Siemens 3UF7946-0AA00-0 USB-Serial Adapter: Configuration, Drive Compatibility, and Replacement Path

The Siemens 3UF7946-0AA00-0 USB-to-Serial adapter is a hardware bridge between the USB port of a modern PC and the legacy RS232 service interface exposed by SIMOCODE motor management devices, 3RK3 Modular Safety System (MSS) central units, and SIRIUS 3RW soft starters. Engineers who still maintain, parameterize, or commission these systems from a Windows 7 Professional 64-bit workstation rely on the 3UF7946-0AA00-0 as the only sanctioned path to a serial service connection when no native DB9 port is available on the PC. This reference documents the product's MLFB decoding, the underlying USB-to-serial conversion principle, drive-system compatibility, RS232 pinout mapping, the Windows 7 64-bit driver installation procedure, the engineering software connection parameters, a field commissioning workflow, the August 2012 replacement 3UF7941-0AA00-0, and a detailed troubleshooting matrix.

Overview

The 3UF7946-0AA00-0 is a passive protocol-conversion dongle that encapsulates a USB device controller on the PC side and a full RS232 level translator on the device side. It is the official Siemens solution for engineers who already own a conventional PC-to-device RS232 service cable (for example, the 3UF7940-0AA00-0 PC cable for SIMOCODE) and need to attach that cable to a USB-only notebook or desktop. The adapter is delivered with a mini-CD containing the Windows driver; the driver is mandatory because Windows 7 64-bit does not ship with a native in-box driver for the converter chipset used by the Siemens adapter.

Field experience places the 3UF7946-0AA00-0 in two configurations. The first is as a stand-alone USB-to-DB9 male adapter attached to a conventional PC-to-device RS232 cable. The second is as part of a service-cable kit supplied with a specific Siemens product. The first configuration remains the more common because it allows the existing RS232 cable inventory to be retained when an engineer's PC is upgraded to a USB-only chassis. The second configuration is more reliable because the integration is sealed at the factory and the cable-to-adapter transition is not exposed to mechanical stress.

For engineers who have not yet purchased a service-cable inventory, Siemens introduced the 3UF7941-0AA00-0 in August 2012. The replacement integrates the USB-to-Serial conversion and the device-side RS232 cable into a single assembly, removing one of the most common point-of-failure in service connections. The two adapters coexist in the Siemens catalog and both remain supported.

Product Identification and MLFB Decoding

The Siemens MLFB (Machine-Readable Product Designation) is the only reliable way to order the correct adapter. The 12-digit order number 3UF7946-0AA00-0 decodes as follows:

The replacement adapter is identified by the source as "3UF79410AA000" without hyphen separators. Following the standard Siemens MLFB convention, the canonical form is 3UF7941-0AA00-0. Confirm the exact order code against the Siemens Industry Online Support portal before placing the order, because the unhyphenated form is ambiguous and could be misread as 3UF79-410AA000 or similar.

USB-to-Serial Conversion: Functional Principle and Chipset Background

A USB-to-serial adapter is a small protocol converter that translates USB data signals to and from a serial communications standard, in this case RS232. The device contains two functional blocks: a USB device controller on the PC-facing side, and a UART-to-RS232 line driver on the device-facing side. A microcontroller on the adapter bridges the two blocks, presenting a virtual COM port to the host operating system and providing true RS232 voltage levels to the downstream device. This class of adapter is sometimes referred to in industry as a "USB CDC adapter," a "USB-to-UART bridge," or simply a "serial dongle." Reference: USB-to-serial adapter (Wikipedia)

The 3UF7946-0AA00-0 enumerates on Windows 7 64-bit as either a USB Communications Device Class (CDC) device or as a vendor-specific USB-to-UART bridge, depending on the converter silicon revision shipped in a given production batch. The most common chipsets historically used in this class of adapter are:

• FTDI FT232R / FT232H — robust, well-supported, signed drivers available directly from FTDI. Typically the most reliable option on Windows 7 64-bit.
• Silicon Labs CP210x — common in industrial adapters, signed VCP drivers available from Silicon Labs. Compatible with Windows 7 64-bit without additional configuration.
• Prolific PL2303 — historically popular but has had driver-signing issues on Windows 7 64-bit; the HXA / TA / RA / SA chip revisions are signed, while the older HXD / HX / TA revisions may not enumerate correctly.
• WCH CH340 — low-cost Chinese-sourced chipset; driver signing varies by manufacturer. Not generally used in Siemens-branded adapters.

Each chipset requires a dedicated driver. Windows 7 64-bit will display a yellow warning triangle in Device Manager if the driver is not present, or if the wrong driver is loaded for the installed chipset revision. The driver supplied on the 3UF7946-0AA00-0 mini-CD is matched to the specific chipset revision that was installed in the adapter at the time of manufacture, which is why field engineers should not arbitrarily replace it with a generic driver from the chipset vendor's website unless the CD is lost or the existing driver is known to be corrupt.

On the device side, the adapter exposes a male DB9 (DE-9) connector with full RS232 levels. The exact voltage swing depends on the converter IC's charge-pump design; FTDI parts typically produce ±5 V to ±9 V swing, while some industrial-grade adapters produce ±12 V. Either is compatible with the Siemens service interface. The adapter does not perform any protocol translation - it is a transparent byte pipe. All SIMOCODE, 3RK MSS, or 3RW protocol framing is generated by the host-side engineering software (SIMOCODE ES, MSS ES, Soft Starter ES) and passed through to the target device unchanged.

Because the conversion is at the electrical level only, the adapter does not appear as a network node, does not require an IP address, and is not subject to firewall rules. It does, however, occupy a COM port number on the PC. Engineers should record which COM port Windows assigns to the adapter (visible in Device Manager under "Ports (COM & LPT)") and select that port in the engineering software's connection dialog.

RS232 Electrical Characteristics Relevant to the Adapter

RS232 is a single-ended, unbalanced voltage-mode signaling standard defined by the TIA (Telecommunications Industry Association). The relevant electrical parameters for service-cable use with SIMOCODE, 3RK MSS, and 3RW soft starters are summarized below. The 3UF7946-0AA00-0 implements the standard DTE (Data Terminal Equipment) side of the interface, with the same voltage swings and pinout as a built-in IBM PC DB9 serial port.

RS232 is not isolated and not differential. The standard is intended for short-distance, point-to-point connection between equipment that shares a common ground reference. The 3UF7946-0AA00-0 is therefore suitable for the laboratory and cabinet-internal connections that characterize SIMOCODE and 3RW commissioning, but it is not suitable for cross-building or cross-ground-plane runs where ground potential differences could exceed the receiver's common-mode range.

Compatible Siemens Drive and Motor Management Systems

The 3UF7946-0AA00-0 is qualified by Siemens for service access to the following product families. The source explicitly identifies three: SIMOCODE, 3RK MSS, and 3RW soft starters. Engineers planning to use the adapter with other SIRIUS devices (for example, 3RT contactor diagnostic interfaces, 3RV motor protector circuit breakers with communication modules, or 3RA load feeders) should verify against the specific device's manual before purchase.

Each target system uses the adapter differently:

• SIMOCODE pro / pro V: the service interface is a female DB9 null-modem connector on the basic unit or on the operator panel. SIMOCODE ES (now integrated into TIA Portal as a HSP) reads the parameter set, performs firmware updates on the basic unit, and writes configuration data over this interface. Service access is typically performed at 19200 bit/s, 8 data bits, no parity, 1 stop bit.
• 3RK3 MSS: the central unit's service interface is reached through a PC cable specific to the 3RK3 family. MSS ES performs the safety logic configuration and diagnostic readout. The bit rate is typically 19200 bit/s with the same 8/N/1 framing.
• 3RW Soft Starters: the 3RW40 and 3RW44 expose a serial service interface for Soft Starter ES. Parameter download, diagnostic log retrieval, and firmware updates are the supported operations. Some 3RW variants also support 9600 bit/s as a fallback; consult the device manual for the exact supported rates.

The adapter is not a replacement for PROFINET or PROFIBUS communications modules - it is exclusively a service and parameterization interface. Live control of the drive or motor management system over the USB-to-Serial adapter is not supported. Only commissioning, parameter download, diagnostic readout, and firmware update operations are within scope.

Windows 7 Professional 64-bit Driver Installation

Windows 7 64-bit (Pro, Enterprise, Ultimate editions) does not include a native in-box driver for the converter chipset used by the 3UF7946-0AA00-0. The driver must be installed before the adapter can enumerate a COM port. The original 3UF7946-0AA00-0 ships with a small CD containing the driver; on modern PCs without an optical drive, the driver can typically be retrieved from the Siemens Industry Online Support portal under the product's entry, or from the converter IC vendor's website for the matching chipset.

Prerequisites

• Local administrator rights on the Windows 7 workstation.
• The 3UF7946-0AA00-0 product CD, or a copy of the driver package.
• A free USB 2.0 or USB 3.0 port. USB 3.0 ports are backward compatible but may enumerate the device on an internal USB 2.0 hub; if enumeration is unstable, plug the adapter directly into a USB 2.0 port.
• Any RS232 PC cable that was previously used with a built-in DB9 serial port.
• The engineering software (SIMOCODE ES, MSS ES, or Soft Starter ES) installed and licensed.

Installation Steps

1. Insert the supplied CD. If autorun is disabled, browse to the CD root and run the setup executable.
2. Allow the installer to copy the driver package to %ProgramFiles% or %ProgramFiles(x86)% as appropriate.
3. Connect the 3UF7946-0AA00-0 to a USB port on the PC.
4. Windows will detect the new hardware and complete the driver install. A notification balloon should confirm "Device is ready to use."
5. Open Device Manager > Ports (COM & LPT) and note the assigned COM port (for example, COM3, COM4, COM5, etc.). This is the port you will select in SIMOCODE ES, MSS ES, or Soft Starter ES.
6. If the device is recognized as an "Unknown device" rather than a COM port, right-click it, select Update Driver Software, and point the wizard to the directory where the setup program installed the driver package.

Driver Signature Enforcement on Windows 7 64-bit

Windows 7 64-bit enforces driver signature enforcement by default. Unsigned drivers are blocked from loading, and a warning dialog appears. The driver supplied on the 3UF7946-0AA00-0 CD is signed by the chipset vendor (or by Siemens) and will load without intervention. If you are installing a generic driver retrieved from the chipset vendor's website, verify that the package is the signed release for Windows 7 64-bit before installing.

Verifying the Installation

After installation, open Device Manager and confirm the following:

1. The 3UF7946-0AA00-0 appears under Ports (COM & LPT) with a name that includes the chipset identifier (for example, "USB Serial Port (COM3)" or "FTDI USB Serial Converter (COM3)").
2. No yellow warning triangle is displayed on the device entry.
3. Right-clicking the device and selecting Properties > Driver shows the driver provider, date, and version, and the digital signature is valid.

Once these checks pass, the COM port is ready for use by the engineering software.

PC-Side Cabling and RS232 Interface Mapping

The 3UF7946-0AA00-0 terminates in a male DB9 (DE-9) connector with the standard IBM PC pinout (DTE). When paired with a Siemens RS232 PC cable, the cable is typically a null-modem (crossover) cable because the target device's service port is also a DTE, and two DTE ends require a null-modem interconnect. Engineers should not assume that any off-the-shelf DB9 cable will work. The Siemens service cables are pre-wired for the null-modem configuration required by SIMOCODE, 3RK MSS, and 3RW soft starters. A straight-through cable will not produce a working connection.

Service tools for SIMOCODE, 3RK MSS, and 3RW soft starters typically run at 19200 bit/s, 8 data bits, no parity, 1 stop bit (19200/8/N/1) or 9600 bit/s with the same framing. Hardware flow control (RTS/CTS) is not used by these service protocols - software (XON/XOFF) is the default, and some tools use no flow control at all. Match the bit rate in the engineering tool's connection settings to avoid framing errors.

Engineering Software Connection Parameters

Each engineering software tool exposes a connection dialog where the COM port and serial parameters are configured. The following table summarizes the typical settings for the three target systems. Always verify against the current release of the engineering software - the exact dialog labels can change between TIA Portal versions.

If the engineering software offers an "Auto-detect" or "Search" option, it can be used to scan available COM ports and identify the one connected to a live target device. This is the recommended approach for first-time commissioning because it eliminates the manual COM-port-selection step.

Field Commissioning Procedure

The following procedure walks an engineer through first-time commissioning of a SIMOCODE, 3RK MSS, or 3RW soft starter over the 3UF7946-0AA00-0 adapter on Windows 7 Pro 64-bit. Adapt the engineering-software step for the specific target device.

1. Install the driver per the procedure above and record the assigned COM port.
2. Power down the target device (SIMOCODE, 3RK MSS, or 3RW soft starter) before making any cabling changes.
3. Connect the 3UF7946-0AA00-0 to a free USB port on the PC.
4. Connect the Siemens RS232 PC cable between the adapter's DB9 connector and the target device's service port.
5. Power up the target device and wait for the device to complete its self-test (typically 3 to 5 seconds for SIMOCODE, 1 to 2 seconds for 3RK MSS and 3RW soft starters).
6. Launch the engineering tool (SIMOCODE ES, MSS ES, or Soft Starter ES).
7. In the connection dialog, select the COM port noted in step 1. Set bit rate to 19200 bit/s (or 9600 if the target device requires it), 8/N/1 framing, no hardware flow control.
8. Click "Connect," "Go Online," or the equivalent toolbar action in the engineering tool. The target device's parameter set should load within a few seconds.
9. Once online, perform the desired parameter download, diagnostic readout, or firmware update.
10. Disconnect from the engineering tool ("Go Offline" or "Disconnect") before unplugging the adapter. Abrupt removal can corrupt the parameter set on the target device.

Verifying the Online Session

After the online session is established, perform the following quick checks to confirm the connection is healthy:

• Read the device's firmware version. The value should match the version stamped on the device's nameplate or in the device's documentation.
• Read one parameter from the device and write the same value back. The engineering tool should report a successful write without raising a fault.
• Trigger a diagnostic readout. The engineering tool should return a populated diagnostic buffer rather than a timeout.

If any of these checks fail, the connection is not healthy and the troubleshooting matrix in the next section should be consulted before proceeding with live parameterization.

Replacement Path: 3UF7941-0AA00-0 and Side-by-Side Comparison

Siemens introduced the 3UF7941-0AA00-0 in August 2012 as the recommended replacement for the 3UF7946-0AA00-0 in new installations. The replacement integrates the USB-to-Serial conversion and the device-side RS232 cable into a single assembly, eliminating the separate RS232 PC cable and the associated failure mode (typically a broken conductor at the DB9 overmold, or a missing cable in a service kit). The source notes that the new adapter is qualified for direct device-to-PC connection to SIMOCODE, 3RK MSS, and 3RW soft starters without any intermediate cable.

Engineers who are still in the buying decision should order the 3UF7941-0AA00-0. Engineers who already own the 3UF7946-0AA00-0 plus the matching RS232 PC cable should retain the existing hardware - both adapters remain in the Siemens catalog and are supported - unless the intermediate cable has failed and a replacement is not immediately available.

Industrial USB-to-Serial Converter Comparison

For applications that require isolation, wider operating temperature, or PROFIBUS / PROFINET integration, third-party industrial USB-to-serial converters are available. These products extend the basic 3UF7946-0AA00-0 functionality with features such as 2.5 kV isolation, screw-down mounting, and IP20 / IP30 protection ratings. Reference: Advantech industrial USB-to-serial converters The 3UF7946-0AA00-0 does not provide isolation; for cabinet-internal commissioning in a controlled environment, this is acceptable. For field commissioning in industrial environments with high electrical noise, an isolated converter is the safer choice.

Generic USB-to-serial adapters are widely available from commercial vendors, including FT232- and CP2102-based units sold through general electronics retailers. Reference: Commercial serial-to-USB adapter offerings These can serve as a fallback if the 3UF7946-0AA00-0 is not available, but they are not Siemens-qualified for SIMOCODE, 3RK MSS, or 3RW service use. Field engineers should treat them as a temporary workaround and replace with the official Siemens adapter for permanent installations.

Known Limitations and Operational Caveats

The following caveats are derived from the field report and from the documented behavior of the underlying converter chipsets. They are not exhaustive; field engineers should always test the connection before scheduling a critical parameterization.

• Driver signature enforcement: Windows 7 64-bit will reject unsigned drivers. The supplied CD contains a signed driver; do not attempt to use a generic driver from the converter IC vendor's website unless you have verified it is signed and tested with the Siemens adapter.
• USB power management: Windows can suspend the USB port to save power, which drops the COM port and terminates the engineering session. Disable USB selective suspend in Power Options if the adapter disconnects during long parameter downloads. This is the single most common cause of mid-session communication failures.
• Maximum cable length: RS232 is specified to 15 m at the rated bit rate. Service cables shipped with Siemens products are typically 3 m to 5 m. Do not extend with passive couplers; use a lower bit rate or a powered extender if longer runs are required.
• No electrical isolation: The 3UF7946-0AA00-0 shares the PC ground with the target device's ground. Ground potential differences greater than a few volts can damage the adapter or the device. Use an isolated USB-to-serial converter if ground loops are a concern.
• Live control not supported: The adapter is a service tool, not a control bus. It is not suitable for real-time drive control and should not be wired into a permanent installation. The control bus for SIMOCODE is PROFINET or PROFIBUS; for 3RW soft starters, it is the analog input or fieldbus module.
• Single-device only: The 3UF7946-0AA00-0 implements a point-to-point link. Multi-drop service over RS232 is not supported by the underlying Siemens service protocols.
• Hot-plug the device side is not supported: Always power down the target device before connecting or disconnecting the RS232 cable on the device side. Hot-plugging RS232 is not specified by the standard and can produce transient voltages that exceed the receiver's common-mode range.

Troubleshooting Matrix

Verification Checklist

Before relying on the 3UF7946-0AA00-0 for a critical parameterization, run the following checks. Each line must pass before proceeding to live commissioning work.

• Driver installed; COM port visible in Device Manager with no warning triangle.
• Driver Properties show a valid digital signature.
• COM port number recorded and entered in the engineering tool's connection dialog.
• Bit rate, data bits, parity, and stop bits set to 19200/8/N/1 (or 9600/8/N/1 if required by the target device).
• Hardware flow control disabled in the engineering tool.
• USB selective suspend disabled in Windows Power Options.
• RS232 PC cable is the correct null-modem type for the target device family.
• Test connection to a known-good target device before going to the live one.
• Engineering tool's online session is closed before physically unplugging the adapter.

Frequently Asked Questions