How to Test a COM Port in Windows

Open Device Manager: press Windows+X and choose Device Manager (or right-click Start). Expand Ports (COM & LPT). The device name shows as COMx (for example, COM3). If you don't see Ports, check Universal Serial Bus controllers or Other devices for unknown entries, install drivers, and then recheck.

If you need to know how to check the COM port in Windows from the command line, open Command Prompt or PowerShell and use commands like "mode" to display active serial ports. "wmic path win32_serialport get deviceid,caption" to list port names and descriptions, or in PowerShell run:

Generic COM port devices:

Get-CimInstance Win32_SerialPort | Select-Object DeviceID,Caption

to see all COM ports and their details on Windows 10 or Windows 11, including temporarily unavailable ones:

Get-PnpDevice -FriendlyName '*COM*' | Select-Object FriendlyName,InstanceId

You can use the free COM Port List utility to list all COM ports on the system in sorted order. It also lets you view port descriptions or display only USB-COM ports. Unpack the ZIP archive and start it with the following command line parameters.

Usage:
 comportlist.exe [/usb] [/desc] [/hwid]
  /usb - output USB-COM devices only
  /desc - output COM port description
  /hwid - output hardware ID for USB-COM devices

Use the Command Prompt to run the mode command and test basic port configuration. Steps: Open Command Prompt as administrator and type mode COM1 (replace COM1 with your port). The output shows baud rate, parity, data bits, and stop bits. To check if the port opens without errors, you can also run type \\.\COM3 (may block if nothing sends data) or use echo to send:

echo test > \\.\COM3. 

If no error is returned, Windows opened the port successfully. This method also works with high port numbers like COM103.

A loopback test checks that the port can transmit and receive. Perform these steps:

1. Power off the device and connect a loopback plug by joining TX (transmit) to RX (receive) for the port pins (for RS-232, connect pin 2 to pin 3 on a DB9).
2. Open a terminal program (PuTTY, Serial Port Monitor, or Windows PowerShell).
3. Configure the session to the correct COM port and serial settings (baud, parity, data bits, stop bits).
4. Type characters; if you see them echoed back, TX and RX work.
5. Remove the loopback before reconnecting the external device.

Download and open PuTTY, select the Serial mode as the connection type, enter the COM port name (e.g., COM3), and set the correct speed (baud) and other serial parameters. Click Open. If the terminal opens without errors, the port is accessible. To verify data flow, use a loopback or a known serial device that sends data, or a null-modem cable to check two ports at once. PuTTY will display incoming bytes and let you send bytes to the device.

PowerShell can open, read, and write serial ports using .NET classes. Example snippet to open COM3, send text, and read response (copy/paste it as a single line):

$port=New-Object System.IO.Ports.SerialPort 'COM3',9600,'None',8,one; $port.Open(); $port.WriteLine('hello'); Start-Sleep -Milliseconds 200; $resp = $port.ReadExisting(); $port.Close(); $resp

Replace COM# or parameters as needed. For a simple existence check, you can list available ports (properly detected, without driver errors) with:

[System.IO.Ports.SerialPort]::GetPortNames()

The command output will include COM ports recognized by Windows OS.