Which USB Is COM1? Find COM Ports in Windows

The short answer: COM1 is a legacy name for the first serial port, and in modern Windows it is usually not a USB port at all — but when a USB-to-serial adapter is plugged in, Windows may assign it COM1, COM3, COM5, or any other number. To find which COM port a USB device is using, open Device Manager, expand “Ports (COM & LPT)”, and look for the device name with “(COM#)” at the end.

Why “COM1” Still Exists

COM ports date back to the original IBM PC in 1981. Back then, a computer had physical 9-pin or 25-pin serial connectors on the back, and the operating system named them COM1, COM2, COM3, and COM4. These were used for mice, modems, and printers. When USB arrived in the late 1990s, it replaced most serial connections, but the COM port naming convention stuck around because many industrial devices, barcode scanners, GPS receivers, and microcontrollers still use serial communication.

Today, a “COM port” is a logical name for a serial interface, not a physical connector. A USB-to-serial adapter creates a virtual COM port that Windows treats like the old hardware. The number assigned (COM1, COM7, COM23) is arbitrary and depends on what Windows decides is free.

How Windows Assigns COM Port Numbers

When you plug in a USB serial adapter, Windows checks which COM numbers are already in use. It then picks the lowest available number. For example:

• If COM1 and COM2 are free, a new adapter gets COM1.
• If COM1 is taken by a built-in serial port, the adapter gets COM3.
• If you plug the same adapter into a different USB port, Windows may assign a new number.

Windows stores a registry entry for each USB serial device and the port it was last assigned. If you unplug the device and plug it back into the same USB port, it usually gets the same COM number. But if you use a different USB port, it may get a new one.

How to Find the COM Port in Windows

Here are the exact steps for Windows 10 and Windows 11:

1. Press Windows + X and select Device Manager.
2. Expand the Ports (COM & LPT) section.
3. Look for a device like “USB Serial Port (COM3)” or “Prolific USB-to-Serial Comm Port (COM5)”.
4. If you don’t see it, unplug and replug the USB adapter and watch the list refresh.

If the device appears under “Other devices” with a yellow exclamation mark, the driver is not installed. You’ll need to install the correct driver from the chipset manufacturer (FTDI, Prolific, or Silicon Labs) before Windows can assign a COM number.

What If You Need a Specific COM Number?

Some software (like old CNC controllers or GPS mapping tools) only works with COM1 or COM2. You can change the assigned number in Device Manager:

1. Right-click the serial port and select Properties.
2. Go to the Port Settings tab.
3. Click Advanced….
4. Click the dropdown next to COM Port Number and pick an unused number.
5. Click OK and restart the software.

Be careful: if you pick a number already in use by another device, Windows will warn you. It’s safer to pick a high number like COM10 or COM20 to avoid conflicts.

Common Problems and Fixes

Here are the most frequent issues people run into:

• Device shows “Unknown Device” — the driver is missing. Install the chipset driver (FTDI, Prolific, CH340, etc.).
• COM port disappears when you unplug — that’s normal; the port only exists while the device is connected.
• COM number keeps changing — plug the adapter into the same USB port every time, or change the port number to a high fixed value.
• Software can’t find the port — check the software’s settings; some programs only show COM1-COM4 by default. Use the “Advanced” settings to set a low number.

USB vs. Native Serial Ports

Most modern computers have no physical serial port at all. If your motherboard has one, it’s usually a 9-pin header on the board, not a rear connector. That built-in port is often COM1. If you add a PCIe serial card, it might be COM2 or COM3. USB adapters then get the next free number.

If you need to know which physical USB port corresponds to which COM number, plug the adapter into each USB port and check Device Manager each time. There’s no built-in label, but you can rename the port in Device Manager to something like “USB Port 1” for clarity.

Alternatives to USB Serial Adapters

If you’re working with older equipment, you have a few options:

• USB-to-serial cable — cheapest, but driver quality varies.
• USB-to-serial adapter with FTDI chip — more reliable, but costs more.
• PCIe serial card — adds a real COM port internally, good for industrial use.
• Ethernet-to-serial server — lets you access a serial device over the network.

Each has trade-offs in speed, reliability, and cost. For most hobbyists, a USB adapter works fine.

Frequently Asked Questions

Is COM1 the same as USB?

No. COM1 is a legacy serial port name. A USB port is a different type of connection. A USB-to-serial adapter creates a virtual COM port that may be named COM1, but it’s still a USB device physically.

Why does my USB device show as COM3 instead of COM1?

Because COM1 and COM2 are often reserved for built-in serial ports or are already in use. Windows picks the lowest free number, so if COM1 is taken, it uses COM3.

How do I check which COM port a USB device is using?

Open Device Manager, expand “Ports (COM & LPT)”, and look for the device name with “(COM#)” at the end. You can also unplug and replug the device to see which entry appears.

Can I force a USB device to use COM1?

Yes, if COM1 is not in use. Go to Device Manager, right-click the port, Properties > Port Settings > Advanced, and select COM1 from the dropdown. If COM1 is already used, Windows will show a warning.

Why does my COM port disappear when I unplug the USB?

Because the virtual COM port only exists while the USB device is connected. When you unplug it, Windows removes the port. The driver stays installed, so it will reappear when you plug it back in.

What if my device doesn’t show up in Device Manager at all?

Check the USB cable and try a different USB port. If it still doesn’t appear, the driver may not be installed. Look under “Other devices” for an unknown device and install the correct driver from the chipset manufacturer.

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