Resolving S7-300 PC Adapter USB A2 Stuck at 1.5 Mbps Transmission

1. Problem Summary

When commissioning a SIMATIC S7-300 CPU 314C-2 DP (Siemens order number 6ES7314-6CG03-0AB0, with selected successor variants 6ES7314-6CH04-0AB0 and the discontinued 6ES7314-6BG03-0AB0 without PtP) together with a PC Adapter USB A2 (6GK1571-1AA00) on a Windows 11 host running TIA Portal V17, the PROFIBUS DP interface in the PG/PC interface assignment only exposes transmission rates up to 1.5 Mbps. The 12 Mbps PROFIBUS baud rate that the CPU and the adapter both support is not selectable, and an attempt to go online over the DP port returns the CPU diagnostic buffer entry 140:17075, with the decoded text "Online: The local MPI address for the programming device/PC is higher than the maximum device address or the wrong transmission rate or wrong interrupt is set."

This symptom is not a hardware defect of the PC Adapter USB A2, the PROFIBUS cable, or the CPU. The cap is produced by the TIA Portal project state in which the X2 PROFIBUS DP port of the CPU has not yet been linked to a PROFIBUS subnet. Until the X2 port is connected to a subnet object in the Devices & Networks editor, TIA Portal falls back to the MPI profile for the interface. The MPI profile is limited by the original MPI specification (EIA-485 physical layer, token ring) to 1.5 Mbps. The PC Adapter USB A2 itself supports 9.6 kbps to 12 Mbps on its RS-485 port; the cap is therefore a software project setting, not a hardware limit.

2. Affected Hardware, Software, and Documentation

Authoritative references used to validate this article: the PC Adapter USB A2 manual (entry ID 109751634), the S7-300 CPU 314C-2 DP operating instructions (entry ID 44240662), the S7-300/S7-400 system and standard functions reference (entry ID 109973530), the STEP 7 Professional V17 system manual (entry ID 109811124), and the PROFIBUS & PROFINET guideline (entry ID 77377610).

3. Root Cause Analysis

The PC Adapter USB A2 enumerates a virtual COM port and a TIA Portal-specific bus interface. TIA Portal's PG/PC interface assignment dialog (Options → Set PG/PC Interface) reads the available transmission rates from the active project subnet profile. If the S7-300's X2 DP port is unlinked, the only profile available in the dialog is the MPI profile. The MPI profile is hard-limited to 1.5 Mbps per the original MPI specification, which used a token ring on EIA-485 with a maximum cycle time defined for that bandwidth.

When the X2 port is linked to a PROFIBUS subnet in the Devices & Networks editor, TIA Portal re-evaluates the bus profile and exposes the full PROFIBUS DP rate ladder:

Selection of 12 Mbps in the PG/PC interface is therefore conditional on the project configuration, not on the PC Adapter hardware. The 1.5 Mbps display in the dialog is a strong indicator that the port has not been linked. Other indicators include the inability to enable "DP master" or "DP slave" mode on the port and the absence of an editable PROFIBUS address (the field is greyed out and shows the default MPI address 0).

4. Diagnostic Entry 140:17075 Decoding

Diagnostic buffer entry 140:17075 is produced by the CPU's operating system when the Online view of the PG/PC interface cannot establish a token-passing connection. The structure of the entry, as defined in the S7-300/S7-400 system and standard functions reference, is:

The fix is to ensure the PG's PROFIBUS address is less than or equal to the configured highest station address (HSA), that the baud rate matches the active subnet, and that no legacy interrupt conflict exists (less likely on USB-attached adapters). The interrupt field is a legacy of the original MPI PC Adapter that used the PC's ISA or PCI IRQ. The PC Adapter USB A2 does not consume a hardware interrupt, so the only meaningful corrective action is to align the baud rate and the address.

5. Prerequisites

1. Administrator rights on the Windows 11 host to install the PC Adapter USB A2 driver and to bind it to the S7ONLINE access point.
2. TIA Portal V17 Update 5 or later installed with the S7-300 Hardware Support Package (HSP) corresponding to the CPU's order number. The HSP is installed via the TIA Portal "Options → Support Packages" menu.
3. A TIA Portal project with the S7-300 station inserted and the CPU 314C-2 DP placed in slot 2 of rack 0. The Devices & Networks editor must be open.
4. PROFIBUS cable (6XV1830-0EH10) with two bus connectors (6ES7972-0BA12-0XA0 or 6ES7972-0BB12-0XA0) installed. Terminating resistors must be ON at both physical end nodes and OFF at every other node.
5. PC Adapter USB A2 connected to the host via the supplied USB cable (6ES7902-1AB00-0AA0). The green POWER LED must be solid and the yellow ACTIVE LED must flash on USB enumeration.
6. CPU in STOP or RUN state with no active password protection that would block online access (CPU Protection tab, configurable in TIA Portal hardware configuration). If a password is configured, have it available.

6. Step-by-Step Resolution

1. Open the TIA Portal project and double-click "Devices & Networks" in the project tree.
2. Select the CPU 314C-2 DP in the network view. The X1 port appears as an orange triangle on the left edge of the device icon (combined MPI/DP), and the X2 port appears as a purple triangle on the right edge (PROFIBUS DP only).
3. Right-click the purple X2 port and choose "Add subnet" → "PROFIBUS_1" (or any descriptive name). Alternatively, drag a PROFIBUS subnet object from the hardware catalog and connect it to the X2 port by drawing a connection line.
4. Click the new PROFIBUS subnet line. In the Inspector window under "Properties" → "General" → "Network settings", set:
   • Transmission rate: 12 Mbps (the full ladder is now visible because the port is linked)
   • Highest PROFIBUS address (HSA): 126 (default; do not lower unless the bus is small)
   • Profile: DP (DP-V0 is the default; DP-V1 and DP-V2 are selectable for advanced diagnostics)
5. Select the X2 port again and open "Properties" → "General" → "Interface assignment". Enable "DP master" (or "DP slave" if the CPU is acting as a slave) and assign a PROFIBUS address such as 2. The address must not conflict with any other node on the bus; the CPU 314C-2 DP defaults to 2, the PC Adapter to 0 or 1.
6. Open "Options" → "Set PG/PC Interface" in the menu bar. Select the access point "S7ONLINE (STEP 7)" and the interface parameterization "PC Adapter USB A2 (MPI/PROFIBUS)". Click "Properties".
7. In the Properties dialog, switch the "Interface Type" dropdown from "MPI" to "PROFIBUS". The transmission rate field now exposes 9.6 kbps through 12 Mbps. Select 12 Mbps to match the subnet.
8. Confirm the PROFIBUS address of the PG/PC is unique and within the bus's address range (default 1, do not assign the same address as the CPU).
9. Click "OK" to save the PG/PC interface assignment and "Save project" in the project tree.
10. Power-cycle the CPU, then attempt an online connection via "Online" → "Go online" or by right-clicking the CPU and choosing "Go online". The status indicator in the project tree shows a green icon when the connection is live at 12 Mbps.

7. PROFIBUS Cable, Connector, and Topology Specification

Topology must be a single linear bus with termination enabled at both physical end nodes and disabled at every other node. For a 12 Mbps DP link between the PG and the CPU, the PC Adapter USB A2 sits at one end and the CPU at the other, with the bus connector's terminating resistor toggled ON at both ends. Star and stub topologies longer than 1 m at 12 Mbps are not permitted. For longer runs, install a PROFIBUS repeater (6GK1500-0AA00) and create a second segment.

8. PC Adapter USB A2 Configuration Reference

The PC Adapter USB A2 supports both MPI and PROFIBUS on its single 9-pin D-sub RS-485 port. The active protocol is determined by the bus traffic it detects. The transmission rate set in the PG/PC interface assignment must match the rate of the live subnet. The LED indicators on the adapter are:

Windows 11 does not require a separate driver installation when TIA Portal V17 is installed, because the driver is delivered as part of the "PC Adapter USB A2" component installed alongside TIA Portal. If the device shows in Device Manager as a generic "CP210x" or "FTDI" USB-to-UART bridge, the TIA driver has not loaded; reinstall TIA Portal with the PC Adapter option enabled. The driver is the Siemens S7USB adapter, not a third-party USB-to-serial bridge.

9. TIA Portal V17 PG/PC Interface Assignment Workflow

The PG/PC interface is the central control for online access. The access point "S7ONLINE (STEP 7)" is the default. For multi-interface laptops, multiple access points can exist (e.g., S7ONLINE_X120 for PN/IE, S7ONLINE_USB for the PC Adapter). The complete assignment flow is:

1. Menu: Options → Set PG/PC Interface.
2. Select "PC Adapter USB A2 (MPI/PROFIBUS)" in the "Interface parameterization used" list.
3. Click "Properties" and verify:
   • Interface Type: PROFIBUS
   • Transmission rate: 12 Mbps (or the project-subnet value)
   • PG/PC address on bus: e.g. 1
   • Highest station address: 126
4. Click "Diagnostics" → "Test" to read the bus statistics. A green status confirms token passing at the selected rate. Repeat-request counters must remain at zero during the test.
5. Click "OK" to commit the assignment. The new setting is stored in the Windows registry under HKLM\SOFTWARE\Siemens\Automation\Interfaces\PCAdapter.

If the "Diagnostics" tab reports "Cannot access bus at 12 Mbps", the issue is the physical bus, not the project. Lower the rate to 1.5 Mbps and retry to isolate whether the bus cable is the limiting factor. If 1.5 Mbps works but 12 Mbps does not, check cable length (≤ 100 m) and termination (two end nodes, both ON, all others OFF).

10. CPU 314C-2 DP Port Reference

The CPU 314C-2 DP has two physical PROFIBUS-compatible interfaces:

The X1 port is a combined MPI/DP interface. When configured as MPI, it is limited to 1.5 Mbps. When reconfigured as DP master or DP slave, it supports the full PROFIBUS DP rate ladder. The X2 port is dedicated PROFIBUS DP and supports DP master and DP slave operation independently of MPI traffic. For a 12 Mbps link to a PC Adapter, X2 is the recommended port because it isolates the engineering traffic from any MPI traffic on X1.

11. Topology Diagram

The diagram shows the canonical commissioning topology: the PC Adapter USB A2 and the CPU 314C-2 DP form the two physical ends of the PROFIBUS segment, both with their bus-connector terminating resistors ON. The 9-pin D-sub RS-485 link between the PC Adapter and the CPU's X2 port carries PROFIBUS DP at 12 Mbps. Stubs off the main run are not permitted at this baud rate.

12. Verification and Commissioning Tests

1. Online connection: Right-click the CPU → "Go online". The status indicator in the project tree shows a green icon when the connection is live.
2. Diagnostic buffer: Open "Online & Diagnostics" → "Diagnostic buffer" and confirm that the 140:17075 entry is no longer present in recent history. Existing entries can be cleared via "Clear buffer".
3. Live monitoring: Open OB1 in the CPU and add a tag monitor. Values must update within 100 ms at 12 Mbps, versus several seconds at 1.5 Mbps. Tag monitor scan rate is configurable under "Online" → "Tag monitor properties".
4. Bus statistics: From the PG/PC interface diagnostic panel, verify zero bus errors and zero repeat-request counters over a 5-minute window.
5. Firmware: In the CPU's "Online & Diagnostics" → "Firmware update" panel, verify the firmware version of the CPU matches the TIA Portal HSP. For the 6ES7314-6CG03-0AB0, firmware V3.3 is the recommended match for TIA V17. Firmware V3.2 is also supported but does not include some of the diagnostic-buffer event codes available in V3.3.
6. Download performance: Use "Online" → "Download to device" to push a small project. At 12 Mbps, a 1 MB project downloads in roughly 1–2 seconds, versus 10–15 seconds at 1.5 Mbps. A useful field benchmark.
7. Project backup: Use "Online" → "Upload from device" to verify full online visibility. If the upload fails, the issue is online access, not bandwidth.

13. Troubleshooting Matrix

14. Decision Flowchart

15. Advanced Topics and Edge Cases

GSD file compatibility. When the CPU 314C-2 DP is used as a DP slave, the master must have a GSD file for the CPU. The GSD file "SIEM814C.GSD" is delivered with TIA Portal V17 and installed automatically. For older masters, import the GSD via "Options → Manage GSD files".

DP-V1 and DP-V2. The X2 port of the CPU 314C-2 DP supports DP-V1 (acyclic data exchange) and DP-V2 (isochronous mode, time stamp). For DP-V2 isochronous mode, configure the constant bus cycle time in the X2 port properties. Note that the PC Adapter USB A2 does not support isochronous mode; it is a programming device, not a real-time master. Use a DP master CPU for isochronous operation.

CPU password protection. If the CPU has a password configured in the CPU Protection tab, Go Online will prompt for the password. The password is case-sensitive and is checked at the start of the online session. Rate selection is not affected by CPU protection.

Time synchronization over PROFIBUS. At 12 Mbps, the CPU's clock can be synchronized from the PG via the PROFIBUS interface. Use "Online → Set time on device" or configure a clock-master in the X2 port properties. Time synchronization over 1.5 Mbps MPI is also possible but slower.

Firmware update over PROFIBUS. The PC Adapter USB A2 can be used to update the CPU firmware at 12 Mbps. Use "Online & Diagnostics → Firmware update". Do not power-cycle the CPU during the update; a power loss during firmware update can render the CPU unusable and require a return-to-factory procedure via the SIMATIC Automation Tool.

S7-300 HSP installation. If the CPU 314C-2 DP order number is not in the TIA Portal hardware catalog, install the matching HSP. The HSP is downloaded from the Siemens support site as a zip file and installed via "Options → Support Packages". Without the HSP, the CPU cannot be added to the project, and the X2 port cannot be linked.

Multi-project commissioning. When the PG/PC is used to commission multiple CPU 314C-2 DP stations on a single PROFIBUS segment, the PG/PC address must be unique and stable. A common practice is to assign PG/PC address 1, CPU 1 address 2, CPU 2 address 3, etc., up to the HSA. The HSA must be ≥ the highest address in use.

16. Frequently Asked Questions