Siemens S7-300 MPI Redundant CPU Network with OP270 Configuration Reference

This reference describes how to design, configure, and troubleshoot a Siemens S7-300 MPI (Multi-Point Interface) network that carries redundant CPU pairs (master/reserve) and multiple OP270/OP370 operator panels. It is written for commissioning engineers who must validate that a four-CPU redundant cluster, two HMI panels, and an engineering PC coexist on a single 187.5 kbps MPI segment without losing tag updates.

1. Reference Topology and Operating Conditions

The canonical problem layout documented in field service reports is:

PC ---- CPU315-2DP (Master A) ---- CPU315-2DP (Reserve A) ---- OP370 ---- OP370 ---- CPU315-2DP (Master B) ---- CPU315-2DP (Reserve B)

Operating parameters assumed by the troubleshooting flow:

2. MPI Physical Layer and Cable Rules

MPI is a shielded two-wire RS-485 variant that shares the PROFIBUS-DP cable part numbers. For redundant CPU applications, the cable plant must satisfy three constraints simultaneously:

1. Termination: Exactly two terminations must be enabled network-wide. The standard 9-pin D-sub PROFIBUS connector ships with an on-board switch; leave it OFF on every intermediate node and ON only at the two physical ends of the segment.
2. Shield bonding: Bond the braided shield to functional earth at both cable ends (large surface area clamp) to dissipate high-frequency noise from the variable-frequency drives that typically share the cabinet.
3. Baud rate versus segment length: 187.5 kbps supports up to 1,000 m without repeaters. Lower rates (19.2 kbps, 187.5 kbps) tolerate the same total length, but the cycle time grows linearly with the number of slaves that must be polled.

3. CPU 315-2DP Hardware Profile Relevant to MPI

The CPU 315-2DP used in the reference design is the workhorse of redundant S7-300 cells. The two part families most commonly encountered in service are:

The CPU 315-2DP exposes a single PROFIBUS-DP physical port that doubles as the MPI interface. Configuring the port is performed in STEP 7 / HW Config:

• Interface: MPI/DP
• Type: MPI
• Address: 2 (default), 3..31 for any additional CPU
• Highest MPI address: 31 (default; can be raised to 126)
• Transmission rate: 187.5 kbps (must match every other node)

Default MPI address is 2 for any S7-300/400 CPU out of the box, per the official TIA Portal reference: MPI network configuration (S7-300, S7-400, PC). Renumber before placing a second CPU on the segment or both will answer to address 2 and behave as a single device to the HMI.

4. OP270 and OP370 HMI Connection Limits

OP270 and OP370 panels are configured with WinCC flexible 2004 SP1 or later (later replaced by TIA Portal / WinCC Comfort for the newer TP/Comfort line). The maximum number of simultaneous S7-300/400 connections supported by these operator panels is:

Therefore a single OP270 can be configured to poll up to six CPU 315-2DP nodes. The reference topology (4 CPUs + 2 HMIs + 1 PG) is well within the HMI connection budget, but the connection budget is per-HMI, not shared.

5. Redundant CPU Topology on a Single MPI Segment

Two distinct redundancy models are compatible with the 187.5 kbps MPI segment described here:

1. Software redundancy (SWR): The most common model on S7-300. Two CPU 315-2DP units share I/O via DP slave ET 200M. One runs as master, the other as reserve. SWR V2.x supports up to four redundant pairs in a single rack pair, with the redundancy link carried over MPI or PROFIBUS.
2. H-system (1+1): Implemented with a CPU 417H in the S7-400 family. The H-system synchronizes over redundant fiber or PROFIBUS links and is not used with CPU 315-2DP. It is included here only to prevent mis-application.

For the S7-300 / CPU 315-2DP case, the redundancy data exchange (link status, swap-over handshake) is transported by the same MPI bus that also carries the HMI traffic. This is the root cause of the latency symptom reported in the field and is the focus of the troubleshooting flow below.

6. Configuring the "Change Connection" Function in WinCC flexible

The OP270/OP370 runtime supports a built-in Change Connection function. The function allows one or more configured S7 connections to be defined as standby; the HMI will use the active connection for tag polling and switch to the standby if the master fails to respond within the configured timeout.

Configure it as follows in WinCC flexible:

1. Open the project, select the panel, open Connections editor.
2. Create one connection per master CPU: Connection_1 = Master A, Connection_2 = Master B.
3. Right-click each connection and select Properties > Change Connection and assign the role of Backup to the connection that targets the redundant partner.
4. For the master connection, leave Cyclic Operation = Enabled (default).
5. For the backup connection, set Only one Master (Disabled) and Cyclic Operation = Enabled so that the panel warms the link in the background and re-routes the moment the master stops responding.
6. Distribute tags across both connections. Do not place all tags on Connection_1 only; the HMI will not see the change-of-master event for tags that are bound solely to the disabled master.
7. Compile, download to the panel, and restart it so that the runtime table is reloaded.

Two parallel Change Connection functions are required when both HMI panels are present. Each HMI must own its own pair of connections; the connections are local to the runtime, not global to the segment.

7. Why the HMI Loses Tags on a 4-CPU Segment

The 187.5 kbps MPI segment is a token-passing bus with a fixed token rotation time that depends on the number of active masters and the number of slaves each master services. The 4-CPU + 2-HMI + 1-PG configuration forces every device to wait for its turn, and the SWR redundancy handshake consumes additional token slots because the reserve CPU is also participating in the link-up exchange.

The dominant failure modes are:

8. STEP 7 Configuration: MPI Addresses, Highest Station, and Profile

Use the following parameter set as the starting baseline. Adjust Highest MPI address only if you need to bring segment-scanning tools online.

STEP 7 procedure:

1. Open SIMATIC Manager > Project > Open > yourproject.
2. HW Config > select the CPU 315-2DP rack > double-click the MPI/DP interface > Properties.
3. Set Address, Transmission rate = 187.5 kbps, and Highest MPI address = 31.
4. Save & Compile and download to the CPU.
5. Repeat for every CPU, ensuring no two nodes share the same MPI address.
6. Open Set PG/PC Interface on the engineering station and bind to PC Adapter (MPI) or CP 5611 (MPI).

9. Bus Termination Rules

The MPI segment is terminated by a resistor network integrated in the PROFIBUS connector. The bus is electrically correct only when the two physical ends of the segment are terminated and every other connector is left unterminated.

• Enable termination on the connector at the first node (left end of the segment) and on the connector at the last node (right end of the segment).
• Disable termination on all intermediate connectors; this is the most common field error.
• If a repeater (6ES7 972-0AA01-0XA0) is used, terminate the two segments individually; the repeater provides its own bias but not its own termination.
• Measure with an ohmmeter across pins 3 and 8 of any connector with the bus unpowered: the value must be approximately 220 Ω only at the two ends, infinite (open) everywhere else.

10. Step-by-Step Commissioning Procedure

1. Inventory the hardware. Verify part numbers, firmware versions, and that the two CPUs of each redundant pair are the same MLFB. SWR will not pair a 6ES7 315-2AF03 with a 6ES7 315-2AG10 without rejecting the reserve.
2. Pre-stage the bus. Lay the cable, terminate both ends, connect shield to earth at both ends.
3. Bring up the engineering PG. Connect the PG with address 0 to the segment; use Accessible Nodes from STEP 7 to confirm a clean 1-node segment.
4. Add the first redundant pair. Connect Master A (address 2) and Reserve A (address 3) one at a time. Verify both are visible from the PG after each insertion.
5. Add the second redundant pair. Connect Master B (address 6) and Reserve B (address 7) the same way.
6. Add the first HMI. Use the OP270's service menu to assign MPI address 4. After restart, verify it is visible from the PG.
7. Add the second HMI. Assign address 5. Verify visibility.
8. Load the WinCC flexible project. On each HMI, transfer the compiled runtime. The runtime will start to poll all four connections.
9. Verify the token round-trip. Use Accessible Nodes from the PG. The four CPUs and the two HMIs must all appear without timeouts.
10. Stress test the redundancy. Stop the Master A CPU. The Reserve A CPU must take over within the SWR configured switch-over time. The HMI must continue to refresh tags through Connection_1 (Master A > Reserve A path) within the WinCC flexible watch-dog.
11. Capture the bus statistics. Record cycle time, communication load, and token rotation time on every CPU. These are the baselines used by the troubleshooting matrix below.

11. Diagnostic and Verification Checklist

12. Troubleshooting Matrix

13. Migration Path: When MPI Is No Longer Sufficient

When the redundant pair, the HMI, and the engineering PG all share a single 187.5 kbps segment, the bus can become the bottleneck for any process that requires sub-second tag updates. The recommended escalation paths are:

1. PROFIBUS-DP at 1.5 Mbps for the CPU-to-CPU SWR link. Re-purpose the MPI/DP port as PROFIBUS-DP on each CPU, set address 2..7, and configure SWR over DP. The HMI traffic can stay on MPI or migrate to DP at the same time.
2. Industrial Ethernet (CP 343-1 Lean) on each CPU for both SWR and HMI traffic. Move the HMI connections to PROFINET. Use CP 343-1 (6GK7 343-1EX11) on each S7-300. Baud-rate limitations vanish; the only constraint is CP 343-1 cycle time.
3. Add a second CP 343-1 to each CPU to physically separate SWR and HMI traffic across two Ethernet subnets. This is the standard solution for high-availability S7-300 cells that exceed the original 187.5 kbps design.

14. Field-Proven Caveats

• OP270 versus OP370. The OP270 is a 6-inch monochrome panel, OP370 is a 10-inch monochrome panel. Both share the same WinCC flexible runtime engine. Always confirm the exact model before assuming the connection budget.
• Default address collisions. Four CPU 315-2DP shipped from the factory all default to address 2. Always renumber at the bench before installing the second CPU in the network. This is the single most common cause of "only two of the four CPUs respond."
• Change Connection tag distribution. When Change Connection is enabled, every tag must be bound to a connection that is part of the active group. Tags bound only to a disabled connection will be reported as "connection failed" even though the bus itself is healthy.
• Cyclic Operation on the disabled connection. Leave it Enabled so that the HMI warms the link; otherwise the first swap-over will stall for the connection-establishment time.
• Token rotation vs watch-dog. The WinCC flexible default watch-dog is 5 s. A 187.5 kbps segment with 8 active devices can exceed 1 s of token rotation; plan for the worst case and shorten the HMI tag list before extending the watch-dog.

15. Quick Reference: Parameter Defaults for a 4-CPU + 2-HMI + 1-PG MPI Segment

For the official Siemens reference on the defaults, address counts, and the active-node limit, consult the TIA Portal / STEP 7 documentation: MPI network configuration (S7-300, S7-400, PC).