Resolving Siemens USB MPI Adapter Communication Failures on S7-300 PLCs in Windows XP Virtual Machines

Field engineers maintaining legacy S7-300 installations frequently encounter a stubborn communication fault when the only available host is a Windows 7 (or later) machine running Windows XP as a guest under VMware Workstation or VMware Player. The PG/PC interface is a Siemens USB-MPI adapter—either the legacy SIMATIC PC Adapter USB (order number 6ES7972-0CB20-0XA0) or the newer CP 5711 (6GK1571-1AA00). STEP 7 V5.5 is loaded in the guest, the PG/PC interface is set to PC Adapter (MPI), and yet the S7-300 CPU remains unreachable. The Device Manager in the XP guest shows the device with a yellow exclamation mark, or it is missing altogether. This article documents the root cause, the proven field fix, and the full configuration matrix required to make a Siemens USB-MPI adapter work reliably inside a Windows XP VMware guest talking to a real S7-300 CPU on the shop floor.

1. Problem Description and Symptoms

The failure pattern is highly consistent across reported field cases:

• Host operating system: Windows 7, Windows 10, or Windows 11 (physical machine).
• Virtualization layer: VMware Workstation, VMware Player, or VMware Fusion with a Windows XP SP3 guest configured for STEP 7 V5.5.
• Adapter: Siemens PC Adapter USB (6ES7972-0CB20-0XA0) or CP 5711 (6GK1571-1AA00) plugged into a physical USB port on the host.
• STEP 7 SIMATIC Manager launched inside the XP guest, project opened, Accessible Nodes invoked, the S7-300 CPU is not found.
• No broadcast response on the MPI bus; Online > Accessible Nodes returns "No nodes found" or hangs.
• Device Manager in the XP guest displays the adapter as SIMATIC PC Adapter USB with a yellow exclamation mark, or shows it as an Unknown USB Device.
• Host Device Manager shows the adapter as working correctly, or the host has a higher-version driver installed that blocks reinstallation.

2. Affected Hardware and Software Stack

2.1 USB-MPI Adapter Variants

Siemens currently offers two USB-attached MPI/DP/PROFIBUS adapters relevant to this scenario. Both implement a virtual COM port and a Siemens-specific device interface, but they use different USB device classes and different driver packages.

Both devices implement RS-232, MPI, and PROFIBUS. The CP 5711 additionally offers PROFINET. For S7-300 MPI access, the legacy PC Adapter USB is by far the most common.

2.2 STEP 7 Version Compatibility

STEP 7 V5.5 is the last release that still installs cleanly on Windows XP SP3. V5.6 is Windows 7 and above. The driver that ships with the SIMATIC Device Drivers.msi package that comes on the STEP 7 V5.5 DVD is the lowest-common-denominator build that works in the XP guest.

* STEP 7 V5.5 SP4 installs on Windows 10 in "compatibility" mode but is not officially supported by Siemens beyond Windows 7. The OEM instruction is to use a VM.

3. Root Cause Analysis

There are three distinct root causes that produce the yellow exclamation mark or "unknown device" symptom in the XP guest. Diagnose the actual cause before applying the fix.

3.1 USB Device Not Routed to the Guest

The default VMware behavior is to attach newly inserted USB devices to the host. The XP guest therefore never sees the adapter, regardless of what the host Device Manager shows. The user must explicitly connect the device to the guest from the VMware menu (VM > Removable Devices > SIMATIC PC Adapter USB > Connect (Disconnect from Host)). The host then releases the device, VMware presents it on the guest's virtual USB bus, and Windows XP in the guest enumerates it for the first time.

3.2 Driver Mismatch in the Guest

Windows XP does not include a built-in driver for the Siemens USB-MPI adapter. If the SIMATIC Device Drivers.msi package has not been installed in the guest, the device enumerates as SIMATIC PC Adapter USB with Code 28 ("The drivers for this device are not installed") or Code 1. Installing the driver package from the STEP 7 V5.5 DVD (or the equivalent standalone package) resolves the yellow exclamation mark.

3.3 Driver Version Conflict Between Host and Guest

If the host has a newer version of the Siemens USB driver installed, and the user attempts to install an older version inside the guest, the Windows installer aborts with a version comparison error. The CP 5711 installer is particularly aggressive about this. The fix is to either: (a) match the versions on both sides, or (b) temporarily disable the host-side driver, allow the guest to install its driver, and then re-enable the host driver if needed.

4. Prerequisites

Before starting the fix procedure, verify the following:

1. VMware Workstation 12 or later (or VMware Player 7 or later) is installed on the host.
2. Windows XP SP3 virtual machine is configured and boots successfully.
3. VMware Tools is installed in the XP guest (provides the USB arbitrator service).
4. STEP 7 V5.5 (any SP) is installed in the XP guest.
5. The Siemens USB-MPI adapter is physically connected to a USB 2.0 port on the host (USB 3.0 ports may require an explicit USB 2.0 controller assignment in VMware VM settings).
6. Administrative credentials are available in both the host and the guest.
7. The S7-300 CPU is powered, terminated (MPI terminator at the bus ends), and the MPI cable is known-good. Verify with a direct PG connection on a non-virtual machine if possible before troubleshooting the VM stack.

5. Step-by-Step Resolution Procedure

Follow these steps in order. Do not skip ahead—if a step fails, the most likely cause is an earlier step that did not complete.

Step 1 — Verify the Adapter on the Host

Open Device Manager on the host. With the adapter plugged in, you should see an entry under Network adapters or Universal Serial Bus controllers labeled SIMATIC PC Adapter USB or CP 5711. If it shows a yellow exclamation mark on the host, fix the host first—the VM will inherit the problem. Right-click the device, choose Update Driver, and point it at the SIMATIC Device Drivers.msi package from the STEP 7 V5.5 DVD.

Step 2 — Connect the USB Device to the Guest

With the VM window focused, navigate to the VMware menu bar (top of the VM window, not the host menu bar). Choose:

1. VM > Removable Devices > SIMATIC PC Adapter USB > Connect (Disconnect from Host)
2. VMware briefly disconnects the device from the host and re-enumerates it on the guest's virtual USB bus.
3. A Windows "ding" plays in the guest as the device is detected.

If SIMATIC PC Adapter USB is not visible in the Removable Devices menu, the host driver is still holding it. Return to Step 1 and resolve the host-side driver issue first.

Step 3 — Install the Guest Driver

Inside the XP guest, open Device Manager (Start > Control Panel > System > Hardware tab > Device Manager). You will see one of two outcomes:

• SIMATIC PC Adapter USB with yellow exclamation mark — driver not installed. Run the driver installer from the STEP 7 V5.5 DVD: D:\\STEP7\\CD_2\\S7drivers\\setup.exe or directly the SIMATIC Device Drivers.msi package. Reboot when prompted.
• SIMATIC PC Adapter USB without yellow mark — driver is already present, skip to Step 4.
• Unknown USB Device — VMware passed the device, but the guest does not recognize the VID/PID. This indicates a VMware USB controller mismatch. Verify that the VM's USB controller is set to USB 2.0 in the VM settings, then reconnect the device.

Step 4 — Configure the PG/PC Interface in STEP 7

Open SIMATIC Manager in the guest. From the menu:

1. Options > Set PG/PC Interface
2. In the Interface Parameter Assignment Used dropdown, select PC Adapter (MPI) for the legacy PC Adapter USB, or CP5711 (MPI) for the CP 5711.
3. Click Properties and configure:
   • Transmission rate: 187.5 kbps (default for S7-300 MPI bus). Higher rates are supported by the adapter and by the S7-300 CPU, but only if all nodes on the segment are rated for that rate.
   • Highest station address (HSA): 31 (MPI default).
   • PC is the only master on the bus: unchecked (allows multi-master operation with other nodes on the bus).
   • Timeout: 30 s for diagnostics, 3 s for normal operation.
4. Click OK to close Properties, then OK again to commit the interface selection.

Step 5 — Test the Connection

Open the project for the target S7-300 station, or use a blank project. From the SIMATIC Manager menu, choose PLC > Accessible Nodes. After a few seconds (up to 30 s on a long MPI bus), the S7-300 CPU should appear in the accessible nodes window with its MPI address and module type.

If the CPU does not appear, see Section 9 for diagnostic steps.

6. VMware USB Controller and Passthrough Configuration

VMware Workstation's USB subsystem consists of three components: the host USB driver, the VMware USB arbitrator service, and the virtual USB controller presented to the guest. Misconfiguration of any of the three causes the symptom pattern described in this article.

6.1 Virtual USB Controller Compatibility

For a Windows XP guest connecting to a Siemens PC Adapter USB, set the USB controller compatibility to USB 2.0. This is configured in VM Settings > USB Controller.

6.2 USB Arbitration Behavior

The VMware USB arbitrator service runs on the host and decides which physical USB device is connected to which running VM. When two VMs (or the host and a VM) both want the same device, the arbitrator applies the following rules:

1. Most recently connected VM wins.
2. If the host is the most recent requester, the host wins by default.
3. The user can manually override by selecting VM > Removable Devices > [device] > Connect.

To force a device to always connect to a specific VM, edit vmname.vmx and add the following lines:

usb.autoConnect.device0 = "vid:0x0908 pid:0x01fb"
usb.autoConnect.device0.present = "TRUE"

Replace 0x0908 and 0x01fb with the actual Vendor ID and Product ID of the Siemens adapter. Find these values in the host's Device Manager under Details > Hardware Ids. For the SIMATIC PC Adapter USB, the typical VID/PID is 0908/01FB; for the CP 5711, it varies by firmware revision. This entry forces VMware to connect the device to the guest automatically at VM power-on.

6.3 Show All USB Input Devices

Some Siemens adapters present a USB Human Interface Device (HID) interface in addition to the bulk data interface. In VMware, navigate to VM > Settings > USB Controller and ensure Show all USB input devices is unchecked unless you specifically need HID pass-through. Leaving it checked can cause input devices (keyboards, mice) to be passed to the guest and removed from the host, which is rarely desired.

7. STEP 7 PG/PC Interface Configuration Reference

The Set PG/PC Interface dialog controls all communication between STEP 7 and physical adapters. The settings shown below are the field-verified values for S7-300 MPI access via PC Adapter USB inside a Windows XP guest.

7.1 MPI Tab

7.2 Diagnostics Tab

For first-time bring-up, enable Diagnostic on the adapter properties. This makes STEP 7 log every bus frame to a trace file, which is invaluable for diagnosing bus contention, address conflicts, and noise. The trace is written to %TEMP%\\siemens\\S7Trace\\ in the guest.

8. Driver Management: Host vs Guest

When the same physical adapter is shared between host and guest operating systems, the driver state must be consistent. The recommended pattern is one of the following:

8.1 Pattern A — Host has STEP 7, guest has STEP 7

Install matching driver versions in both environments. The SIMATIC Device Drivers.msi from the same STEP 7 DVD (or the same standalone download) must be used on both host and guest. The auto-connect option in the .vmx file ensures the device is always routed to the guest when the VM is running.

8.2 Pattern B — Host has no Siemens software, guest has STEP 7

This is the cleanest configuration. The host does not "claim" the device, the guest installs the driver from the STEP 7 V5.5 DVD, and the adapter is always available to the guest when the VM is running. This is the pattern that should be used for dedicated engineering VMs.

8.3 Pattern C — Host and guest share, with driver isolation

Use VMware Workstation's USB passthrough in "shared" mode and ensure the device is never claimed by the host. The host can still see the device appear in the host Device Manager, but the VMware USB arbitrator will hand it to the guest when the VM requests it.

9. Verification Tests and Diagnostic Procedure

After completing the configuration, run the following verification tests in order. If any test fails, do not proceed to the next—fix the current test first.

Test 1 — Device enumeration

Open Device Manager in the guest. Confirm SIMATIC PC Adapter USB appears under Network adapters with no yellow exclamation mark.

Test 2 — Virtual COM port assignment

The PC Adapter USB presents itself as a virtual COM port. In the guest, open Device Manager and expand Ports (COM & LPT). Note the COM number assigned to SIMATIC PC Adapter USB (COMx). The Set PG/PC Interface dialog must reference this exact COM port via the PC Adapter (MPI) selection.

Test 3 — Accessible nodes

From SIMATIC Manager, choose PLC > Accessible Nodes. The target S7-300 CPU should appear within 5 – 30 seconds, depending on bus length and timeout settings.

Test 4 — Online connection

Open the project containing the target S7-300 station, right-click the station, and choose PLC > Download to Target System or PLC > Online > Monitor/Modify. The online view should populate with the actual CPU state within 5 seconds.

Test 5 — Bus diagnostic

Use PLC > Diagnostic > Module Information in STEP 7. This sends an MPI frame to the CPU and validates the round-trip. A successful module information read confirms full bidirectional bus health.

Troubleshooting matrix

10. Compatibility Matrix and Adapter Comparison

For S7-300 MPI access inside a Windows XP VMware guest, the PC Adapter USB (6ES7972-0CB20-0XA0) remains the most field-proven choice. The CP 5711 is a viable upgrade if PROFINET is required, but it requires a more recent driver and the version-conflict issue is more pronounced.

11. Alternative Solutions and Migration Paths

If the VM-based workflow continues to cause problems, the following alternatives eliminate the virtual machine altogether.

11.1 Native STEP 7 on a Windows 7 Host

STEP 7 V5.5 SP4 installs and runs on Windows 7 in compatibility mode. This is the simplest solution for engineering workstations that do not need Windows XP for legacy reasons. The driver stack is identical, and the USB-MPI adapter works without any VMware involvement.

11.2 Native STEP 7 on Windows 10 with Compatibility Mode

STEP 7 V5.5 SP4 can be installed on Windows 10 with the compatibility shim published by Siemens. The official Siemens FAQ on this topic describes the registry entries required to bypass the Windows 7 lock-out. This approach is supported by Siemens Industry Online Support as a "workaround" configuration.

11.3 Migration to TIA Portal

If the project allows migration, TIA Portal V16 or V17 with a PC Adapter USB A2 or CP 5711 provides a modern workflow that does not require a Windows XP guest. The S7-300 migration tool in TIA Portal converts STEP 7 V5.5 projects with full program, symbol, and hardware retention.

11.4 Ethernet CP at the S7-300

Installing a CP 343-1 Lean or CP 343-1 Ethernet communications processor in the S7-300 rack and using ISO-on-TCP or S7 communication over Ethernet eliminates the MPI bus entirely. STEP 7 inside the VM (or on the host) connects via TCP to the CP's IP address. This is the most robust solution for permanent installations.