Resolving MPI Connection Error on Siemens CPU 315T-2DP

Problem Description and Error Message Analysis

When attempting a hardware download from STEP 7 to a Siemens CPU 315T-2DP (catalog number 6ES7315-6TH13-0AB0 / 6ES7315-6TG10-0AB0 family) using a USB PC Adapter over MPI, the following error is generated in the SIMATIC Manager Online dialog:

Online: The local MPI address of the PG/PC is longer than the maximum size permitted for node addresses or an incorrect baud rate or incorrect interrupt has been set.

The dialog is reported by SIMATIC Manager > Options > Set PG/PC Interface > PC Adapter (MPI) and the underlying WSH DLL S7onlinx.dll. The message has three distinct, stacked root causes, all of which must be verified:

1. PG/PC MPI node address exceeds the maximum station address set on the bus (Highest Station Address, default 31, range 1..126).
2. Configured MPI baud rate of the PG/PC interface does not match the bus baud rate of the CPU.
3. The driver assigned to the PC Adapter uses an I/O address or interrupt that conflicts with another device (relevant for PCI/PCMCIA cards, not USB).

In a T-CPU scenario, root cause (2) is the most common and is the focus of this article. The standard text is misleading because the displayed parameters (address 0, 187.5 kbit/s) appear correct in the Set PG/PC dialog yet the download still fails.

Root Cause: T-CPU MPI Default of 12 Mbit/s vs. USB Adapter Limit of 1.5 Mbit/s

The CPU 315T-2DP boots in two phases. The first phase loads the system part and the MPI node negotiates at 187.5 kbit/s for a short window (typically < 1 s) and then re-negotiates to 12 Mbit/s once the technology part is recognized. A USB PC Adapter (any variant under catalog 6ES7972-0CB20-0XA0 / 6ES7972-0CB35-0XA0) supports a maximum of 1.5 Mbit/s on the MPI/DP side. The S7-300 MPI interface of the T-CPU cannot be down-negotiated to 1.5 Mbit/s by software once the technology part is loaded.

The Set PG/PC interface window displays 187.5 kbit/s because that is the PG/PC interface rate, not the active bus rate. The error message is therefore triggered by the time-out on establishing the active MPI connection (BSEND/BRECV, OPC-DBA, or PG download) which fails because the CPU is transmitting at 12 Mbit/s and the USB adapter never sees a valid token frame.

Field Verification of the Root Cause

To confirm the 12 Mbit/s T-CPU default without access to STEP 7 diagnostics, observe the MPI LED (SF/BF) on the front of the CPU. When a USB PC Adapter is attached at 187.5 kbit/s but the CPU is at 12 Mbit/s, the BF (bus fault) LED will flicker at 2 Hz and the SF (system fault) LED will remain off because the bus fault is physical-layer-only, not logical.

STEP 7 diagnostics can be read in the online buffer via PLC > Diagnostics/Settings > Module Information when an Ethernet connection succeeds. The diagnostic buffer typically contains event 0xE0B1 "Bus fault on MPI" followed by 0x4302 "Sign-of-life failure on the MPI sub-network".

Hardware Architecture: CPU 315T-2DP MPI Interface Specifications

The CPU 315T-2DP exposes two physical interfaces on the front X1 / X2 connectors:

The X2 connector is shared between MPI and PROFIBUS DP. In the default HW Config, the T-CPU ships with X2 set to MPI, address 2, highest station address 31, transmission rate 12 Mbit/s once the technology part is loaded. The 187.5 kbit/s value is only valid when the MMC contains no technology part (factory-fresh or after MMC erase).

Component List of the Affected Configuration

MPI Baud Rate Matrix and Adapter Compatibility

The compatibility table below maps each commercially available PC adapter to the maximum supported MPI/DP baud rate. Only adapters that can sustain 12 Mbit/s will reach a T-CPU once the technology part is loaded.

Solution Path A: Ethernet via CP 343-1 (Recommended)

The CP 343-1 is already present in the affected configuration (slot 4). Using Industrial Ethernet side-steps the MPI baud rate issue entirely and is the recommended path because it does not require new hardware.

Prerequisites

• STEP 7 V5.5 + SP4 or later (CP 343-1 firmware ≥ V2.0 for full S7 communication).
• SIMATIC NET software (only required for advanced routing; PG functions work with plain STEP 7).
• Ethernet cable (Cat 5e or higher) with RJ45 connectors.
• Free Ethernet port on the engineering PG/PC.

Step-by-Step Procedure

1. Assign IP address to the CP 343-1. The default IP is 0.0.0.0. Open SIMATIC Manager and select PLC > Ethernet > Edit Ethernet Nodes (or, in newer STEP 7 versions, Options > Edit Ethernet Nodes). Click Browse to discover the CP on the network. Highlight the CP and click Assign IP Address. Use an address on the same subnet as the PG, e.g. 192.168.0.10, subnet mask 255.255.255.0, no router.
2. Configure the PG/PC interface. In Set PG/PC Interface, select S7ONLINE (STEP 7) > TCP/IP > <your network card>. The access point S7ONLINE is the one used by all STEP 7 online operations.
3. Add the CP to the project HW Config. If the project hardware configuration does not yet contain the CP 343-1, drag it from the hardware catalog (path: SIMATIC 300 > CP 300 > Industrial Ethernet > CP 343-1) into slot 4. Open its properties and assign the same IP address 192.168.0.10. Save and compile (Station > Save and Compile).
4. Configure S7 connection in NetPro. Open NetPro. Right-click the CPU 315T-2DP and select Insert New Connection. Choose connection partner (unspecified), type S7 connection. Set the local endpoint to the CPU and the partner IP to 192.168.0.10 (CP 343-1). Save and compile.
5. Download via Ethernet. Select the CPU 315T-2DP in SIMATIC Manager and choose PLC > Download to Target System. The download dialog will use the S7 connection established in step 4. The full technology DBs transfer at 100 Mbit/s, typically completing in < 30 s.

Verification

After download, open PLC > Diagnostics/Settings > Module Information. The diagnostic buffer should show event 0x13A1 "Startup completed" followed by 0x4542 "STOP due to internal error" only if the project does not match. The CP 343-1 should report RUN on its LEDs (LINK, RX/TX, RUN all steady green).

Solution Path B: Replace USB-MPI Adapter with 12 Mbit/s Capable Adapter

If a wired MPI connection is required (e.g. for PROFIBUS DP commissioning or S7 routing through MPI), the USB PC Adapter must be replaced.

Recommended Adapters

• CP 5711 (6GK1571-1AA00): USB 2.0, supports 12 Mbit/s MPI/DP, integrated PROFIBUS connector. Recommended for field engineering laptops.
• CP 5623 (6GK1562-3AA00): PCI Express, HARDNET, DP master capable. Recommended for stationary engineering stations.
• CP 5611 A2 (6GK1561-1AA01): PCI, SOFTNET-S7 license, 12 Mbit/s. Standard for STEP 7 PG operations.

Step-by-Step Procedure

1. Install the SIMATIC NET CD included with the adapter. Use the version compatible with STEP 7 V5.5 (SIMATIC NET 2008 SP6 or later; SIMATIC NET V14 for STEP 7 V5.6).
2. Insert the adapter. Windows Plug & Play detects it and prompts for the SIMATIC NET driver.
3. Open Set PG/PC Interface. Under S7ONLINE (STEP 7), select the new CP, e.g. CP5611.A2.1 > PROFIBUS.
4. Click Properties and set MPI address 0 (PG default), transmission rate 12 Mbit/s, highest station address 31. The CP uses its own PG address; this is not the bus master address.
5. Connect the PROFIBUS cable between the CP and the X2 MPI/DP port of the CPU. Verify that the terminating resistor is ON at the CP and OFF at the CPU (the CPU is in the middle of the bus, not the end).
6. Download from STEP 7. The error should no longer appear because the bus rate now matches.

Solution Path C: Erase the MMC to Restore 187.5 kbit/s MPI Default

If no replacement adapter is available, the T-CPU can be temporarily forced back to the 187.5 kbit/s default by erasing the Micro Memory Card. This removes the technology part from boot media; the T-CPU will then boot in "standard" S7-300 mode and the USB PC Adapter can connect at 187.5 kbit/s.

Step-by-Step Procedure

1. Power down the S7-300 station. Remove the MMC from the CPU 315T-2DP.
2. Insert the MMC into a PG/PC with a SIMATIC MMC reader (e.g. field PG M2 / M3 with built-in MMC slot, or external USB MMC reader 6ES7792-0AA00-0XA0).
3. Open SIMATIC Manager > PLC > S7 Memory Card > Delete or use the S7ONLINE API PS7DelMMC. Confirm the erase.
4. Re-insert the MMC into the CPU. Power up. The CPU will detect an empty MMC, enter STOP with SF LED blinking at 0.5 Hz, and boot the MPI at 187.5 kbit/s because no technology part is present.
5. Download the project from STEP 7 via the USB PC Adapter at 187.5 kbit/s. The download re-creates the technology DBs and the bus re-negotiates to 12 Mbit/s automatically after the technology part is recognized.

Alternative: Offline MMC Programming

STEP 7 V5.5+ supports offline programming of the MMC: PLC > S7 Memory Card > Create writes the project to the MMC while it is mounted in the PG. Insert the programmed MMC into the CPU, power up, and the boot will use 12 Mbit/s from the start. This is the preferred workflow for T-CPU commissioning because it avoids the 187.5 kbit/s intermediate state.

Configuring PG/PC Interface in STEP 7: Reference Values

The following table summarizes the parameters of the Set PG/PC Interface dialog for each viable solution. Save the configuration with OK > OK; the setting is stored in the Windows registry under HKLM\SOFTWARE\Siemens\S7ONLINE.

STEP 7 NetPro Configuration and Topology Verification

When the connection is made via Ethernet, NetPro must contain a valid S7 connection object. Open the project in NetPro (SIMATIC Manager > Options > NetPro or click the NetPro button in the toolbar). The bus topology should look as shown in the diagram below.

For the Ethernet path, the yellow line in NetPro must connect the PG/PC node to the CP 343-1 node, not to the CPU MPI node. Right-click the CPU and verify that the "PG/PC is the only master on the bus" is unchecked; in mixed MPI/DP networks, a checkmark forces all PG traffic to MPI and will trigger the same error.

Verification Procedures and Diagnostic Indicators

After applying any of the three solution paths, run the following verification sequence. A successful download is defined as the CPU entering RUN with the project online and all technology DBs loaded.

1. Online view test. In SIMATIC Manager, right-click the project and select PLC > Display Accessible Nodes. The CPU 315T-2DP must appear with its MPI/PROFINET address and a green status icon.
2. Diagnostic buffer read. PLC > Diagnostics/Settings > Module Information. Confirm event 0x13A1 "Startup completed" with the timestamp matching the download.
3. Cycle time check. In Module Information → "Performance" tab, OB1 cycle time should be < 50 ms. A cycle time > 100 ms typically indicates the technology DBs were loaded but the technology part is not initialized — power-cycle the station to complete the boot.
4. CPU LEDs. RUN = steady green, STOP = off, SF = off, BF = off. A flickering BF at 2 Hz with SF off still indicates a bus baud-rate mismatch even after "successful" download — re-check the adapter.
5. Compare offline/online. Right-click the S7 program and choose PLC > Compare Offline/Online. The result must be "Identical". Any divergence indicates a partial download caused by a transient baud rate mismatch during the technology part transfer.

Diagnostic Flowchart

Common Related Issues and Edge Cases

Error Returns After Successful Ethernet Download

If the MPI error returns when the operator next inserts a USB PC Adapter, it indicates that an MPI/DP project was previously compiled and downloaded while the USB adapter was selected. The CPU's MPI bus parameters in HW Config still list 12 Mbit/s. To re-enable USB adapter use, open HW Config → CPU properties → MPI → change transmission rate to 187.5 kbit/s and re-download via Ethernet. The MPI rate is part of the system data and persists across power cycles.

Multiple T-CPUs on the Same MPI Bus

When more than one T-CPU is on the same MPI bus, only the master CPU can run at 12 Mbit/s. Slave T-CPUs must be set to 187.5 kbit/s in HW Config, otherwise the bus will segment-fault (BF LED steady red) within 5 s. Use the MPI → Properties → Network Settings dialog to assign the rate per node.

CP 343-1 With Firmware Below V2.0

CP 343-1 modules with firmware < V2.0 do not support S7 routing through the backplane to the CPU. In this case, the CPU is reachable only as an Ethernet node, not as a STEP 7 target with the full project tree. Update the CP firmware via the SIMATIC Manager (PLC > Module Information → Firmware Update) or replace the module with a 6GK7343-1EX30-0XE0 (V2.4) or later.

MPI Address Conflict With a Touch Panel

If a TP 177B or OP 77B is on the MPI bus with default address 1, and the CPU is at address 2, the PG at address 0 will still be rejected if the Highest Station Address is incorrectly set to 1 in NetPro. Symptom: "Online: node not reachable" even though the bus LEDs are healthy. Always set Highest Station Address to ≥ (largest node address on the bus + 1). The default 31 is correct for almost all installations.

USB Driver Issue on Windows 10/11

The legacy SIMATIC PC Adapter USB driver (V1.x) is not signed for Windows 10 64-bit and may install as a generic USB serial device. This causes the Set PG/PC Interface dialog to show "Unknown" interface and the error message to appear even though the baud rate is set correctly. Install SIMATIC NET V14 SP1 or later to obtain the signed Windows 10 driver; the current SIMATIC PC Adapter USB A2 (6ES7972-0CB35-0XA0) ships with a signed driver.

FAQ

Reference documentation: SIMATIC S7-300 CPU 31xT Manual, SIMATIC NET PC Adapter USB Manual, CP 343-1 Product Information, SIMATIC NET CP 5711 Commissioning, STEP 7 V5.5 Set PG/PC Interface Help.