Configuring S7-300 MPI Communication with VIPA CPUs in STEP 7

1. Problem Overview

Engineers integrating a VIPA CPU with a Siemens SIMATIC S7-300 controller (for example, CPU 314C-2 DP, order number 6ES7314-6CG03-0AB0) frequently encounter a STEP 7 V5.6 download error such as "cannot find slave CPU on bus" or "does not see slave PLC in the bus" when attempting to configure Multi-Point Interface (MPI) peer-to-peer data exchange between the two racks. The error is not caused by a defective MPI cable or by STEP 7 configuration syntax, but by a fundamental hardware limitation of the VIPA CPU and a misunderstanding of the MPI protocol model.

This article clarifies why the native MPI port on most VIPA 300S/300V CPUs cannot carry the S7-300/400 S7 communication services, and it documents two field-proven replacement architectures:

1. Ethernet S7 communication using a Siemens CP 343-1 Lean (6GK7343-1CX10-0XE0) in the S7-300 station and the on-board Ethernet port of the VIPA CPU, exchanging data with PUT / GET or BSEND / BRECV.
2. PROFIBUS-DP master/slave using a Siemens CP 342-5 (6GK7342-5DA03-0XE0) as a DP slave in one of the racks, configured per the official Siemens application DP link between CP 342-5 (slave) and S7-300/400 (master) with STEP 7 V5.1.

2. Root Cause: Why MPI Download Fails on VIPA CPUs

The Siemens S7-300 MPI port is a multiprotocol interface that carries three families of services:

Most VIPA CPUs in the 300S and 300V families expose an MPI/DP combo port that supports PG functions only. They do not implement the S7 communication server that the Siemens CPU needs in order to register a passive or active S7 connection endpoint over MPI. The symptom is twofold:

• STEP 7 can go online to the VIPA over MPI for programming, so the cable and addresses appear correct.
• STEP 7 cannot establish an S7 connection from the Siemens CPU to the VIPA, so the configured connection in NetPro stays in the "not established" state, and the download error "does not see slave PLC in the bus" is reported when the configuration is downloaded to the master.

Adding to the confusion, the Siemens CPU 314C-2 DP has a combined MPI/DP interface that is being used as a DP master for a diameter gauge machine. If the engineer repurposes that port for MPI, the DP master functionality is lost, and the gauge machine loses its I/O. The correct approach is to leave the integrated port on the Siemens side dedicated to its DP master role and add a dedicated communication module on the VIPA side.

3. Prerequisites

• STEP 7 V5.6 (or V5.5 SP2 as the minimum supported for CP 343-1 Lean firmware 3.x) installed on the engineering station.
• Siemens SIMATIC S7-300 station with at least one free slot in the rack for a CP (slot 4–11, addressing the CP as a logical S7-300 module).
• VIPA CPU with an on-board Ethernet port (typical of 300S+ and 300V+ variants) or free slot for a CP 342-5 if the PROFIBUS route is chosen.
• MPI/DP cable terminated on both ends with 9-pin sub-D connectors (Siemens order code 6ES7901-0BF00-0AA0) — required only for the initial STEP 7 download to the VIPA if no Ethernet path exists yet.
• Siemens GSD file for the CP 342-5 in DP slave mode: SIEM804D.GSE (shipped with STEP 7 V5.6, folder ...\S7DATA\GSD).
• VIPA GSD/GSDML file matching the CPU firmware version, available from the VIPA support download portal.

4. Solution A — Ethernet S7 Communication via CP 343-1 Lean

4.1 Hardware installation

1. Power down the S7-300 rack and insert the CP 343-1 Lean in a free slot. Tighten the front connector screws and record the slot number (for example, slot 6).
2. Connect the CP 343-1 Lean port (RJ45) with a Cat 5e patch cable to the same Ethernet subnet as the on-board Ethernet port of the VIPA CPU.
3. Power up the rack; the CP 343-1 Lean should display the STOP LED briefly and then transition to RUN with the LINK LED on the RJ45 lit.

4.2 STEP 7 V5.6 hardware configuration

1. Open the SIMATIC Manager project, expand the S7-300 station, and double-click Hardware.
2. In the hardware catalog, navigate to SIMATIC 300 \ CP 300 \ Industrial Ethernet \ CP 343-1 Lean and drag the module into the same slot you used in step 4.1.1.
3. Open the CP 343-1 Lean properties dialog. On the General tab set:
   • IP address: 192.168.0.10
   • Subnet mask: 255.255.255.0
   • Ethernet interface: Ethernet(1)
4. Add the VIPA CPU Ethernet port as a second node on the same Ethernet(1) subnet, IP 192.168.0.20, mask 255.255.255.0.
5. Save and compile the hardware (HW) configuration, then download it to the S7-300 station.

4.3 NetPro connection configuration

1. In SIMATIC Manager, open NetPro. The CP 343-1 Lean appears on Ethernet(1) with IP 192.168.0.10; the VIPA CPU appears on the same subnet with IP 192.168.0.20.
2. Select the CP 343-1 Lean, right-click and choose Insert New Connection.
3. In the connection dialog, set:
   • Connection partner: VIPA CPU Ethernet port
   • Connection type: S7 connection
   • Active connection establishment: checked (the Siemens side initiates)
   • Local ID (hex): 1
   • Partner port: 0 (any ISO-on-TCP port — the Lean CP picks one automatically)
4. Confirm the dialog. NetPro will report a green line between the two nodes indicating the S7 connection is configured.
5. Download the NetPro configuration to the S7-300 station. The CP 343-1 Lean RUN LED should remain steady; check CP Diagnostics > S7 Connections in STEP 7 to confirm "Established".

4.4 S7 communication function blocks

Once the S7 connection is established, data exchange is performed with the standard S7 communication FBs. The two most common choices are:

Example: PUT to write 10 bytes of control data from the S7-300 to the VIPA every 100 ms.

// OB1 - PUT call (S7-300 master, VIPA is passive partner)
CALL  FB 8 , DB 8
  REQ        := M 10.0          // rising edge starts a new PUT
  ID         := W#16#1          // local connection ID from NetPro
  DONE       := M 10.1
  ERROR      := M 10.2
  STATUS     := MW 12
  ADDR_1     := P#DB20.DBX 0.0 BYTE 10  // local source
  ADDR_2     := P#DB50.DBX 0.0 BYTE 10  // partner destination in VIPA
  SD_1       := P#DB30.DBX 0.0 BYTE 10  // data to send
  RD_1       :=                          // not used for PUT
  LEN        := 10

For the partner side (VIPA CPU), create an equivalent SFB 8 instance, set REQ := FALSE, and use the same ID value. The PUT is initiated from the Siemens side; the VIPA side only needs the connection to be present in NetPro.

5. Solution B — PROFIBUS-DP via CP 342-5

If the VIPA CPU is later-model 300V+ and exposes a PROFIBUS interface, or if Ethernet is not available on the plant network, the second option is to add a Siemens CP 342-5 in DP slave mode. This is the architecture documented in the official Siemens application DP link between CP 342-5 (slave) and S7-300/400 (master) with STEP 7 V5.1.

1. Insert the CP 342-5 into the VIPA rack (logical slot). In HW Config, drag SIMATIC 300 \ CP 300 \ PROFIBUS \ CP 342-5 into the station.
2. On the Operating Mode tab of the CP 342-5 properties, select DP Slave and assign it a unique PROFIBUS address (for example, 3). The Siemens CPU 314C-2 DP remains the master on the gauge bus with address 2; either the CP 342-5 joins that bus, or it joins a separate PROFIBUS subnet terminated with a PROFIBUS connector with terminating resistor ON at both ends.
3. Configure the I/O slot mapping: define input slots (CP 342-5 reads from the master) and output slots (CP 342-5 sends to the master). Up to 32 slots, each 0–32 bytes, total 244 bytes in each direction.
4. In the Siemens master station (CPU 314C-2 DP), use HW Config to insert the CP 342-5 from the PROFIBUS DP / Siemens AG catalog as a slave. The slave's slots are now visible as I/O in the master's process image.
5. Data exchange is performed with the standard DPRD_DAT (SFC 14) and DPWR_DAT (SFC 15) calls on the master. These functions are not required on the VIPA side; the CP 342-5 firmware handles the I/O update.

6. NetPro and HW Config — S7 Connection States

After downloading the configuration, monitor the connection state via PLC > Diagnostics/Setting > Diagnostics of S7 Connections in NetPro or via the CP's online diagnostics. A healthy S7 connection shows the following SF/Status codes:

7. Hardware Diagnostic LEDs on CP 343-1 Lean

8. Commissioning and Verification

1. From the engineering station, ping 192.168.0.10 and ping 192.168.0.20. Both must reply with <2 ms on a copper subnet.
2. In NetPro, right-click the S7 connection and choose Status > Connection Status. The status should be "Established" with status code 0000.
3. On the S7-300, open VAT view of the test DBs created in §4.4. Toggle a few M bits that are written to the VIPA via PUT and confirm they change at the partner DB.
4. On the VIPA side, use the VIPA SPEED7 Studio or TIA Portal (if applicable) to monitor the same DBs. Confirm the data round-trips within the 100 ms cycle time of the OB35 call.
5. Force a CPU restart on the VIPA side. Within 30 seconds, the S7 connection must re-establish automatically — the CP 343-1 Lean initiates the connection on every restart of the partner.
6. Capture a 60-second trace of STATUS from the PUT/GET FBs. The error word must remain W#16#0000; if STATUS <> 0, refer to the table in §6.

9. Troubleshooting Matrix

10. Alternative Controller Notes

For engineers on TIA Portal V17 or later, the CP 343-1 Lean can be replaced by the CP 343-1 (full version, 6GK7343-1EX30-0XE0) or by the on-board PROFINET interface of newer S7-300 CPUs (315-2 PN/DP, 317-2 PN/DP). The NetPro-equivalent is the Devices & Networks editor in TIA Portal, where the S7 connection is created with the same parameters (local ID, partner IP, active/passive flag). The SFB 8/9/FB 8/9/10/11 calls remain identical.

For S7-1200/S7-1500 replacements, the VIPA CPU 015PN or 021PN implements the same S7 connection server on its PROFINET port, and PUT/GET are invoked as the standard PUT/GET instructions in the TIA Portal program blocks.

11. Field-Proven Caveats

• CP 343-1 Lean firmware < V2.0 does not support S7 connections with a local ID other than 1. Keep the local ID at W#16#1 on every project to avoid surprises when an older firmware module is reused as a spare.
• The 314C-2 DP integrated DP port and the CP 342-5 cannot both be DP masters on the same physical PROFIBUS segment. Use a repeater (Siemens 6GK1500-0AA10) to isolate the segments if both masters must coexist.
• On the VIPA side, the Protection tab must allow PUT/GET access. This setting is CPU-firmware dependent; in SPEED7 Studio it is under CPU Properties > Security > Connection mechanisms.
• Maximum cycle time for a 100-byte PUT over the CP 343-1 Lean at 100 Mbit/s is approximately 12 ms. Increase OB35 priority or use BSEND/BRECV with a hand-shake for larger payloads.
• If the project is migrated to TIA Portal in the future, the CP 343-1 Lean must be re-inserted from the TIA hardware catalog — it is not back-compatible with the STEP 7 V5.6 GSD file.

12. Summary Recommendation

For new installations, deploy the Ethernet S7 communication option (Solution A) using the CP 343-1 Lean in the S7-300 and the on-board Ethernet port of the VIPA CPU. The hardware is cheaper, the bus speed is 100 Mbit/s versus 12 Mbit/s for PROFIBUS, and the configuration maps directly to the S7 basic communication model that STEP 7 V5.6 documents. Reserve the PROFIBUS-DP CP 342-5 option (Solution B) for plants that already run a PROFIBUS backbone and need deterministic <1 ms cycle times for safety-relevant I/O. Both solutions are documented in official Siemens application notes and require no changes to the existing DP master role of the 314C-2 DP.

Frequently Asked Questions