USB PC Adapter Not Detected in STEP 7 V5.5 SP3 on Windows 7

1. Problem Overview

The Siemens USB PC Adapter (catalog 6ES7972-0CB20-0XA0) is the standard commissioning interface for S7-300, S7-400, C7, and the legacy SIMATIC Field PG family. When STEP 7 V5.5 SP3 is installed for the first time on a clean Windows 7 (32-bit or 64-bit) engineering station, the adapter frequently fails to appear in the Local Connection dropdown of Set PG/PC Interface. The USB radio button is missing, and the Set PG/PC Interface window shows only the legacy RS-232 / TCP / MPI options.

The failure is not a hardware fault and not a missing driver in the STEP 7 image: the S7USB64.SYS and S7USB32.SYS files are copied to the workstation during the SIMATIC Manager setup. The root cause is that Windows PnP has never been allowed to complete the driver binding because the adapter was never enumerated while powered from the S7-CPU. The fix is procedural: connect the adapter to a powered S7-CPU MPI port, confirm the ACT LED, and let Windows bind the driver automatically.

2. Affected Environment

3. Root Cause Analysis

The PC Adapter USB is a composite device. The USB portion is bus-powered (5 V from the host) but the MPI/PROFIBUS side is galvanically isolated and powered from the 24 V DC present on the S7-CPU MPI connector. Until the adapter sees that 24 V supply, the MPI controller stays in reset, the full USB descriptor set is not advertised to the host, and Windows sees only a partial enumeration that the bundled S7USBCOM.INF cannot match.

STEP 7 V5.5 SP3 Setup pre-stages the driver files but does not force a PnP bind. The first hot-plug event triggers driver installation - but only if the adapter is already connected to a powered S7 CPU. Engineers who follow the natural workflow of "install software first, then plug the adapter, then connect MPI" are caught in a loop: the adapter never lights the ACT LED, the driver never binds, the Local Connection dropdown never offers USB.

A secondary divergence is observed between upgrade installations and clean installations. When STEP 7 V5.5 SP3 is installed as an update on top of V5.4 or earlier, the legacy Setup already pre-loaded the USB driver (added in STEP 7 V5.4 + Hotfix 12 or in the separate PC Adapter USB V5.x driver package). On a clean Windows image, no such pre-staging exists, and the missing binding becomes visible immediately.

4. Driver Architecture and File Layout

The driver is installed by STEP 7 Setup under the following paths. Both 32-bit and 64-bit binaries are copied regardless of the host OS, to support 32-bit STEP 7 instances launched on 64-bit Windows.

• %ProgramFiles(x86)%\Siemens\Automation\Simatic Manager\Add-On\USB\ (32-bit driver S7USB32.SYS)
• %ProgramFiles(x86)%\Siemens\Automation\Simatic Manager\Add-On\USB\x64\ (64-bit driver S7USB64.SYS)
• %ProgramFiles(x86)%\Siemens\Automation\Simatic Manager\Add-On\USB\S7USBCOM.INF
• Runtime copy under %SystemRoot%\System32\drivers\S7USB64.SYS (after PnP bind)

The INF registers the following hardware IDs:

• USB\VID_0908&PID_0001 - Siemens PC Adapter USB, MPI channel
• USB\VID_0908&PID_0002 - reserved (not used by 6ES7972-0CB20-0XA0; used by 6GK1571 series DP/USB)

Vendor 0x0908 is Siemens AG assigned by the USB Implementers Forum. The driver exposes a virtual COM port (typically COM4 or COM7) to the S7USB upper layer of the SIMATIC Manager, which then tunnels MPI/PROFIBUS frames over USB.

5. LED State Reference

The 6ES7972-0CB20-0XA0 has four status LEDs on the top housing. Their state is the fastest diagnostic in the field.

6. Diagnostic Procedure

1. Open Device Manager (Win+R, devmgmt.msc) before plugging in the adapter.
2. Plug the USB cable into the host. Inspect Universal Serial Bus controllers and Other devices.
3. If the adapter appears as Unknown Device with hardware ID USB\VID_0908&PID_0001, the S7USB driver is not bound. Do not select "Search automatically for updated driver software" - that points to Windows Update and never resolves the device.
4. Connect the MPI cable from the adapter to a powered S7-CPU MPI port. The ACT LED must light.
5. Refresh Device Manager. The device should rebind automatically to the S7USB driver within 30-60 seconds. If it does not, see Manual Driver Install below.
6. Test alternate USB ports if the adapter does not enumerate at all. Some USB 3.0 hubs have known enumeration issues with full-speed (USB 1.1) devices. Use a USB 2.0 port directly on the motherboard.

7. Step-by-Step Resolution

1. Verify the STEP 7 version. Open Programs and Features (appwiz.cpl) and confirm the entry SIMATIC STEP 7 V5.5 with version 5.5.3.x or higher. If V5.5 without SP3 is installed, install SP3 first; the integrated USB driver was added in SP3.
2. Insert the STEP 7 V5.5 SP3 DVD or mount the ISO so that the Add-On\USB folder is accessible on a local or mapped drive.
3. Connect the PC Adapter USB to the S7-CPU MPI port and to a USB port on the engineering station. The adapter must be the last device on the MPI segment or the TERM switch must be ON. The MPI segment must be terminated at both ends per PROFIBUS standard EN 50170.
4. Confirm that both PWR and ACT LEDs are ON. If ACT is off, the CPU is not supplying 24 V - check the MPI connector pinout (pin 2 is +24 V, pin 7 is data plus, pin 9 is data minus) and verify the CPU is in RUN or STOP, not faulted.
5. Allow Windows to detect and install the driver. This takes 30-60 seconds on a clean install.
6. Open Set PG/PC Interface (Start > SIMATIC > STEP 7 > Set PG/PC Interface, or run s7pgpcfg.exe from the STEP 7 install directory).
7. In the Access Path list, select PC Adapter (MPI) and click Properties. In the Local Connection tab, the USB radio button is now present.
8. Assign the node address (default 0) and the transmission rate (default 187.5 kbps) to match the MPI network.
9. Test the connection from SIMATIC Manager with PLC > Accessible Nodes. The CPU should appear within 5-10 seconds.

8. Manual Driver Install (When Auto-Install Fails)

If Windows still does not bind the driver after the S7 CPU is connected and the ACT LED is lit, force the install manually:

1. Open Device Manager and locate the Unknown Device with hardware ID USB\VID_0908&PID_0001.
2. Right-click > Update Driver > Browse my computer for driver software.
3. Point to %ProgramFiles(x86)%\Siemens\Automation\Simatic Manager\Add-On\USB\. Tick Include subfolders.
4. Click Next. Windows will install Siemens PC Adapter USB and bind S7USB64.SYS.
5. Repeat the path Properties > Local Connection check in Set PG/PC Interface.

If the INF is rejected, check the file S7USBCOM.INF for the correct Windows version signature. Windows 7 64-bit requires the [Manufacturer] section to declare %Siemens% = Siemens,NTamd64 in addition to NTx86. The bundled INF in SP3 contains both; older driver packages (V5.4 hotfix era) do not.

9. Upgrade Installation vs. Clean Installation Divergence

The STEP 7 V5.5 SP3 Setup tool checks for the existence of the S7USB*.SYS files in %SystemRoot%\System32\drivers before staging. Behavior differs as follows:

Engineers migrating to a fresh Windows 7 image, a new engineering laptop, or a SIMATIC Field PG that was shipped with a base OS should follow the procedure in Step-by-Step Resolution before commissioning begins. Do not assume a working adapter on a different machine implies a working adapter on the new one.

10. Verification and Commissioning Checks

1. Open SIMATIC Manager and create a new project (or open an existing offline project).
2. Set the online access path to PC Adapter (MPI) via Options > Set PG/PC Interface.
3. Select PLC > Accessible Nodes. The S7-CPU should appear with its MPI address within 5-10 seconds. If the scan returns nothing, drop the baud rate from 187.5 kbps to 19.2 kbps (default for older S7-300 CPUs) and rescan.
4. Go online (PLC > Online). The project must be offline-compiled first or the Go online wizard will prompt for the CPU by MPI address.
5. From Set PG/PC Interface > Diagnostics, run the bus scan. The MPI segment will report all active stations and their addresses. The diagnostics window also shows the bus profile, error count, and token rotation time.
6. Cycle power on the CPU and confirm the MPI link recovers within 30 seconds without re-plugging the adapter. This validates that the driver binding is stable and not dependent on a one-time enumeration.

11. MPI Connector Pinout and Termination

The 9-pin Sub-D MPI connector on the adapter follows the PROFIBUS DP standard. Termination is critical for stable communication.

12. Common Pitfalls

• Driver search pointed to Windows Update. The S7USB driver is not in the Windows Update catalog. Always point to the STEP 7 Add-On\USB folder.
• Adapter connected to a non-powered MPI port (a passive MPI repeater, an unpowered DP segment, or an S7-CPU in fault state with 24 V off). The 24 V supply must come from the CPU backplane.
• USB 3.0 hub in between. Use a direct USB 2.0 port on the motherboard. Some USB 3.0 hubs and front-panel connectors drop USB 1.1 full-speed devices during enumeration.
• STEP 7 V5.5 without SP3. The integrated driver was added in SP3. Earlier versions need the standalone "S7USB Driver" package, available from the Siemens Industry Online Support portal.
• 32-bit / 64-bit mismatch. 64-bit Windows needs S7USB64.SYS; 32-bit Windows needs S7USB32.SYS. STEP 7 Setup installs both, but verify by inspecting %SystemRoot%\System32\drivers after install.
• Missing termination. A floating MPI segment will appear to connect, then drop token randomly. Enable termination at both physical ends.
• Confusion with 6GK1571-0BA00 (USB/PROFIBUS dongle, discontinued). The 6GK1571 series has a different driver (SIMATIC NET PC Software) and is not addressed by the S7USB driver in V5.5 SP3.

13. Reference Documentation

For deeper configuration context, refer to:

• SIMATIC PC Adapter USB (6ES7972-0CB20-0XA0) manual - covers hardware description, LED states, MPI pinout.
• Product entry for 6ES7972-0CB20-0XA0 - firmware notes, download of STEP 7 V5.5 SP3, certificates.
• STEP 7 V5.5 SP3 readme - documents the integrated USB driver as a new feature of SP3.

14. Field-Engineer FAQ