Resolving PC Adapter USB A2 Greyed-Out Install in STEP 7 V5.5

1. Problem Statement

The Siemens PC Adapter USB A2 (MLFB 6ES7 972-0CB20-0XA0) is recognized by the operating system after driver installation, and the device is visible in the SIMATIC STEP 7 V5.5 SP3 "Set PG/PC Interface" dialog under Interface Parameter Assignment Used, but the Install and Uninstall buttons remain permanently disabled (greyed out). Selecting the USB interface entry does not enable the action buttons, which prevents the user from binding the adapter to the STEP 7 communication stack.

2. Affected Environment

3. PC Adapter USB A2 Hardware Reference

The PC Adapter USB A2 is a USB-to-MPI/PROFIBUS converter designed for programming, diagnostics, and online functions on SIMATIC S7/C7 systems. It replaces the legacy PC-Adapter (RS-232) and provides galvanic isolation between the USB bus and the MPI/PROFIBUS segment.

4. Root Cause Analysis

On Windows 7 (and Windows Vista/8/10 in legacy mode), Siemens changed the Set PG/PC Interface architecture. The classic Install / Remove mechanism historically bound a device to the S7DOS / S7ONLINE access point by writing registry entries under HKLM\SOFTWARE\Siemens\S7ONLINE. In Windows 7, however:

1. The adapter is enumerated as a USB Plug & Play device using the bundled .inf driver.
2. STEP 7 reads the Plug & Play device list dynamically; the Install/Uninstall buttons are intentionally disabled in the dialog because manual binding is no longer required.
3. When the USB device is enumerated and the matching STEP 7 access point exists, the entry becomes auto-assigned to S7ONLINE.

5. Pre-Installation Verification (Mandatory)

Before assuming the adapter is correctly bound, perform the following checks. The Install button will not enable if any of the conditions below fail.

1. Open Device Manager ("devmgmt.msc"). Expand the SIMATIC devices group. Verify that PC-Adapter USB (MPI/PROFIBUS) appears without a yellow exclamation mark.
2. Confirm driver load: Right-click the device → Properties → Driver. Driver provider must be Siemens AG, driver date and version must be present. Driver files: s7usb2.dll, s7wtrapi.dll and the user-mode s7epsm.sys filter driver must be loaded.
3. Verify the virtual COM/USB pipe: Under Universal Serial Bus controllers, the device SIMATIC PC-Adapter USB must be listed.
4. Check Windows services: Ensure SIMATIC S7DOS Help Service (service name S7DOS) and SIMATIC S7ONLINE are running. Restart: sc query S7DOS → sc start S7DOS.
5. Check the registry access point: HKLM\SOFTWARE\Siemens\S7ONLINE\<your machine GUID>\S7ONLINE_ACCESS_POINT should contain the entry pointing to the USB adapter. This is auto-populated.
6. Reboot after driver install: Several registry values under S7ONLINE are only evaluated at boot. If the driver was installed while STEP 7 was open, close STEP 7 and reboot.

6. Step-by-Step Resolution Procedure

1. Step 1 — Driver installation. Insert the PC Adapter USB A2 into a USB 2.0 port. Windows 7 should auto-launch the New Hardware wizard. If the wizard does not appear, point it manually to C:\Program Files\Siemens\Automation\S7DATA\PCADAPTER (path applies to STEP 7 V5.5 default install) or to the downloaded driver folder.
2. Step 2 — Verify in Device Manager. Confirm the device is listed as PC-Adapter USB (MPI/PROFIBUS) under SIMATIC devices with no warning icon.
3. Step 3 — Launch STEP 7 and open Set PG/PC Interface. In SIMATIC Manager: Options → Set PG/PC Interface.... The dialog opens with the current interface parameterization list.
4. Step 4 — Select the USB entry. Highlight PC-Adapter (MPI/PROFIBUS) -> PC Adapter USB A2 in the Interface Parameter Assignment Used list. The Properties... button becomes active.
5. Step 5 — Configure bus parameters. Click Properties... and assign the desired MPI or PROFIBUS profile:
   • MPI: 187.5 kbit/s default (range 19.2 kbit/s to 12 Mbit/s)
   • PROFIBUS-DP: 1.5 Mbit/s, 3 Mbit/s, 6 Mbit/s, 12 Mbit/s for PC Adapter use
   • Highest station address (HSA): default 31 / 126 for PROFIBUS
6. Step 6 — Do not click Install. Because the device is Plug & Play on Windows 7, clicking OK in the properties dialog writes the assignment directly. The greyed Install button is the expected state.
7. Step 7 — Re-launch the dialog. Close Set PG/PC Interface and re-open it. The selected entry is now persistent across STEP 7 sessions.

7. Confirming the Active Access Point

After following the procedure above, verify the binding is active by inspecting the registry or by running the PG/PC Diagnostic tool.

HKLM\SOFTWARE\Siemens\S7ONLINE\OLE2\S7ONLINE_ACCESS_POINT
Value: "PC-Adapter.USB.A2" (REG_SZ or REG_MULTI_SZ depending on version)

Alternatively, from the STEP 7 start menu, open SIMATIC → PC PG → Set PG/PC Interface → Diagnostics.... The PC Adapter USB A2 should report Status: OK when the bus is connected and terminated.

8. Diagnostic LED Patterns on the Adapter

9. MPI / PROFIBUS Parameter Mapping

10. Troubleshooting Matrix

11. Verification Procedure

1. Open STEP 7 → Options → Set PG/PC Interface... and confirm PC Adapter (MPI/PROFIBUS) -> PC Adapter USB A2 is checked.
2. Open Accessible Nodes from the STEP 7 start menu. The connected S7-300/400 station must appear with its MPI/PROFIBUS address.
3. Open the project online with the assigned station and check PLC → Diagnostic / Information. The online view must show the actual S7 module order code, e.g. 6ES7 315-2EH14-0AB0 V3.2.
4. Read the diagnostic buffer (PLC → Diagnostic Buffer) to confirm time-stamped entries are being retrieved — this proves the MPI/DP handshake is healthy.

12. Version Compatibility and Firmware

13. Driver-Only Manual Install (Workaround for Persistent Grey-Out)

If the Install button remains grey even after driver verification, force re-bind the access point manually:

1. Close STEP 7 completely.
2. Open regedit as Administrator. Navigate to HKLM\SOFTWARE\Siemens\S7ONLINE.
3. Locate the subkey for the local machine. Create or update the value S7ONLINE_ACCESS_POINT (REG_SZ) and set it to PC-Adapter.USB.A2.
4. Restart the S7DOS service: net stop S7DOS & net start S7DOS.
5. Re-open STEP 7 and re-verify in Set PG/PC Interface.

14. Common Diagnostic Log Entries

15. Safety and Commissioning Notes

• Always power down the S7 station before inserting or removing the PROFIBUS connector to avoid bus faults that can corrupt the adapter's bus controller.
• Connect the PROFIBUS cable shield to the grounding bar at the cabinet entry; the PC Adapter USB A2 is not a substitute for proper EMC termination.
• On Windows 7, disable USB selective suspend under Power Options when running long-term online traces to avoid the adapter being put to sleep during a download.
• When commissioning across a corporate firewall, ensure UDP port 137 and TCP 102 are open if Accessible Nodes must traverse routing.

16. Frequently Asked Questions