purely a software/configuration flag.

The CPU 312's diagnostic buffer does not contain an entry about the BF LED because the LED is driven by the CP 342-5 firmware and is not propagated to the CPU's diagnostic buffer in this state. You must look at the CP's own diagnostic buffer, which is accessible through STEP 7's HW Config → CP 342-5 → Diagnostics tab or via SIMATIC Manager → Accessible Nodes.

Root Cause: The Test, Commissioning, Repeating Checkbox

The Test, Commissioning, Repeating checkbox lives on the DP slave properties dialog of the CP 342-5 inside STEP 7 HW Config. The path is:

1. Open the S7-300 station that contains the CP 342-5.
2. Double-click the CP 342-5 in the rack.
3. Switch to the DP Slave operating mode in the Operating Mode tab.
4. In the same dialog, find the section labeled Test, Commissioning, Repeating. The checkbox controls whether the CP shall respond to PROFIBUS-DP test, commissioning, and repeating telegrams that the master (or the Y-Link acting on behalf of the master) may broadcast.
5. The Operating Mode tab also has a DP-Slave checkbox just above this option. Both checkboxes must be set consistently: enable DP Slave mode and disable Test/Commissioning/Repeating for normal redundant operation.

This is a configuration flag on the slave side, not the master side. The Y-Link and the H-system are functioning correctly. The fault is in the CP 342-5's properties in the S7-300's HW Config, not in the master station.

Step-by-Step Resolution Procedure

The corrective action is a one-click configuration change followed by a download to the S7-300 station. Use the following procedure exactly as written.

Prerequisites

• STEP 7 V5.4 SP3 or later (V5.5 or V5.6 recommended for HSP support of newer CP 342-5 GSD revisions).
• Project file (.s7p) containing both the H-station and the S7-300 station with the CP 342-5.
• Online connection to the S7-300 (MPI, PROFIBUS, or Ethernet via CP 343-1 Lean) so the changed HW Config can be downloaded.
• The CP 342-5 must be in RUN, and the S7-300 CPU must be in RUN-P or STOP. Download to RUN is allowed for this configuration change.

Procedure

1. In SIMATIC Manager, open the S7 project and expand the S7-300 station that contains the CP 342-5.
2. Double-click Hardware to launch HW Config.
3. In the station window, double-click the CP 342-5 module slot to open its properties dialog.
4. Click the Operating Mode tab.
5. Confirm that DP slave is selected (radio button).
6. In the same tab, locate the Test, Commissioning, Repeating checkbox. Uncheck it.
7. Click OK to close the properties dialog.
8. Press Ctrl+S to save the HW Config, then Station → Save and Compile.
9. Download the compiled HW Config to the S7-300 station: PLC → Download to Target Station. Confirm the target module address.
10. After the download, the CP 342-5 performs a brief initialization cycle (typically 5 to 15 seconds). The BF LED on the CP should extinguish within one PROFIBUS cycle of the DP2 segment (typically under 100 ms for a 1.5 Mbps segment with a small slave count).
11. The Y-Link's EX fault and BF LEDs on slave 17 should also stop flashing within the same window.

Code: S7-300 DP_SEND/DP_RECV pattern (unchanged)

The user program blocks for the CP 342-5 do not change. For reference, the canonical call pattern in OB1 looks like this:

// CP 342-5 DP-Slave cyclic data exchange (FBD/ST)
// LADDR = configured I/O start address of the CP 342-5 (e.g. W#16#0100)

CALL  "DP_SEND"  // FC 50 for CP 342-5
     CPLADDR  := W#16#0100     // CP 342-5 base address
     SEND     := P#DB100.DBX0 BYTE 32   // 32 bytes of output data
     DONE     := M10.0          // 1 = send complete
     ERROR    := M10.1          // 1 = error
     STATUS   := MW12           // status word, 0000hex = OK
     ;

CALL  "DP_RECV"  // FC 49 for CP 342-5
     CPLADDR  := W#16#0100     // CP 342-5 base address
     RECV     := P#DB101.DBX0 BYTE 32   // 32 bytes of input data
     NDR      := M11.0          // 1 = new data received
     ERROR    := M11.1          // 1 = error
     STATUS   := MW14           // status word, 0000hex = OK
     ;

If STATUS returns 0000_hex from both calls, the CP 342-5 is healthy. The BF LED alone is not a sign of a data-plane fault; it is a link-layer status indicator and can be cleared by configuration as described above.

Verification After Configuration Change

After deselecting the Test, Commissioning, Repeating checkbox and downloading, perform the following checks before declaring the fault closed.

LED verification

STEP 7 online diagnostic verification

1. In HW Config, right-click the CP 342-5 → Diagnostics → Module Information.
2. In the Diagnostic Buffer tab, confirm no new entries after the configuration download timestamp.
3. Switch to the DP Slave Diagnostics view. Confirm station status byte 1 = 00_hex (no fault) and station status byte 2 = 0C_hex (slave present, ready for data exchange, watchdog active).
4. For the Y-Link (slave 17), confirm the same DP-slave diagnostic pattern.

Long-tail stability check

Leave the segment running for at least one hour and re-read the diagnostic counters. The NPC collision counter on the CP 342-5 should remain at zero, and the Y-Link should report zero faults. If the BF returns, the most likely secondary cause is cable reflection due to missing or duplicated termination, or a duplicate PROFIBUS station address on DP2. Use an oscilloscope on the PROFIBUS segment to confirm clean 5 V differential signaling.

CP342-5 DP Slave Configuration Parameters

The full DP-slave property dialog of the CP 342-5 has a number of parameters that interact with the Test, Commissioning, Repeating option. The table below summarizes the parameters and their recommended values for this configuration.

Y-Link Coupler Configuration Notes

The Y-Link (IM 157, MLFB 6ES7 157-1AA00-0AB0 or successor) has its own configuration pitfalls that interact with the CP 342-5 fault pattern. The most common false leads when chasing the BF LED are:

• Y-Link station address: The Y-Link must be assigned a unique PROFIBUS address on the H-system master segment. The factory default is 17, but if you have multiple Y-Links on the same H-system, all must be unique. Duplicate addresses on the H-system side surface as intermittent EX faults on the Y-Link and are independent of the CP 342-5 checkbox issue.
• Lower segment termination: The Y-Link's lower DP segment (DP2) requires PROFIBUS termination at both ends. The Y-Link itself is one end, and the last physical slave on the bus is the other end. Missing termination causes reflections that the CP 342-5 reports as collisions (NPC counter). This is a separate issue from the checkbox but can mask it.
• Y-Link diagnostic mapping: In HW Config on the H-station side, expand the Y-Link and confirm the lower-segment slaves are mapped correctly. The Y-Link presents itself as a slave with one slot per attached lower slave. If the CP 342-5 mapping is missing, the H-system reports slot fault rather than the BF LED pattern.
• Y-Link operating mode: The Y-Link must be set to DP Slave mode with DP Master below. If accidentally configured as DP Slave without master, all lower slaves will be invisible. This is also distinct from the BF LED symptom.

Diagnostic Tools and Buffer Interpretation

The CP 342-5 and the Y-Link each maintain their own diagnostic buffer. The buffers are independent; an entry on one does not automatically surface in the other.

CP 342-5 diagnostic buffer entry types

When the checkbox is enabled, the typical sequence in the CP 342-5 buffer is:

1. 0xA125 NPC collision
2. 0xA126 Diagnostic echo received
3. 0xA124 Bus fault (subsequent, secondary)

After deselecting the checkbox, the buffer should no longer accumulate new entries of types 0xA125 or 0xA126.

Y-Link diagnostic buffer entry types

For the Y-Link, the relevant events are Slave exists change, Slave diagnostic, and Bus fault on lower segment. The Y-Link's diagnostic buffer can be read in HW Config by right-clicking the Y-Link and selecting Module Information → Diagnostic Buffer.

NPC counter readout

The NPC (Network Protocol Counter) collision counter is accessible via:

• STEP 7 HW Config → CP 342-5 → Diagnostics tab → Statistics sub-tab.
• SFC 51 / SZL read of partial list W#16#0132 index W#16#0001 (PROFIBUS diagnostic statistics) — not all CP 342-5 firmware versions support all SZL indices, so the HW Config path is the most portable.

Related Configuration Pitfalls to Avoid

Beyond the Test, Commissioning, Repeating checkbox, several other configuration parameters on the same CP 342-5 property dialog cause similar symptoms. Use the following matrix as a checklist whenever a CP 342-5 reports BF in a Y-Link topology.

Field-Commissioning Checklist

Use this checklist on every new S7-400H → Y-Link → CP 342-5 deployment to avoid returning to site for the same fault.

1. HW Config on H-station: Confirm CPU 417-4H PROFIBUS master system is configured with the correct baud rate (typically 1.5 Mbps).
2. Y-Link in H-station: Drag the IM 157 from the catalog into the H-master system. Confirm its PROFIBUS address (default 17, change if conflict). Confirm lower-segment DP master is enabled and shows up in the Y-Link's slot view.
3. CP 342-5 in S7-300 station: Place the CP in the S7-300 rack, confirm PROFIBUS address is unique on DP2, and assign I/O lengths matching the H-station slot configuration.
4. CP 342-5 DP-Slave properties: Select DP Slave operating mode. Confirm Test, Commissioning, Repeating is unchecked. Save and compile.
5. Download S7-300 station: Download HW Config to the S7-300 CPU.
6. Download H-station: Download HW Config to the H-CPU(s) and confirm the Y-Link appears online with no SF.
7. LED verification: CP 342-5 BF off, RUN green; Y-Link BF off, EX off.
8. Online diagnostic: Open CP 342-5 module information, confirm diagnostic buffer is empty since download.
9. User data: Verify DP_SEND/DP_RECV RETVAL = 0000_hex and a test byte round-trips correctly between H-station and S7-300.
10. Stability observation: Leave the system running for at least 30 minutes, re-check LEDs and diagnostic buffers. No new entries should appear.

Why This Fault Is Easily Overlooked

The combination of data exchange OK + DP_SEND/DP_RECV no error + BF LED lit + Y-Link flashing points commissioning engineers in two wrong directions:

1. Cable / electrical hypothesis: Engineers check shield grounding, segment termination, baud rate, and cable length. None of these are the cause.
2. Firmware / hardware hypothesis: Engineers suspect a CP 342-5 firmware bug (V5.7.3 has had rare reported issues, none of which produce this exact LED pattern) and may attempt firmware downgrade. The firmware is fine.

The correct hypothesis is the Test, Commissioning, Repeating checkbox. Siemens added this option to support a specific test-and-commissioning workflow that is not compatible with redundant master operation. In redundant operation, the H-system's failover mechanism generates broadcast-style test frames during the H-CPU master switch-over, and the CP 342-5 with the checkbox enabled treats these as repeated test frames and enters diagnostic-echo mode.

Summary

The persistent red BF LED on a Siemens CP 342-5 (firmware V5.7.3) configured as a DP slave behind a Y-Link connected to an S7-400H master is caused by the Test, Commissioning, Repeating checkbox being enabled in the CP's DP-Slave properties in STEP 7 HW Config. Disabling the checkbox and re-downloading the configuration to the S7-300 station clears the BF LED, stops the Y-Link EX flashes, and eliminates the NPC collision diagnostics. No electrical or firmware changes are required. The Siemens Industry Online Support entry ID 19951154 is the authoritative reference for this configuration and explicitly identifies this checkbox as a critical parameter. Apply the commissioning checklist in this article on every new deployment to avoid the same fault recurring.

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