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You’ve connected a precision gage—maybe a digital scale, a DRO, a height gage, or another shop-floor device—to a Windows PC and expected measurements to start flowing into your software. Instead, your data collection program (or terminal utility) is waiting for one detail: which “COM Port” to use.
This guide explains what a COM port is and why many USB devices present themselves as virtual serial ports or virtual COM ports. It shows how to find the correct COM port number in Windows Device Manager, then test it with ComTestSerial by matching communication parameters (baud rate, data bits, parity, stop bits). You’ll learn to interpret results—successful opens, readable replies, garbled characters, or “port in use” errors—and adjust settings accordingly. Finally, you’ll apply the verified COM port and parameters in your software to establish a reliable connection.
Note: If you’re looking to connect your device to a program such as Excel or a cloud-based application there is an easier way. Read our guide How to Interface with RS-232 Devices.
You’ve been asked to choose a ‘COM port’ and have no idea which one is right. The best way to think of a COM port is as a dedicated digital mailbox. Short for “Communication Port,” it’s a private channel your computer uses to exchange information with one specific device at a time, ensuring messages don’t get mixed up with other connected devices.
But why does your modern USB device need this old-sounding port? For compatibility. A special piece of software called a virtual serial port driver—such as the FTDI driver or the serial.sys driver MicroRidge utilizes—creates a virtual COM port for your USB device. A virtual COM port is created because the device is utilizing USB communication and not an RS-232 connection through a connector such as a DB9. This clever trick lets your new device talk to a wide range of serial communication software or any serial communication tool that expects this classic, reliable connection method.
That number you see in the list, like ‘COM3’ or ‘COM4’, is simply the address Windows assigned to that virtual mailbox. The key to getting your device working is finding out which specific port number belongs to it.
Before you can test a serial port, you need to find out which “mailbox number” Windows assigned to your device. This number can change each time you plug your device into a different USB port. Windows has a built-in tool called Device Manager that tells you exactly what you need to know.
Device Manager also acts as a quick com port checker when you’re verifying which device maps to which port.
Now that you have your port number, you need a program to see if it’s actually working. This is where a COM port tester comes in. Think of it as a digital walkie-talkie; it’s a simple piece software that lets you send a message like “Hello World” or a serial command such as “<A” out through the port and listen for a reply. It’s the fastest way to confirm your communication channel is open before you go back to your main software.
To make a connection, there’s one critical setting you must match: the communication parameters. Devices communicating on a COM port must be configured with matching communication parameters on both ends, so the software can correctly interpret the data coming from the device. Think of it as the “language,” and both sides of the connection must be speaking the same language. The key settings are baud rate (bit timing), data bits (typically 7 or 8), parity (none/even/odd as a basic error check), and stop bits (usually 1 or 2) that mark the end of each character frame. A common default you’ll see in gaging and industrial devices is 9600-N-8-1 (9600 baud, no parity, 8 data bits, 1 stop bit). The communication parameters can be found in your device’s manual; if you can’t find the communication parameters, don’t worry—you can start at 9600-N-8-1 and work your way to the correct values.
When choosing the best free serial port tester, you can skip the PuTTY vs ComTestSerial debate. We’ll use ComTestSerial. It’s an excellent serial com port tester for precision gages and industrial equipment because its straightforward, no-frills interface makes it easy to select a COM port, set the key parameters, and start sending/receiving data without writing a line of code or digging through advanced terminal options. That simplicity speeds up com port troubleshooting and reduces setup mistakes, especially in shop-floor measurement workflows.
Download ComTestSerial (free serial COM port testing software for Windows)
You’re ready for the moment of truth. Let’s use a COM port tester to open a direct line to your device. While each interface has many options, we only need to focus on a few specific settings to see if your connection works.
Follow these exact steps (example with ComTestSerial, a lightweight com port utility):
Seeing a stream of jumbled characters from the device like {\x9f}b${\xa4}? Believe it or not, this is good news. It means the COM port is open and you can monitor serial port data traffic, but your communication parameters are wrong. Select “Serial Port” on the bottom right of the ComTestSerial main page and try another common set of communication parameters, such as 4800-E-7-1, 19.2k-N-8-1 or 115.2k-N-8-1.
If you’re seeing the black window which means your device is connected but you can’t get a response or you’re still getting a response of jumbled characters from your device, follow this trail and error method to find the correct communication parameters.
Send a command to the device after each change of the following communication parameters.
Keep in mind that you should be able to fix a jumbled response with the correct communication parameters. If you’ve exhausted all common comm parameter combinations and still can’t resolve the garbled response, the issue may be a pinout mismatch rather than a parameter mismatch — a ProAdapter allows you to remap any RS-232 DB9 signal without soldering to test alternative wiring configurations. If you’re unable to fix the jumbled response or you still can’t get any response, reach out to the manufacturer of the device and inquire about the communication parameters and supported commands. It is very possible that you’re sending a command that the device doesn’t recognize and won’t respond to.
If you saw the blue window and instead got an “Unable to open serial port xxx. This port is not installed or is already in use.” error, another program has likely claimed the port. To fix this error, close any other software that communicates with your device (like its official app) and try again. Tools like ComTestSerial can be handy for quick com port diagnostics during this step.
You started with a device that wouldn’t connect, but now you have its digital address (the COM port) and its language (the communication parameters). By testing the connection, you’ve confirmed the hardware is working and turned a frustrating mystery into a solvable puzzle.
Your final step is to take this information back to your original software. Open its connection settings and enter the exact COM port and communication parameters you just verified. Your device should now connect successfully.
You haven’t just fixed one problem; you’ve learned a repeatable skill. This simple “Find, Test, Apply” process with a COM port tester—whether you think of it as a serial port tester or an RS-232 COM port tester—gives you a practical framework for COM port troubleshooting with any gadget, empowering you to solve the problem yourself.
Riley Tronson is President and owner of MicroRidge Systems, a role held since 2023. Riley brings a strong technical foundation to leadership in measurement solutions. An experienced entrepreneur, Riley has founded and grown multiple software companies, including a venture focused on developing iPhone applications, blending engineering expertise with innovative product development.
2 Responses
Great article
Thank you! Glad you found it helpful. If you want to learn more about the RS-232 protocol and how to get data into cloud-based software or programs like Excel, check out our guide on How to Interface with RS-232 Devices.