Problem Overview
Engineers frequently encounter a "not connecting" or "cannot establish online connection" fault when attempting to commission, troubleshoot, or backup a Siemens SIMATIC S7-300 CPU from a Windows 10 laptop running STEP 7 V5.7 using a USB-to-MPI/DP programming cable. The typical symptom is that the adapter is recognized in Device Manager, STEP 7 V5.7 starts without complaint, but the Accessible Nodes window returns no nodes, or the Online > Connect to Target System command times out.
The most common root causes are:
- An obsolete PC Adapter USB (MLFB 6ES7972-0CB20-0XA0) is being used instead of the current PC Adapter USB A2 (MLFB 6ES7972-3CB20-0XA0).
- Drivers for the original PC Adapter USB were never published for Windows 10.
- The PC Adapter is plugged in before STEP 7 is installed, so the S7DOLine or PC Adapter driver is not registered.
- The MPI/DP port on the S7-300 CPU cannot deliver the 24 V supply required to power the adapter.
- The Set PG/PC Interface access point is not bound to
PC Adapter (MPI)orPC Adapter (PROFIBUS).
Identifying the Correct Hardware
Siemens has shipped three generations of the PC Adapter over the MPI/DP bus. The part number printed on the label is the only reliable identifier.
| MLFB (Order Number) | Designation | Status | Windows 10 Support |
|---|---|---|---|
| 6ES7972-0CB20-0XA0 | PC Adapter USB | Discontinued 2015 | No (up to Windows 7 only) |
| 6ES7972-3CB20-0XA0 | PC Adapter USB A2 | Active product | Yes (STEP 7 V5.7 / V5.6 SP2+) |
| 6ES7972-3CB30-0XA0 | PC Adapter USB A2 (variant) | Active product | Yes |
The original PC Adapter has a beige/light-gray plastic housing with a single USB-A pigtail and a 9-pin sub-D connector for the MPI/DP cable. The A2 is distinguished by a darker anthracite housing and a more compact molded USB cable. If the label reads "PC_Adapter_USB_V12" or any other non-standard suffix, treat it as a counterfeited or third-party clone and discard it - Siemens never released a V12 designation.
Driver Installation and STEP 7 V5.7 Prerequisites
STEP 7 V5.7 (and V5.6 SP2 and later) bundles the PC Adapter USB A2 driver inside the installation media under CD_1\Drivers\PC_Adapter_USB_A2. The driver is the Siemens PC-Adapter USB A2 S7DOS variant and is not distributed as a stand-alone download from the Siemens support portal.
Prerequisites
- STEP 7 V5.7 installed and licensed with at least one valid Authorization (mini-LIC or classical License Key).
- Windows 10 (64-bit, build 1607 or later) or Windows 11 with the original Microsoft USB drivers allowed.
- Administrator rights for the first USB enumeration of the PC Adapter.
- .NET Framework 4.6.2 and Microsoft Visual C++ 2015-2022 redistributables, installed automatically by the STEP 7 setup.
Installation Order
- Install STEP 7 V5.7 before the PC Adapter USB A2 is plugged in. The driver catalog is registered at installation time.
- Reboot the laptop when prompted by the STEP 7 setup - the S7DOS service is registered at this point.
- Power on the S7-300 CPU, then connect the PC Adapter USB A2 to a free USB 2.0/3.0 port on the laptop.
- Windows should enumerate the device as Siemens PC-Adapter USB A2 under Universal Serial Bus controllers in Device Manager.
Hardware Connection Procedure
The PC Adapter USB A2 draws its 5 V operating power from the USB port but its RS-485 transceiver is powered from the MPI/DP bus on the CPU side. The S7-300 CPU must be in RUN, STOP, or power-up state and must supply 24 V on pin 7 of the MPI connector.
Wiring Steps
- Verify the S7-300 CPU is de-energized. Touch a grounded metal point to discharge any static.
- Connect the 9-pin sub-D plug of the supplied MPI cable to the adapter; lock the screws.
- Plug the MPI cable into the S7-300 CPU's MPI (X1) port. The MPI port is the leftmost sub-D connector on the CPU 312, 314, 315, 317 family.
- Connect the USB-A pigtail to a USB port on the laptop.
- Apply 24 V to the PS 305 / PS 307 power supply and switch the CPU to STOP (or leave it in RUN if commissioning in online mode).
- Boot Windows and launch SIMATIC Manager.
PG/PC Interface Configuration
Once the hardware is connected, the Set PG/PC Interface tool must be configured. This is the single most common configuration fault.
- Open Start > Siemens Automation > SIMATIC > STEP 7 > Set PG/PC Interface.
- In the Access Point of the Application dropdown, select S7ONLINE (STEP 7).
- In the Interface Parameter Assignment Used list, select PC Adapter (MPI) for MPI-only networks or PC Adapter (PROFIBUS) if the bus is a PROFIBUS-DP segment.
- Click Properties and confirm the following default values:
| Parameter | MPI Default | PROFIBUS Default |
|---|---|---|
| Transmission rate | 187.5 kbps | 1.5 Mbps |
| Highest MPI/PB address | 31 | 126 |
| Highest own address | 0 (PG) | 0 (PG) |
| Timeout | 30 s | 30 s |
| Bus profile | DP (universal) | DP (universal) |
- If the CPU is a 317 or 319 with PROFIBUS integrated, the bus may operate at 12 Mbps; set the transmission rate to 12 Mbps in that case.
- Click OK and then OK on the main dialog. The settings are written to the registry under
HKLM\SOFTWARE\Siemens\SINEC.
Establishing the Online Connection
With the PG/PC interface assigned, open or create the STEP 7 project offline, then:
- Select PLC > Accessible Nodes from SIMATIC Manager. After 5-15 seconds a list of detected stations should appear.
- If no nodes are listed, switch to PLC > Diagnostics > Online and confirm whether the adapter's bus status LED is flashing green (active bus scan).
- Right-click the detected CPU and select Connect to Target System > Online. The CPU's current operating mode, diagnostic buffer, and module status appear.
MPI Bus Power and Termination
The PC Adapter USB A2 is a bus node and requires a properly terminated, powered segment. Termination is built into the PC Adapter and the CPU's MPI port. If you have any other device in the segment (HMI panel, drive, ET 200), confirm:
- No more than two terminating resistors are active in the segment (one at each physical end).
- The segment's total length does not exceed 50 m at 187.5 kbps or 1,000 m at 19.2 kbps (the standard MPI baud-rate table applies).
- The shield of the PROFIBUS cable is bonded to ground at both ends through the sub-D backshells.
Troubleshooting Matrix
| Symptom | Likely Root Cause | Corrective Action |
|---|---|---|
| Yellow triangle in Device Manager ("S7DOS") | STEP 7 not installed or corrupted | Repair STEP 7 V5.7, then re-plug the adapter |
| Device Manager shows "USB Serial Port" with no driver | Wrong adapter (PC Adapter USB V12 clone) | Replace with 6ES7972-3CB20-0XA0 |
| LED off, no enumeration at all | No bus power, MPI cable reversed | Measure 24 V on MPI pins 2-7, check cable orientation |
| LED flashing red | Bus error / wrong baud rate | Lower transmission rate to 19.2 kbps first, then step up |
| Accessible Nodes returns no nodes | PG/PC interface mis-assigned | Reassign S7ONLINE access point to PC Adapter (MPI) |
| Online stops with "Connection to target system failed" | CPU in wrong operating mode or bus fault | Set CPU to STOP, check SF LED, clear diagnostic buffer |
| Connection works once, fails after reboot | USB suspend enabled in Windows power plan | Disable USB selective suspend in Power Options |
Verification Checklist
After applying the corrective actions, perform the following verification steps in order:
- Open Device Manager and confirm the entry Siemens PC-Adapter USB A2 with no warning icon.
- Launch Set PG/PC Interface and run Diagnostics on the PC Adapter. The bus status should read OK and report the connected node count.
- In SIMATIC Manager, run PLC > Accessible Nodes and confirm the CPU's MPI address appears with its correct type and firmware version.
- Open PLC > Module Information and verify the diagnostic buffer shows a normal power-on entry, not a bus fault.
- Upload the project to the laptop via PLC > Upload Station to PG to confirm a sustained, error-free MPI link.
Frequently Asked Questions
Can I still use the original PC Adapter USB (6ES7972-0CB20-0XA0) with STEP 7 V5.7?
No. The original PC Adapter USB was discontinued in 2015 and its drivers only support up to Windows 7. On Windows 10/11, you must upgrade to the PC Adapter USB A2 (6ES7972-3CB20-0XA0), which is fully compatible with STEP 7 V5.7.
Does the PC Adapter USB A2 get its power from the USB port or from the MPI connector?
The adapter's logic draws 5 V from the USB port, but the RS-485 transceiver is powered from the 24 V supply on pin 7 of the MPI/DP connector. If the S7-300 CPU is not energized, the adapter will enumerate in Windows but will not communicate with the bus.
What transmission rate should I select for a standard MPI network?
Start with 187.5 kbps, which is the default for S7-300 MPI networks. If the network includes older CPUs or HMI panels that do not support auto-baud, drop to 19.2 kbps. PROFIBUS-DP segments typically run at 1.5 Mbps or 12 Mbps depending on the CPU and slaves installed.
Where do I find the driver for the PC Adapter USB A2?
The driver ships with STEP 7 V5.7 (and V5.6 SP2 or later). It is installed automatically when STEP 7 is set up. Do not attempt to install it manually unless the bundled installation is corrupted - in which case, run a Repair on the STEP 7 installation.
My accessible nodes window is empty even though the adapter LED flashes green. What is wrong?
The PG/PC access point is most likely bound to a different interface (TCP/IP, ISOTCP, or another COM port). Open Set PG/PC Interface, set the S7ONLINE (STEP 7) access point to PC Adapter (MPI), and try Accessible Nodes again.
Is there a difference between PC Adapter USB A2 and the variant 6ES7972-3CB30-0XA0?
Yes, but functionally both are equivalent for MPI/DP programming. The 3CB30 variant is a regional order number used by Siemens for specific markets. Both run the same S7DOS driver set included in STEP 7 V5.7.
Can I use the PC Adapter USB A2 with TIA Portal as well?
Yes. TIA Portal (V15 or later) supports the PC Adapter USB A2 as a programming device interface for S7-300/S7-400 CPUs. It is selected under Online > Online Access > PC Adapter.