You connect a USB-to-Serial adapter to your laptop.

Windows detects it correctly.

Your software connects successfully.

Your Arduino board appears.

Your 3D printer controller responds.

Your industrial equipment communicates normally.

Then you disconnect the USB-C hub and reconnect it later.

Suddenly, the device is no longer available.

You check Device Manager and find:

Before:

COM3

After reconnecting:

COM7

Next time:

COM9

The adapter is the same.

The cable is the same.

The software configuration has not changed.

So why does Windows keep assigning a different COM port?

The answer is:

Windows does not identify a USB serial adapter only by the adapter itself. The USB connection path and device instance also affect COM port assignment.

This is why the problem appears more frequently when using:

• USB-C hubs;
• docking stations;
• different USB ports;
• extension cables;
• multiple USB serial devices.

Why Does Windows Change COM Port Numbers?

Many users expect:

“Same adapter = same COM port.”

However, Windows Plug and Play manages USB devices based on more than the physical hardware.

The system considers:

USB Device

↓

USB Hub

↓

USB Port

↓

Device Instance

↓

COM Port Assignment

When you connect the serial adapter directly to your laptop, Windows may create one device identity.

When you connect it through a USB-C hub, Windows may see another device path.

For example:

Direct Connection

Laptop
 ↓
USB Serial Adapter
 ↓
COM3

USB-C Hub Connection

Laptop
 ↓
USB-C Hub
 ↓
USB Port
 ↓
USB Serial Adapter
 ↓
COM7

The adapter did not change.

The connection identity changed.

Microsoft explains that Windows Plug and Play uses device identification information and device instances when installing and matching hardware drivers through the Windows device installation process.

First Check Whether Windows Created Multiple COM Devices

Before reinstalling drivers, check what Windows already remembers.

Open:

Device Manager

Then select:

View → Show Hidden Devices

Expand:

Ports (COM & LPT)

You may find entries like:

USB Serial Port (COM3)

USB Serial Port (COM7)

USB Serial Port (COM9)

Some entries may appear faded.

These are previous device instances created by Windows.

They do not automatically mean the adapter is damaged.

The important question is:

Is Windows creating a new COM assignment because the device path changed?

Check the USB Serial Adapter Identity

Right-click the serial device:

Properties → Details

Check:

Hardware Ids

Example:

USB\VID_1A86&PID_7523

This identifies the USB chipset.

Common USB-to-Serial chipsets include:

• FTDI;
• Silicon Labs CP210x;
• Prolific;
• CH340/CH341.

Then check:

Device Instance Path

This identifies the specific connection instance Windows created.

If:

Hardware ID stays the same

but:

Device Instance Path changes

after moving through the USB-C hub, the adapter itself is probably working correctly.

The USB topology is what changed.

Why USB-C Hubs Trigger COM Changes More Often

A direct USB connection has fewer variables:

Laptop
 ↓
Serial Adapter

A USB-C hub adds another layer:

Laptop
 ↓
USB-C Hub
 ↓
USB Port
 ↓
Serial Adapter

Windows now needs to identify:

• the hub;
• the hub port;
• the serial adapter;
• the complete USB device path.

Changing any of these may result in another COM assignment.

Common triggers include:

• plugging the adapter into another hub port;
• moving the hub to another laptop USB-C port;
• using another dock;
• connecting through a monitor hub.

For normal USB devices, this change may not matter.

For serial applications, it can break the workflow because many programs remember a fixed COM number.

Common Examples

Arduino Development

Your Arduino IDE is configured for:

COM3

After reconnecting:

COM7

The board appears disconnected.

The board is not necessarily faulty.

Windows simply assigned another serial port.

3D Printer Connection

Your slicer software remembers:

COM5

After reconnecting the USB-C hub:

COM8

The printer software cannot connect.

Again, the issue is mapping—not necessarily hardware failure.

Industrial Serial Equipment

Fixed COM assignments are often required for:

• PLC controllers;
• CNC machines;
• measurement devices;
• console cables.

A changing COM number can cause:

• connection failures;
• software configuration errors;
• automation interruptions.

For these systems, a stable USB connection path is often more important than maximum USB speed.

Keep the USB Connection Path Consistent

The simplest solution is also usually the most reliable.

Keep the same:

• USB-C hub;
• hub port;
• laptop port;
• USB serial adapter;
• cable.

For example:

USB-C Hub Port 1
        ↓
USB Serial Adapter
        ↓
COM3

Avoid moving the adapter between different USB ports.

Every new connection path gives Windows another opportunity to create a different device instance.

Assign a Fixed COM Port Manually

Windows allows you to manually choose a COM number.

Open:

Device Manager

→ Ports (COM & LPT)

→ Right-click USB Serial Port

→ Properties

→ Port Settings

→ Advanced

Choose your preferred COM number.

Example:

COM3

This can help when the device identity remains consistent.

However:

Manual COM assignment cannot completely prevent changes if Windows sees the adapter as a new device instance every time.

The best solution is still maintaining a consistent USB topology.

Remove Old COM Port Entries Carefully

If you see many old entries:

COM3
COM4
COM8
COM12

from the same adapter, you can clean unused device records.

Steps:

1. Open Device Manager.
2. Enable:

View → Show Hidden Devices

3. Remove only old inactive serial devices.

Do not remove:

• USB Root Hub;
• USB Host Controller;
• all USB devices.

The goal is to clean outdated COM mappings, not rebuild the entire USB system.

Check the USB Serial Driver

If the adapter repeatedly changes identity or shows errors, check the driver.

Open:

Device Manager → Driver

Review:

• Driver Provider;
• Driver Version;
• Device Status.

Use drivers from:

• adapter manufacturer;
• chipset manufacturer;
• Windows Update.

For example, FTDI provides official USB serial drivers for its USB-to-serial interface chips.

Microsoft also recommends using Windows Update or official manufacturer sources when updating device drivers through Device Manager.

Avoid third-party “automatic driver updater” tools.

Test Without the USB-C Hub

This is the fastest way to determine whether the hub is involved.

Test A

Laptop
 ↓
USB Serial Adapter

Test B

Laptop
 ↓
USB-C Hub
 ↓
USB Serial Adapter

Compare the results.

If the entire USB device disappears, rather than only the COM number changing, check the USB-C hub not recognized troubleshooting guide because the problem has moved from COM mapping to USB detection.

Use a Stable USB Hub Layout for Serial Devices

For development boards and USB serial equipment, a predictable USB layout is often more important than adding more ports.

A USB-C hub with four USB 3.0 data ports provides a simple setup for USB-A serial adapters and development accessories.

The LENTION CB-C31 includes:

• 4 × USB 3.0 data ports;
• up to 5Gbps USB transfer speed;
• up to 60W Power Delivery pass-through.

It is suitable for keeping devices such as:

• USB serial adapters;
• Arduino boards;
• development tools;

connected through a fixed hub port.

However:

The hub does not control COM numbering. Windows still manages device identity and COM assignment.

For setups using multiple USB-C peripherals, a USB-C hub with four 10Gbps USB-C data ports provides a higher-speed USB-C data environment.

The LENTION CE31s provides:

• four USB-C USB 3.2 Gen 2 data ports;
• up to 10Gbps data transfer;
• up to 100W PD pass-through.

It can be useful for development workstations that combine:

• USB-C devices;
• storage;
• multiple peripherals.

But:

Higher USB speed does not automatically keep COM ports stable. A consistent device path does.

Do Not Confuse COM Changes With Serial Communication Problems

These are different issues.

COM Port Changed

Example:

Software expects COM3
Windows shows COM7

The device works.

The mapping changed.

Serial Communication Failed

Example:

COM3 exists
Device connected
No data received

Now check:

• baud rate;
• parity;
• stop bits;
• application settings;
• device firmware.

Do not change serial communication settings to fix a COM assignment problem.

Quick Answers

Why does my USB serial adapter keep changing COM ports?

Windows assigns COM numbers based on device identity and connection path. Using different USB ports or hubs can create a new device instance.

Does the USB-C hub cause the problem?

Not always. The hub changes the USB topology, which can affect how Windows identifies the adapter.

How do I keep the same COM port?

Use the same hub port, clean old hidden COM entries, and manually assign a preferred COM number.

Why does Arduino IDE lose my board after reconnecting?

The board may have received a different COM number. Select the new COM port in Arduino IDE or stabilize the Windows mapping.

Will a faster USB-C hub fix COM changes?

No. COM stability depends on Windows device identity, not USB bandwidth.

Final Takeaway

When a USB-to-Serial adapter changes from:

COM3

to:

COM7

after reconnecting through a USB-C hub, the adapter is not necessarily failing.

The real chain is:

USB Device Identity

↓

Windows Device Instance

↓

COM Port Assignment

To keep serial devices stable:

Same hub

↓

Same USB port

↓

Same adapter

↓

Same software configuration

For Arduino, 3D printers, CNC controllers, and industrial serial devices, a predictable USB connection path is often more valuable than maximum USB speed.

Once Windows recognizes the adapter as the same device every time, your COM port mapping becomes stable.