Configuring S7-300 Profibus CPU-CPU Communication via CP 342-5

Overview

CPU-to-CPU communication over PROFIBUS DP is a standard requirement when two SIMATIC S7-300 stations must exchange process data deterministically. The configuration described here is a typical mixed configuration:

• Station 1: S7-300 CPU 315-2 DP (6ES7315-2AG10-0AB0) plus a CP 342-5 communications processor (e.g., 6GK7342-5DF00-0XE0) on the backplane.
• Station 2: S7-300 CPU 315-2 DP (6ES7315-2AG10-0AB0) with the integrated DP interface on the CPU.

The CP 342-5 is an external PROFIBUS interface module that the CPU 315-2 DP cannot use as a passive "backplane extension" for I/O. The CP must be configured either as a DP master or DP slave and the two stations then exchange data using the CP services or, for master-to-master, an interposed DP/DP coupler.

This article consolidates the four realistic options, identifies the correct system function blocks (SFCs) and function blocks (FBs), and shows the hardware and software configuration flow inside STEP 7 V5.5 / SIMATIC Manager.

Prerequisites

Communication Method Comparison

Four practical options exist. The choice depends on whether the two CPUs must act as peers or whether one side is purely a data source.

Method 1 - Master/Slave on the DP Port (Recommended for S7-300)

The simplest and most common configuration is to run the CP 342-5 in Station 1 as a DP master and the integrated DP interface of CPU 315-2 DP in Station 2 as a DP slave. This avoids the need for a DP/DP coupler, but the two stations must share the same PROFIBUS segment and bus address space.

Hardware Configuration in STEP 7

1. Insert a SIMATIC 300 station and add the CPU 315-2 DP (rack 0, slot 2).
2. Open the DP port properties and set the operating mode to DP master. Define the PROFIBUS subnet with the bus parameters profile "Standard" or "DP" and assign a unique master address (default: 1).
3. Add the CP 342-5 to the same station. Right-click the CP, choose Properties > Operating Mode, and select DP master if the CP is in Station 1, or DP slave if it is in Station 2.
4. Assign the CP 342-5 to the same PROFIBUS subnet as the CPU's integrated DP port. Each node must have a unique PROFIBUS address on the segment (1...125).

Configuring the Slave I/O Slots

1. Open the DP master system of Station 1 in HW Config.
2. Drag the Station 2 CPU 315-2 DP from the catalog into the master system. The slave is inserted at the next free PROFIBUS address.
3. Insert the desired I/O slots: from the catalog select CPU 315-2 DP > Slave > Universal module or use the predefined modules (DI, DQ, AI, AQ). Each slot can carry up to 16 bytes.
4. For each slot, choose alignment: "Input" = data read by the master, "Output" = data sent by the master to the slave, "Input/Output" = bidirectional.

Programming the Data Exchange (FC 1 / FC 2)

Unlike PROFIBUS DP that uses the I/O image directly, communication via the CP 342-5 is "DPV1 services"-based and must be triggered in the user program. Two function blocks are provided in the STEP 7 library "SIMATIC_NET_CP > CP 300 > Blocks":

• FC 1 "DP_SEND" (also called "AG_SEND" in older projects): transfers data from the CPU to the CP for transmission.
• FC 2 "DP_RECV" (also called "AG_RECV"): receives data from the CP and writes it to the CPU.

Sample call in STL (master station, write 8 bytes to slave PROFIBUS address 2):

// Call FC 1 - DP_SEND
CALL  FC    1
  CPLADDR  := W#16#100   // Logical address of the CP 342-5
  SEND     := P#DB10.DBX0.0 BYTE 8
  DONE     := M 50.0
  ERROR    := M 50.1
  STATUS   := MW 52

// Call FC 2 - DP_RECV
CALL  FC    2
  CPLADDR  := W#16#100
  RECV     := P#DB11.DBX0.0 BYTE 8
  NDR      := M 53.0
  ERROR    := M 53.1
  STATUS   := MW 54
  RD_LEN   := MW 56

The CPLADDR must match the logical base address of the CP 342-5 module; this is visible in HW Config under the module's I/O addresses (e.g., 256 decimal = W#16#100). The STATUS word follows IEC 61131-3 conventions: bit 15 set = error class, bit 7..0 = event number.

Programming the Slave Side (Station 2)

On the DP slave CPU 315-2 DP, the data simply appears in the process image configured for the slots. No FC 1/FC 2 call is required because the slave is a "passive" DP slave. Reading and writing is done with normal L/T operations on the configured I/O addresses (or via the partial process image).

Method 2 - Master-to-Master via DP/DP Coupler

When both stations need to remain DP masters - typically because each side also owns local distributed I/O - a DP/DP coupler must be installed. The Siemens part number is 6ES7158-0AD01-0XA0 (current variant). It has two electrically isolated DP interfaces and is configured as a DP slave to each master on its respective segment.

Configuration

1. Install the GSD file "SIEM80F0.GSD" (for the 6ES7158-0AD01) into STEP 7: Options > Install GSD File.
2. Refresh the hardware catalog; insert the "DP/DP Coupler" into the master system of each station.
3. For each side, configure the maximum of 16 input bytes and 16 output bytes (or up to 244 bytes each if using the larger DPV1 variant). The byte offset on one side must mirror the offset on the other side - data byte 0 on master A is byte 0 on master B, and so on.
4. Compile and download the hardware configuration to both stations.

Programming on Both Sides

The coupler is "transparent" to the application. On each side, the configured input/output areas of the coupler appear in the I/O area of the corresponding master. Data exchange is performed using FC 1 / FC 2 on each CP 342-5 in the same way as Method 1, except the CP now talks to a slave whose slave address is the coupler's address on that segment.

Method 3 - MPI Global Data Communication

When the two CPUs are on the same MPI bus and only small amounts of data need to be broadcast, the built-in global data (GD) mechanism is the simplest option. No CP 342-5 is required; the data rides on the integrated MPI/DP port of the CPU.

1. In SIMATIC Manager, right-click the MPI/DP subnet and choose Define Global Data.
2. Add both CPUs to the GD table.
3. For each CPU row, specify the GD packet: choose a memory area (M, DB, I, Q) and length (1 to 22 bytes per packet, max. 32 packets total).
4. Mark the data source (publisher) and the receivers (subscribers). The CPU automatically handles the broadcast - no SFCs are called in the user program.

The transmission cycle is configured separately for each GD packet (1 to 255 MPI scan cycles). Status words can be assigned to monitor the transfer; SF (send fault) and RF (receive fault) bits indicate bus problems.

Method 4 - S7 Communication over MPI (X_GET / X_PUT)

For larger or more structured exchanges on the MPI port, the X-routines implement a client-server model. The CP 342-5 manual distinguishes them from the FC 1 / FC 2 DP services.

Example: reading 20 bytes from DB 5 of the partner CPU at MPI address 2:

CALL  SFC  67
  REQ     := TRUE
  CONT    := TRUE
  DEST_ID := W#16#2       // MPI address of the partner
  VAR_ADDR:= P#DB5.DBX0.0 BYTE 20
  RET_VAL := MW 100
  BUSY    := M 102.0
  RD_LEN  := MW 104

Each SFC call is non-blocking; the BUSY bit must be checked, and the call must be repeated with the same REQ=FALSE until BUSY=FALSE. Up to 4 parallel jobs are permitted per CPU.

Configuration of the PROFIBUS Subnet

Common parameter recommendations for a two-station CPU-to-CPU link:

Diagnostics and Error Code Reference

The CP 342-5 returns diagnostic information in the STATUS word of FC 1 / FC 2 and in the module's diagnostic buffer (accessible via PLC > Module Information). The most common events:

LEDs on the CP 342-5 indicate the link state:

• SF (red): Group error - PROFIBUS wiring, slave diagnostics, or configuration error.
• BF (red): Bus fault - cable break, missing terminator, or slave power off.
• RUN (green): CP is in RUN, PROFIBUS active.
• STOP (yellow): CP is in STOP - no cyclic exchange takes place.

Verification

1. In HW Config, click "Save and Compile" (Ctrl+S). No errors should be reported for the configured PROFIBUS system.
2. Download the hardware configuration to both stations.
3. Switch the CP 342-5 to RUN using the mode selector: MRES → STOP → RUN (hold RUN until the green RUN LED stays on).
4. Trigger a test data write from Station 1 (master) and read the value at the configured I/O address in Station 2. Use a VAT or watch table to monitor both directions simultaneously.
5. In PLC > Module Information > Diagnostic Buffer of the CP 342-5, verify that no diagnostic events are pending.

Troubleshooting Matrix

Field-Proven Caveats

• The CP 342-5 has its own processor; it does not extend the CPU's I/O. The CPU must explicitly call FC 1 / FC 2 for any data to be moved - the bus will not run on its own.
• When a CPU 315-2 DP uses its integrated DP port, the maximum number of DP slaves is 32, not counting the CP 342-5 in Station 1 if the CP is on a separate segment.
• MPI global data is broadcast-only; no acknowledgement, no retry, no error recovery. Use SFC 67 / SFC 68 for reliable peer-to-peer on MPI.
• The CP 342-5 requires the optional package "SIMATIC S7-CP PtP/DP" installed in STEP 7 to display its blocks (FC 1, FC 2) in the library.
• For data volumes above 244 bytes per cycle, use S7 communication (SFC 15 / SFC 14 for PUT/GET) over Industrial Ethernet (CP 343-1 Lean) instead of PROFIBUS.