Siemens S7-313C MPI PLC-to-PLC Communication Configuration Guide

Overview

The Siemens SIMATIC S7-300 CPU 313C integrates an MPI (Multi-Point Interface) port on the front connector. This port supports three documented peer-to-peer communication methods that allow two S7-313C CPUs (or one S7-313C and another MPI-capable S7-300/400 station) to exchange process data without any additional CP module:

• Global Data (GD) - cyclic, broadcast-style data exchange configured entirely from STEP 7; no code changes required.
• S7 Basic Communication - event-driven data exchange via SFC 72/73/74 in the user program.
• S7 Communication - acknowledged, larger-payload data exchange via SFB/FB 8, 9, 12, 13, 14, 15.

All three methods ride on the same MPI physical layer (RS-485, terminated, 187.5 kbps default). The selection is driven by data volume, scan determinism, and whether acknowledgments are required.

S7-313C Hardware and MPI Interface Specifications

The CPU 313C family is shipped in several MLFB (order) variants. The two most common are:

Key MPI specifications for the integrated port:

• Physical layer: RS-485, isolated, 9-pin Sub-D female connector.
• Baud rates supported: 19.2 kbps and 187.5 kbps (187.5 kbps is the default and the only rate valid for inter-CPU traffic when mixed with newer stations).
• Maximum number of nodes on a single MPI segment: 32 (addresses 0-31), of which addresses 0 and 1 are reserved for the programming device (PG) by default.
• Maximum segment length without repeaters: 50 m for 187.5 kbps with standard PROFIBUS cable.
• Termination: bus termination resistors must be enabled at both physical ends (switch on PROFIBUS connector or terminator plug).

MPI Network Topology for Two S7-313C CPUs

Two CPUs are addressed with unique MPI addresses. The default MPI address of an S7-313C out of the box is 2, which collides if both stations ship from the factory unchanged. One station must be reassigned before commissioning.

Topology rule: connect all MPI nodes linearly. Use only PROFIBUS cable (violet, 6XV1 830-0EH10 or equivalent) and 9-pin Sub-D PROFIBUS connectors with switchable termination (e.g., 6ES7 972-0BA12-0XA0).

Hardware Cabling and Connectors

For PC-to-MPI programming during commissioning, the PC needs an MPI adapter:

Termination must be enabled at the two physical ends of the segment, never in between.

Comparison of the Three Communication Methods

For the user's stated requirement - "I want to use first PLC's sensor information in second PLC" - any of the three works. Global Data is the smallest implementation effort and is recommended for sensor-status broadcasts. S7 Basic Communication is recommended when read/write confirmation and modest payload are needed. S7 Communication is appropriate for buffered block transfer.

Option 1 - Configuring Global Data (GD)

GD exchanges data cyclically on the MPI bus between configured stations without any user code. STEP 7 builds the GD table at compile time and downloads it with the S7 program.

Prerequisites

1. STEP 7 V5.x installed (e.g., STEP 7 V5.6) and licensed.
2. Both CPU 313C stations online with their HW Config loaded.
3. MPI bus connected, terminated, and PG online to both CPUs.

Step-by-step

1. Open the STEP 7 project containing both S7-300 stations.
2. Assign unique MPI addresses (CPU #1 = 2, CPU #2 = 3) in HW Config -> Properties of the CPU -> Interface.
3. Establish an MPI online connection to CPU #1 (address 2) via the PG.
4. From the menu, choose Options -> Define Global Data. The GD editor opens.
5. Right-click in the GD table and choose New; the CPUs participating in the GD must be inserted. Use CPUs in the GD Circle from the menu to add CPU #2 (address 3) and any other station.
6. Click into the first row of the source column (CPU #1) and enter the sender area, e.g. DB1.DBD0. Click into the matching receiver cell (CPU #2) and enter the destination, e.g. DB2.DBD0.
7. Set the transmission rate per row: factor 1 sends on every CPU scan; higher factors spread the rows across scan cycles to keep bus load low.
8. Compile (GD Table -> Compile) and download (PLC -> Download to all CPUs in the GD circle).

Verification

1. Open the GD editor and select View -> Status. Each row shows GD STATUS per partner with the bits: transmitter fault, receiver fault, transmission fault.
2. In receiver CPU #2, monitor the destination DB in VAT or in the program; values must update cyclically.

Option 2 - Configuring S7 Basic Communication

S7 Basic Communication uses SFCs in the user program. The MPI side of the connection is configured by assigning the partner's MPI address.

Function blocks / SFCs

The SFCs require a configured connection object of type S7 connection on each side, defined in NetPro.

Step-by-step

1. In NetPro of CPU #1, right-click the CPU symbol and select Insert New Connection -> S7 connection. Partner = "Unspecified"; MPI interface; partner address = 3.
2. Repeat on CPU #2 with partner = CPU #1, partner address = 2.
3. Compile and download both NetPro configurations.
4. In the user program of CPU #2 (the consumer), call SFC 72 (I_GET):

   CALL "I_GET" // SFC 72
     REQ    :=M10.0
     CONT   :=TRUE
     ADDR_1 :=P#DB1.DBX 0.0 BYTE 10  // partner DB1 in CPU #1
     ADDR_2 :=P#DB20.DBX 0.0 BYTE 10 // local target
     RD_I   :=0
     SD_I   :=0
     RET_VAL:=MW12
     BUSY   :=M10.1
     DONE   :=M10.2
     ERROR  :=M10.3
   

5. To write back, call SFC 73 (I_PUT) with ADDR_1 = local source and ADDR_2 = partner destination.

Each call references an explicit connection identifier (e.g., connection ID 1). The SFC instance stays in BUSY until done; DONE = 1 means success, ERROR = 1 means check RET_VAL.

Option 3 - Configuring S7 Communication

S7 Communication uses the SFB/FB blocks and supports both unbuffered (PUT/GET, USEND/URCV) and buffered (BSEND/BRCV) transfer.

STEP 7 / TIA Portal: configure the S7 connection in NetPro the same way as for S7 Basic Communication. The SFBs/FBS are contained in the standard library and are available for S7-300 from the moment a connection is created.

STEP 7 Project Configuration Sequence

1. Create a STEP 7 project with two S7-300 stations in HW Config (one CPU 313C each).
2. Assign MPI addresses in the CPU properties (Interface -> MPI/DP -> Address).
3. In the project tree, open NetPro and add an S7 connection between the two CPUs; use "Unspecified" partner if the partner is not part of the same project.
4. Compile and download both station HW Config + NetPro + S7 program.
5. For GD: build the GD table in Options -> Define Global Data and download.
6. For S7 Basic / S7 Communication: insert SFC / SFB calls in OB1 or a cyclic OB, then download.

Diagnostics, Error Codes, and Troubleshooting

MPI problems typically manifest as BUSY staying set, RET_VAL returning non-zero, or GD STATUS showing fault bits. The following matrix covers the recurring field failures.

For a complete list of SFC 72/73 RET_VAL codes refer to the S7-300 Communication manual entry at Siemens Support 78028908.

Verification Procedure

1. Connect the PG to either CPU; verify Accessible Nodes lists both CPU 313C stations at their configured addresses.
2. Open PLC -> Monitor / Modify on each CPU. Force a known value into the source area on CPU #1 and read the destination area on CPU #2.
3. Open the GD Status view (for GD) or watch RET_VAL/BUSY/DONE in VAT (for SFC/SFB) and confirm DONE pulses without ERROR.
4. Cycle power to both CPUs; communication must re-establish automatically without redownload.
5. Disconnect the MPI cable mid-run to confirm the diagnostic blocks (GD STATUS fault, RET_VAL = 8084) react correctly; reconnect and verify recovery.

Edge Cases and Field-Commissioning Notes

• Mixed baud rates: All MPI nodes on a segment must operate at the same baud rate (187.5 kbps). A legacy 19.2 kbps node will lock the bus off-line.
• PG during runtime: Disconnecting the PG does not break the GD circle, but hot-swapping of MPI stations on a live segment can disturb traffic for one scan cycle.
• CPU 313C-2 PtP: The PtP variant replaces the second MPI port with a serial port. Only X1 is MPI. Configuring an MPI connection on X2 will fail.
• Routing to another subnet: Two S7-313C stations cannot route through each other over MPI; route capability on MPI is restricted. For routing, use an Ethernet CP (CP 343-1).
• Buffered vs. unbuffered blocks: BSEND/BRCV maintain data on power-down of either partner; USEND/URCV do not. Choose buffered for sensor logs and unbuffered for live control signals.
• Connection resources: Each S7 Basic / S7 Communication uses one connection resource per CPU. The CPU 313C supports up to 6 S7 connections over MPI; keep the count below this limit.

FAQ