Siemens 6ES7 972-0CB20-0XA0 PC Adapter USB Firmware Update Guide

1. Problem Scope and Adapter Identification

The Siemens PC Adapter USB with order code 6ES7 972-0CB20-0XA0 is the standard SIMATIC programming interface used to connect an engineering station (PC/laptop) to S7-200, S7-300, S7-400, ET 200, and SINAMICS drive controllers through MPI or PROFIBUS DP. It replaces the legacy RS-232 PC Adapter (6ES7 972-0CA23-0XA0) by replacing the COM port with a USB 1.1/2.0 interface, eliminating the need for a physical serial port on the host.

Firmware updates on this adapter are normally non-intrusive: they patch the internal microcontroller firmware that handles PROFIBUS DP-V0/V1 framing, MPI token handling, RS-232/USB bridging, and bus termination logic. However, the update process fails silently or with execution errors when the adapter cannot establish a stable handshake with the host because of insufficient power on the PROFIBUS segment, mismatched driver versions, or interrupted USB enumeration.

Siemens documents this adapter, the firmware update tool, and known revision history in the official support entry: Siemens Support Entry 51806786 – PC Adapter USB 6ES7 972-0CB20-0XA0.

2. Adapter Specifications and Order Code Decoding

The MLFB decoding is critical when ordering replacement parts or downloading firmware: 6ES7 = SIMATIC, 972 = PC Adapter family, 0CB20 = USB variant, 0XA0 = standard release without options.

3. Root Cause: Why Firmware Updates Fail

The single most common root cause of a stalled or error-flagged firmware update on the 6ES7 972-0CB20-0XA0 is missing 24 V supply on the PROFIBUS/MPI connector. The USB 5 V line powers only the USB transceiver and the bridge controller; the PROFIBUS RS-485 transceiver and the bus terminator require the 24 V rail provided through pin 7 (+24 V) and pin 2 (0 V) of the 9-pin sub-D socket.

Without 24 V, the adapter enumerates on USB, shows up in Set PG/PC Interface, and even reports a working USB-side connection, but the firmware update loader cannot complete the PROFIBUS-side handshake, which causes the EXECUTION button to abort immediately or hang.

3.1 Visualizing the Power and Signal Path

Figure 1 – Power and data topology. The PC supplies 5 V over USB; the PLC supplies 24 V over the PROFIBUS connector. A passive PROFIBUS socket on a non-powered device starves the adapter.

4. Prerequisites

1. Host PC running Windows 7 SP1 or Windows 10 (32-bit recommended for legacy driver compatibility). Windows 11 is not officially supported for the 0CB20 firmware updater.
2. Local administrator account. The firmware update tool installs a temporary WinUSB replacement driver and must run elevated.
3. Siemens PC Adapter USB driver set installed first (so the adapter is recognized in Set PG/PC Interface as PC Adapter (USB) or PC Adapter (MPI/DP USB)).
4. Original Siemens 6ES7 972-0CB20-0XA0 connected via a 6ES7 901-1BF00-0XA0 or equivalent MPI cable to a powered S7 CPU or PROFIBUS master with active 24 V terminator.
5. Firmware update tool downloaded from Siemens Support Entry 51806786. The file is shipped as a self-extracting archive (*.exe or *.zip) containing FWUpdate.exe and a *.hex / *.bin image.
6. Disconnection from any active bus. Remove other DP slaves if the firmware loader cannot establish a quiet bus during programming.

5. Pre-Update Diagnostic Procedure

Run these checks in order before launching the firmware update. Every step is a precondition; skipping one is the most common reason for the EXECUTION button to fail.

5.1 Confirm Adapter Enumeration

1. Connect the adapter to the PC. Windows should emit a USB enumeration tone.
2. Open Device Manager → Universal Serial Bus controllers. Look for Siemens PC Adapter USB.
3. Open Start → SIMATIC → Set PG/PC Interface (or control.exe /name Microsoft.PCAdapter in newer TIA Portal shells).
4. Select PC Adapter (MPI/DP USB), click Properties → Diagnostics.
5. Click Test. The dialog must report OK on both USB and MPI/DP sides. If the MPI/DP side reports Error or No response, the 24 V rail is missing – stop here.

5.2 Confirm Bus Voltage at the Adapter

Use a multimeter on the 9-pin sub-D socket as seen by the adapter (i.e. the bus side). Measure between pin 2 and pin 7:

5.3 Capture Current Firmware Version

Open Set PG/PC Interface → PC Adapter (MPI/DP USB) → Properties → Diagnostics → Read. Note the firmware string returned (e.g. FW 1.2.0). This is your baseline. If the firmware you are flashing is older or identical, the update tool will refuse to run.

6. Firmware Update Procedure

1. Close STEP 7, TIA Portal, Starter, Scout, and any other process that opens the COM/USB handle of the adapter.
2. Right-click the extracted FWUpdate.exe and choose Run as administrator. On Windows 7, this is mandatory; on Windows 10 it is strongly recommended.
3. The tool scans the USB bus and lists every detected PC Adapter. The entry matching MLFB 6ES7 972-0CB20-0XA0 is the target. Click Select.
4. The tool shows the current firmware version and the target image version from the loaded *.hex. Confirm the target version is strictly greater than the current version.
5. Click Read to capture the current firmware block. The tool should successfully read back the existing image; this verifies the PROFIBUS-side link is alive. If Read fails, abort and re-check the 24 V supply.
6. Click Execute (or Program, depending on the tool revision). The progress bar advances through erase, write, and verify stages. Do not disconnect USB or 24 V during this window.
7. When the tool reports Update successful or Programming finished, power-cycle the adapter by unplugging USB for at least 5 seconds.
8. Reconnect and re-run the diagnostics in Set PG/PC Interface to confirm the new firmware string.

6.1 Firmware Update State Machine

Figure 2 – The READ step is the gatekeeper. If READ succeeds, EXECUTE almost always succeeds. If READ fails, the failure is on the power/24 V side, not on the tool side.

7. Error Matrix and Resolution

8. Firmware Version History (Confirmed 0CB20 Revisions)

Siemens has issued incremental firmware updates to address DP-V1 timing, USB enumeration on newer chipsets, and the SINAMICS Starter download handshake. The exact revision list is maintained on Siemens Support Entry 51806786. The field-proven highlights:

9. LED Status Indicators

The PC Adapter USB has two LEDs on the housing:

10. Use-Case Context: SINAMICS Drive Commissioning

A common reason to update the 0CB20 firmware is to fix download failures in Starter or Startdrive when commissioning a SINAMICS G120, S120, or V90. Symptom pattern: Go online works (drive answers PG/PC polling), but Download to drive aborts partway through parameter write. This is a known handshake timing issue on early 0CB20 firmware and is resolved by updating to the latest 1.x image referenced on Siemens Support 51806786.

11. Verification After Update

1. Open Set PG/PC Interface → PC Adapter (MPI/DP USB) → Diagnostics. Verify the firmware string in the device response now matches the target version.
2. From STEP 7, perform a CPU Accessible Nodes scan. The target CPU should appear with its MPI or DP address.
3. From Starter/Startdrive, repeat the SINAMICS Online → Download workflow. It must complete without abort.
4. Run a sustained DP cycle of 2–3 hours with a real slave to confirm no regression in bus stability.

12. Common Pitfalls and Field Tips

• Counterfeit adapters: third-party "PC Adapter USB" units often share the housing but use a different MCU. The Siemens firmware update tool will detect an unknown USB PID/VID and refuse to flash. Buy MLFB 6ES7 972-0CB20-0XA0 from authorized Siemens distributors.
• USB hub in the path: pass the USB cable directly to a rear-panel root port. Front-panel ports and hubs sometimes drop packets during the 60-second flash window.
• Long MPI cable: cables beyond 20 m with no repeater can corrupt the loader handshake. Keep the cable under 5 m for the firmware update, then restore the production topology after.
• Laptop on battery: aggressive power management on some laptops drops the USB 5 V rail during a long flash. Plug in the AC adapter.
• Multiple adapters plugged in: unplug any other USB-Serial or PC Adapter variants to avoid PID collisions in the loader.
• Antivirus / EDR: some endpoint security products block unsigned driver replacement. Temporarily disable real-time protection for the duration of the flash.

13. Quick Reference: Update Checklist