Resolving PC677 6AV7801-0BA00 MPI Embedded Driver Installation

The SIMATIC Panel PC 677 (MLFB 6AV7801-0BA00-0AC0) integrates an onboard MPI (Multi-Point Interface) port used to connect directly to S7-300/S7-400 PLCs without an external PC adapter. Unlike USB or standard RS-232 COM ports, the embedded MPI interface is not enumerated as a Plug-and-Play device on a clean Windows Embedded Standard installation. The MPI driver stack is shipped only with Siemens engineering software (STEP 7, ProTool/Pro, WinCC flexible, WinCC, or SIMATIC Net). If the panel is restored from a system image, reflashed with a new operating system, or commissioned without first installing one of those packages, the MPI port will appear in Device Manager as an unknown device or will be completely missing, and ProTool/Pro Runtime will report "Cannot access MPI interface" or "Online: Interface not installed" (German: "Schnittstelle nicht installiert").

This reference walks through the diagnosis, the root cause, the supported recovery path using Siemens-supported software, and the field-verification sequence for a Panel PC 677 running Windows XP Embedded SP2 with ProTool/Pro 6.0 SP3 RT 256 PowerTag against an S7-315-2DP.

1. Affected Hardware and Order Numbers

The Panel PC 677 is a 15-inch or 19-inch flat-panel industrial PC with a CPU board based on Intel Pentium M or Celeron M, depending on configuration. The MPI variant carries a passive RS-485 transceiver isolated from the chassis ground. The relevant Siemens order numbers (MLFB format 6AV7801-0xxxx-xAC0) are documented in the entry at SIMATIC PANEL PC 677 - 6AV7801-0AA00-1AC0 - Support - Siemens. Confirm the exact MLFB on the type plate at the rear of the unit before ordering replacement parts or recovery media.

The "0AC0" suffix typically designates a software bundle (operating system + ProTool/Pro Runtime PowerTag); the runtime license is bound to the MLFB and survives a motherboard swap only via the CF card or the recovery media. Always record the COA sticker and the ProTool Runtime authorization disk before any service action.

2. Symptoms: How the Fault Manifests

The fault is observed during commissioning or after a Windows image restore. Operators and engineers typically see one of the following behaviors:

• ProTool/Pro RT 256 PowerTag starts in transfer mode but cannot establish an online connection; the status bar reads "No connection to PLC" or "DP/MPI fault".
• In Windows Device Manager, the MPI port appears under "Other devices" with a yellow exclamation mark, or under a missing "Siemens AG" category with a yellow triangle.
• The 9-pin D-sub MPI connector on the rear of the PC 677 has no entry in Device Manager → Ports (COM & LPT) at all; this is the most common case on a fresh Windows XP Embedded SP2 image.
• Set PG/PC Interface (German: "PG/PC-Schnittstelle einstellen") shows only TCP/IP and IE-PG; the "MPI" and "PROFIBUS (PC Adapter)" tabs are greyed out.
• WinCC flexible / TIA Portal transfer from engineering station to the panel fails with "The connection to the HMI device could not be established. Check the HMI device's interface configuration."

3. Root Cause Analysis

The root cause is the absence of the Siemens MPI driver package on the system. The embedded MPI interface is implemented as a dedicated UART on the PC 677 CPU board, paired with a galvanically isolated RS-485 transceiver. On the Windows XP side, this interface is exposed through a virtual COM port enumerated only when the Siemens s7hcom_xp.sys and s7hcom_xp.inf driver stack is present in the driver store.

The driver files are NOT shipped with:

• Windows XP Embedded installation media (any language, any service pack).
• Siemens Restore CD/DVD for the PC 677 (these re-image the OS but do not include engineering software drivers).
• Third-party backup/restore tools (Acronis, Symantec Ghost, Paragon) when restoring an image created on a PC that never had STEP 7 / ProTool / WinCC / SIMATIC Net installed.

The driver files ARE shipped with the following Siemens products (at minimum):

1. STEP 7 V5.x (any service pack) — the most common source; installs under C:\Program Files\Siemens\Automation\S7wincf\drivers or %WINDIR%\system32\drivers.
2. ProTool/Pro V6.0 SP3 or later — relevant here because the panel runs ProTool RT 256.
3. WinCC flexible 2004 SP1 or later.
4. WinCC V6.0 SP3 or later.
5. SIMATIC Net V6.x or V7.x (PC-Software edition).

Any one of these packages, when installed on the PC 677, registers the MPI driver and creates the entry in Device Manager. The same driver can therefore be installed by running the SETUP of any of the products above even if the user does not intend to use the engineering functionality. This is the officially supported workaround.

A second class of root cause is BIOS-related. The Siemens knowledge base entry for the PC 677 (referenced above) explicitly states that using an older BIOS on a new PC platform can lead to faults, and that the USB stick or USB disk drive must be the first drive in the boot sequence when running a recovery. A wrong BIOS configuration can also mask the onboard UART and cause the same symptoms. Always check the BIOS revision (typically displayed on POST screen as Vxx.xx.xx) against the current version in the Siemens product support page.

4. Siemens Knowledge Base Entry 21866535

The Siemens support entry ID 21866535 documents a known corruption issue with the MPI driver set installed by certain STEP 7 / ProTool combinations. The legacy URL pattern at support.automation.siemens.com is the entry point; the redirected entry can be found by searching the Siemens Industry Online Support with entry ID 21866535 and the keyword MPI driver corrupt. The symptom in that entry is a driver signature mismatch after a Windows XP hotfix (notably KB925902, the USB stack update) overwrites the s7hcom_xp.sys driver file with a generic Windows RS-485 driver.

If Device Manager shows the MPI port as present but the driver date or version is wrong, or if ProTool logs ERROR: s7hcom DLL returned 0x8013, follow the deinstallation procedure in section 5 before re-installing the driver.

5. Resolution Procedure

The procedure is sequential. Do not skip step 1; do not attempt a driver-only reinstall while the corrupted entry is still in the registry.

5.1 Prerequisites

• Administrator rights on the Panel PC 677 (built-in Administrator account, password per plant policy).
• One of the Siemens software installation media: STEP 7 DVD, ProTool/Pro CD set, WinCC flexible DVD, or SIMATIC Net DVD.
• USB keyboard and mouse (the touch screen is disabled during BIOS/Setup changes).
• An MPI cable (Siemens 6ES7 901-0BF00-0AA0 or equivalent) and a powered S7-300 CPU, e.g. S7-315-2DP, for end-to-end verification.
• The current BIOS version, ideally pre-loaded on a USB stick in case a BIOS flash is required.

5.2 Step-by-Step Recovery

1. Remove the corrupted driver. Open Device Manager, right-click the MPI entry (usually shown as "Siemens AG MPI/DP" or under "Other devices" → "PCI Serial Port"), select Uninstall, tick Delete the driver software for this device, confirm. Reboot.
2. Clean registry keys. Open regedit (with admin rights). Delete the following keys if they exist:
   • HKEY_LOCAL_MACHINE\SOFTWARE\Siemens\Automation\S7HCOM
   • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\s7hcom_xp
   • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\S7HCOM_XP
   Back up the registry first using File → Export.
3. Install the driver via Siemens media. Insert the STEP 7 / ProTool / WinCC / SIMATIC Net DVD and run SETUP.EXE with admin rights. When the installation wizard reaches the "Communication" or "Drivers" step, ensure MPI Driver for SIMATIC Panel PCs is selected. Allow the install to complete; do not cancel.
4. Reboot. The system must reboot before the new driver binds. After reboot, the MPI port should appear in Device Manager → Ports (COM & LPT) as Siemens AG MPI/DP Interface (COMx) with no warning indicator.
5. Configure PG/PC Interface. Open Start → SIMATIC → PC-based Automation → Set PG/PC Interface. Select PC Adapter (MPI) for the local interface and confirm. The MPI tab must now be active (no longer greyed out).
6. Set MPI address. The PC 677 defaults to MPI address 0 (zero), PLC address 2. Adjust as required for the plant topology using the same Set PG/PC Interface tool, "Properties" of the MPI interface.

5.3 Command-Line Alternative

If the Siemens media is not at hand but a copy of the s7hcom_xp driver package exists on a network share, the driver can be installed by hand:

pnputil -i -a "\\fileserver\siemens_drivers\s7hcom_xp\s7hcom_xp.inf"
sc create s7hcom_xp type= kernel start= auto binPath= system32\drivers\s7hcom_xp.sys DisplayName= "Siemens MPI/DP Driver"
sc start s7hcom_xp
pnputil /scan-devices

This is an advanced path; the Siemens SETUP install is always preferred because it also registers the CP card database entries that ProTool/Pro and WinCC flexible look up at runtime.

6. Verification Procedure

After the driver installation, perform the following sequence to verify the embedded MPI interface is fully operational. Each step must pass before moving to the next.

1. Device Manager check. Confirm Siemens AG MPI/DP Interface (COMx) is listed without a yellow triangle. Note the COM port number; it is needed for ProTool configuration.
2. Loopback test (no PLC). Open Set PG/PC Interface → Properties → MPI → Test with the MPI cable disconnected from any PLC. The test must report No partner connected — Timeout. If it reports Hardware fault, the transceiver or the cable is suspect.
3. Online connection to PLC. Connect the MPI cable between the PC 677 and the S7-315-2DP. From STEP 7 (on the engineering PC or the panel itself), use PLC → Accessible Nodes. The S7-315-2DP must appear with its MPI address (default 2).
4. ProTool/Pro Runtime test. Start the ProTool RT 256 PowerTag project. Switch the project to "Online". All configured tags must update; the status bar must read "Online — Connected".
5. WinCC flexible transfer test (if WinCC flexible is installed): from the engineering station, use Transfer → MPI/DP Transfer and confirm the panel accepts a project download.

7. Performance and Bus Parameter Notes

The PC 677 MPI port supports bus speeds of 19.2 kbps to 12 Mbps, with 1.5 Mbps as the typical default for an MPI-only segment. For an HMI segment that connects only to one CPU, leave the bus profile at "MPI 187.5 kbps" or "DP 1.5 Mbps" — the PC 677's onboard port auto-detects the speed on first contact with the CPU. After the first successful handshake, do not change the bus profile without rebooting both the panel and the CPU.

MPI cable length is limited to 50 m per segment without repeaters, and to 1000 m total across a multi-segment topology. The PC 677 must be the last node on one segment if no termination is fitted at the connector end. The 9-pin D-sub on the PC 677 does not contain termination resistors; termination must be supplied by the cable or by a connector block.

8. Troubleshooting Matrix

The matrix below maps the most common field symptoms to the corresponding root cause and the corrective action. Use it as a quick-reference after the basic driver install has been completed.

9. BIOS and Hardware-Related Checks

Per the Siemens support entry for the 6AV7801-0AA00-1AC0 (which shares the same driver baseline as the -0BA00), the BIOS revision must match the CPU board. Symptoms of a BIOS mismatch include the MPI port not enumerating at all, the system hanging at POST, and the USB stick not being recognized as a boot device. The supported recovery path is:

1. Update BIOS to the latest release available in the Siemens Industry Online Support entry for the exact MLFB.
2. Ensure the USB stick or USB disk drive is the first drive in the boot sequence if running a recovery.
3. Disable Secure Boot / Fast Boot if the Siemens restore image is older than the BIOS.
4. Confirm the date/time in BIOS after the flash; an incorrect date prevents driver signature verification.

For older boards, the BIOS flash is performed from a DOS-bootable USB stick with the AWDFLASH utility and the BIOS binary. Never interrupt power during a BIOS flash; the unit can be bricked if the flash fails.

10. Licensing and Authorization Considerations

The PC 677 -0BA00-0AC0 ships with a ProTool/Pro RT 256 PowerTag authorization. The authorization is tied to the Automation License Manager (ALM) or the legacy License Key (parallel port or USB dongle) depending on the bundle. Installing STEP 7 or another Siemens software does not consume the ProTool runtime authorization. Installing ProTool/Pro a second time, however, may move the authorization between licenses — record the original license key number before any re-installation. STEP 7 and ProTool/Pro can coexist on the same Windows XP Embedded partition because they target the same MPI driver, but do not install two versions of the same product (e.g. STEP 7 V5.4 SP5 and STEP 7 V5.6) on the same image without uninstalling the previous version first; the MPI driver versions can conflict.

11. Long-Term Maintenance Recommendations

• Create a system image (e.g. Acronis True Image, Symantec Ghost) immediately after the driver install and verification, so a future image restore does not remove the driver stack. The image must include the %WINDIR%\system32\drivers\s7hcom_xp.sys file and the matching registry keys.
• Document the COM port number assigned to the MPI interface; record it in the panel's project documentation. If a future image restore enumerates the COM port to a different number, ProTool RT may need to be reconfigured.
• Disable automatic Windows Update for the system root, or whitelist only critical security updates. Driver updates from Windows Update can overwrite s7hcom_xp.sys and reintroduce the fault.
• Schedule an annual verification of the MPI connection using the Accessible Nodes test.
• Keep the Siemens support entry 21866535 in the maintenance binder; it is the canonical reference for re-installation in case of a recurrence.

12. Migration Path to Newer Hardware

The Panel PC 677 reached end of active marketing, and its successors (SIMATIC IPC477E, IPC677E) use PROFINET onboard instead of MPI/DP. If a migration is planned, the ProTool/Pro project can be converted to WinCC flexible and then to TIA Portal using the ProTool / WinCC Converter, but the communication driver must change from "PC onboard MPI" to "PROFINET IO". Plan the migration with at least one parallel-run phase where both the PC 677 (MPI) and the new panel (PROFINET) are connected to the same PLC through the integrated MPI/DP port of the S7-300 CPU, with PROFINET traffic via CP343-1 or the onboard PN port of a newer CPU. The legacy MPI project remains valid for the duration of the parallel run.