Resolving CP342-5 Bus Fault When Connected to YOKOGAWA ALP111

1. Problem Overview

Field engineers integrating a Siemens SIMATIC S7-300 station with a YOKOGAWA CENTUM VP / DCS subsystem routinely use the CP 342-5 communications processor in DP-Slave mode behind a YOKOGAWA ALP111 PROFIBUS-DP master module. The most common symptom at first power-up is a continuously flashing BF (Bus Fault) LED on the CP 342-5 with no diagnostic entry in the CPU diagnostic buffer, while the ALP111 reports a CABA alarm ("Illegal data / wrong type of data") on the Yokogawa side. Because the CPU has no entry of its own, the fault is purely on the CP 342-5 PROFIBUS interface and on the master's view of the slave GSD.

This reference documents the field-proven diagnostic path, the operating-mode and GSD pitfalls, the FC12 / FC13 programming required on the S7-300, and the parameter mapping that eliminates the CABA error. It is written for the engineer who is online with STEP 7 (HW Config) and has a Yokogawa engineering workstation with SyCon-PB / YOKO available.

2. Hardware and Software Reference Set

The YOKOGAWA ALP111 is documented in the manufacturer manual GS33K50G80-50E as a PROFIBUS-DP master module installed on the Field I/O (FIO) bus of a CENTUM VP / CS 3000 system. The manual states that the ALP111 exchanges cyclic I/O data with PROFIBUS-DP slave devices and that the master configuration, GSD assignment, and slot/offset definition are performed from the Yokogawa System View / Resource Configurator and from SyCon-PB.

3. Reading the LEDs Correctly

The CP 342-5 has four relevant indicators on the front bezel:

A flashing BF on the CP 342-5 specifically means the slave has detected the bus, has heard the master's parameterisation telegram, but has not received a valid Chk_Cfg / Set_Prm handshake. The CPU diagnostic buffer is typically empty because the CP 342-5 buffers the DP-slave diagnostics locally; you must open the CP-specific diagnostic buffer, not the CPU one.

4. Diagnostic Procedure (Step-by-Step)

1. Open SIMATIC Manager > PLC > Online > Accessible Nodes, double-click the CPU 312, and switch to the Diagnostics view.
2. Open HW Config and place it in Online > Online. The CP 342-5 will show a yellow diagnostic icon if the BF is latched.
3. Double-click the CP 342-5 in the online rack. The Module Information window shows the slave diagnostic, the Special Diagnostics tab, and the DP-Slave Diagnostics tab.
4. Read the DP-Slave Diagnostics bytes 7…10 — these carry the PROFIBUS Diag_Buffer flags that identify whether the master sent valid parameterisation.
5. Open CP 342-5 Properties > Operating Mode and verify that Test, Commissioning, Routing is OFF. With this option enabled the CP accepts only configuration from STEP 7 and rejects the ALP111 parameterisation, which is the single most common root cause of the flashing BF.
6. Capture the DP_SEND / DP_RECV return codes by reading DONE, ERROR, and STATUS outputs. If DONE = TRUE and ERROR = FALSE the CP is communicating with the S7 CPU, isolating the fault to the DP cable / master side.
7. On the Yokogawa side, open System View > IOM and inspect the CABA log to determine whether the alarm is raised on parameterisation (Set_Prm) or on the first Data_Exchange cycle.

5. Root Causes and Their Symptoms

6. CP 342-5 Configuration for Slave Operation

The CP 342-5 must be configured as a DP-Slave in HW Config, not as a DP-Master. Configure the slot as follows:

1. From the hardware catalog, drag CP 342-5 > DP-Slave into the S7-300 rack.
2. Open CP 342-5 Properties > PROFIBUS Interface and assign a unique PROFIBUS address (typical 2…32).
3. In Operating Mode, set DP-Slave and confirm Test, Commissioning, Routing is not checked. With the option checked the CP is forced into a test mode that does not accept third-party master parameterisation, which is the most common cause of the persistent BF flash described in the field incident.
4. In DP-Slave Properties define at least one input slot and one output slot. Field experience shows that a CP 342-5 configured with only outputs and no input word can be parameterised by an S7 master but rejected by a non-Siemens master until at least one input word is declared. Define a 2-byte input and 2-byte output slot to be safe.
5. Save and download the hardware configuration to the CPU.

7. FC12 (DP_SEND) and FC13 (DP_RECV) Programming

Once the slave is parameterised, the S7-300 application must use the SIMATIC function blocks FC12 DP_SEND and FC13 DP_RECV from the standard library "CP 342-5 / CP 343-5". They move data between a configured DB and the internal buffer of the CP 342-5.

7.1 Input / Output Interface of FC12 DP_SEND

7.2 OB1 Call Template (Structured Text equivalent)

// OB1 - cyclic call of FC12/FC13 for CP 342-5
CALL  FC   12  // DP_SEND
 CPLADDR  := W#16#100,    // base address of CP 342-5
 SEND     := P#DB100.DBX0.0 BYTE 2,
 DONE     := M 100.0,
 ERROR    := M 100.1,
 STATUS   := MW 102

CALL  FC   13  // DP_RECV
 CPLADDR  := W#16#100,
 RECV     := P#DB101.DBX0.0 BYTE 2,
 NDR      := M 104.0,
 ERROR    := M 104.1,
 STATUS   := MW 106

7.3 DP_SEND / DP_RECV Status Codes (subset)

8. ALP111 Master Configuration (Yokogawa Side)

According to the ALP111 manual GS33K50G80-50E, the module is a PROFIBUS-DP Class-1 master that the Yokogawa resource configurator presents as an I/O module occupying one slot on the FIO node. The configuration steps are:

1. From System View, drag the ALP111 into the FIO node and assign it to a logical node (e.g. NODE01 / SLOT05).
2. Open SyCon-PB / YOKO and add a new DP network. Import the CP 342-5 GSD (file SIEM814D.gsd for part number 6GK7342-5DA02-0XE0). If the GSD is missing, SyCon falls back to a generic DP-V0 profile which is the most common origin of the CABA "wrong data type" alarm.
3. Add the CP 342-5 (Siemens) slave, assign the PROFIBUS address matching HW Config, and define a single I/O block of 2 bytes input / 2 bytes output on slot 1 — these correspond to the FC12/FC13 transfer area.
4. Set the master to DP-V0 mode. The CP 342-5 supports DP-V0; if the ALP111 is configured for DP-V1 with slot-based acyclic services the slave will reject the parameterisation and re-trigger the BF flash.
5. From the Yokogawa System View IOM tab, register a data item pointing to the slave's input area. A typical entry: Area: INPUT, Station: 2, Offset: 0, Length: 16, Data type: Unsigned.
6. Download the configuration to the ALP111 and the FCS and reset the bus.

9. GSD Selection Matrix for CP 342-5 Variants

Always use the GSD file that matches the printed MLFB on the module. A different revision GSD can declare a different I/O length maximum or new diagnostic slots, which the ALP111 will treat as a parameterisation error.

10. Field-Confirmed Workaround When BF Persists

Engineers who have been unable to resolve the BF flash — for example, when the customer has not provided a recent GSD or when the ALP111 firmware predates DPV0 acyclic handling — have deployed the DP/DP Coupler (6GK1571-0BA00-0AA0) as a buffer. The DP/DP coupler acts as a slave to the ALP111 and as a master to the S7-300's CP 342-5 (in master mode). It is configured with two independent DP segments and a fixed input/output length.

1. Configure the left segment of the DP/DP coupler as a slave to the ALP111 with a fixed 16 byte in / 16 byte out data block.
2. Configure the right segment of the DP/DP coupler as a master with the CP 342-5 in slave mode and 2 byte in / 2 byte out.
3. Wire the coupler to map its left input area to its right output area, and its left output area to its right input area, using the coupler's internal mapping table.

The DP/DP coupler isolates the protocol idiosyncrasies of each segment and converts the CABA alarm into a clearly mapped status byte, which is usually accepted by the Yokogawa side. The trade-off is an additional 90 mA load on the S7-300 backplane and a small bus-cycle delay (typically 1.5 ms at 1.5 Mbps).

11. Verification and Commissioning Checks

Once the BF LED goes off and the Yokogawa CABA alarm clears, run the following verification sequence:

1. LED pattern: CP 342-5 RUN = solid, BF = off, SF = off, STOP = off. ALP111 "READY" LED = solid, "COMM" LED = solid or flashing 1 Hz depending on vendor revision.
2. STEP 7 side: In HW Config online view, the CP 342-5 diagnostic icon must be grey (no fault). The DP_SEND / DP_RECV DONE bits must toggle each OB1 cycle.
3. Yokogawa side: In the System View, the IOM data item must show a fresh Good quality stamp. CABA must be absent from the message log.
4. Live data test: Force a value in DB100.DBW0 from STEP 7 and confirm the corresponding Yokogawa data item updates within one master poll interval (typical 5–20 ms at 1.5 Mbps, depending on the number of slaves).
5. Bus health: Connect a PROFIBUS analyser (e.g. Softing PROFINET/PROFIBUS Diagnostic Tool or a vendor equivalent) and confirm the line is at 1.5 / 3 / 6 / 12 Mbps, the waveform is symmetric, and the slave addresses are unique on the segment.

12. Engineering Notes and Edge Cases

• The CP 342-5 in slave mode does not participate in DP-V1 acyclic services. If the Yokogawa application uses Read/Write Record on the slave, the CP 342-5 will respond with Access Denied. Use the FC12/FC13 cyclic area or migrate to a CP 343-5 / CM PROFIBUS DP-V1 module.
• The CP 342-5 consumes 16 bytes of input and 16 bytes of output in the S7-300 process image regardless of the configured user data. These 16 bytes are the internal channel between the CP and the CPU and are not visible on the PROFIBUS segment.
• If the same S7-300 station is also a master on the same segment via a second CP 342-5, address the two CPs in different segments or with different addresses; the ALP111 cannot co-exist with a foreign master on the same physical bus without a repeater.
• The BF LED will also flash when the bus terminator is missing on a segment with more than two devices. Always enable termination on the physical end nodes only and switch it off on the middle nodes.
• When a Yokogawa CABA alarm is raised together with a Yokogawa STBY flag, the master has switched the slave to fail-safe output handling. The S7 application must accept that the Yokogawa holds the last valid output. Resetting the CABA does not require a CPU restart on the S7 side; the BF LED will clear within one master poll once the parameterisation is accepted.
• For S7-300 stations running STEP 7 V5.4 or earlier, the CP 342-5 GSD must be the one bundled with that STEP 7 version. Newer GSDs may declare slots not recognised by older SyCon-PB revisions.

13. Summary of the Fix Path

1. Open HW Config online and clear all CP 342-5 diagnostics.
2. Disable Test, Commissioning, Routing on the CP 342-5 operating mode tab.
3. Confirm at least one input slot and one output slot are configured in the CP 342-5 DP-Slave properties.
4. On the Yokogawa side, import the matching CP 342-5 GSD into SyCon-PB and align the slot I/O length byte-for-byte with the S7-300 configuration.
5. Force DPV0 on the ALP111 master slot for the CP 342-5.
6. Register a Yokogawa IOM data item whose size, offset, and data type match the slave announcement exactly.
7. Verify: BF LED off, DP_SEND DONE = 1, CABA absent, live data round-trip OK.

If the BF LED still flashes after these steps, the remaining causes are usually a physical-layer issue (cable, connector, termination, baud rate mismatch) or a GSD file that does not match the part number printed on the module label. Capture a PROFIBUS trace and check the master's Set_Prm telegram against the slave's Get_Cfg reply to identify the exact disagreement.

FAQ