Serial Port Testing Guide

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Updated on: October 5, 2026
Time to read: 6 minutes

Ways to check a port’s health include hardware COM port testers, null-modem connection, loopback connectors, or even something as simple as a paperclip. In most cases, all you need is just one computer and maybe a few serial cables. Learn how to run each test and maximize the accuracy of the data you get.

Table of Contents

Try Serial Port Test Software

Before you start testing, consider trying our serial port testing software – Serial Port Monitor. This app has numerous display modes to get the most out of testing data. It allows you to save logs and compare previous sessions. Multiple ports can be tracked at the same time.

Serial Port Monitor is free to try – just download the app, and you can use it for 14 days, with all the port tracking features. However, by purchasing the Pro license, you can also start using the Advanced Send Dialog – a dialog that allows you to send data in any format, directly send files, and repeat messages at a fixed interval. This will provide you with the full set of tools for data emulation. In addition, you will be able to repeat any messages sent during a previous session – the ultimate form of automated testing!

Serial Port Monitor is compatible with both older and newer versions of Windows, starting with Vista and all the way up to Windows 11, and additionally supports Windows Server 2008-2022 and ARM Windows.

How to Use Our Serial Port Testing Software

1. Download and install Serial Port Monitor.

Serial Port Monitor being installed

2. Start a new session from the menu bar.

Starting a new session from the Session menu

3. Set up the session by choosing all necessary vizualizers.

New session options

4. Send a message or multiple messages to the port that is being tracked. In this case, we will use the “echo” command.

Sending an echo to the port

5. Check the output of the chosen vizualizers.

Output from Serial Port Monitor

6. Save the session from the menu bar so you can re-check and compare logs later.

Saving the Serial Port Monitor session

Port Loopback Testing

Serial port testing via a loopback connection allows you to check the functionality of its data line and signal lines without the need for additional devices or ports. In essence, the port will be communicating with itself, because of its TX-RX (transmit – receive) lines being connected.

Port loopback connection diagram for RS-232 and RS-422

The image above shows which contacts or pins need to be shorted out. The thinner lines are optional, since they connect lines used for handshaking. Note that the female interfaces – the ones with a plastic inset in the shield – are easier to test this way, since the pin holes can be connected with a paperclip or a piece of wire. On the other hand, male interfaces will need a jumper wire, or for the tester to hold a screwdriver up to the necessary pins – which will also disrupt the test if the screwdriver slips.

A successful outcome for a port loopback test is receiving the same message that you’ve sent. It’s preferable to send a lot of data over a span of time, to stress-test the port and catch any possible defects that won’t show up with a single send.

How to Run a Port Loopback Test

1. Find out the type of serial port that you’re testing.

2. Connect the data lines, and, optionally, handshake lines, according to the diagram.

3. Open Serial Port Monitor and start tracking the port.

4. Send a message to the port and check the response.

Host Loopback Testing

An average null-modem cable

A host loopback test can be ran if you have two serial ports on your motherboard, or a USB-to-serial adapter. There are two benefits – first, you can track the input and the output separately, and send messages both ways, to test the ports separately. Second, you won’t need to fiddle with household objects and risk damaging the port.

For most computers and adapters, you will need a female-to-female null-modem cable, as pictured above.

Hardware COM Port Testers

An OONO breakout tester

In cases where the port doesn’t belong to a computer, or a computer is inaccessible, it may be necessary to use a hardware serial tester, also known as a breakout tester. These devices can be inserted between two serial devices and will show which lines are active. Additionally, more complicated devices (like the OONO F-1023 pictured above) allow lines to be blocked, and provide a pin hole interface for direct connection.

The role of such a device can also be fulfilled by portable microcontrollers like Arduino. However, specialized devices have been optimized to run off of the power provided by signal lines, and thus don’t require additional external power. On the other hand, a microcontroller can be programmed to perform more intricate and specific tests, in addition to being upgradeable with WiFi capability for remote testing.

FAQ

RS232 ports can break down as a result of some negative impacts or misuse. Unfortunately, it’s not always obvious if the problem is with the COM interface or with the external device attached to it.

A handy COM Port Test program can help you check the operability of any COM port and prevent any possible problems.

✔ Versatility. The software can be applied in a great variety of usage scenarios. It’s able to monitor, read, record, save, and analyze serial data going through a specified COM port or several ones at a time. Plus, the app will allow you to emulate sending data to the required port in different formats (string, binary, octal, decimal, hexadecimal, or mixed).

✔ Reliability. Serial Port Tester is a stable utility that works equally well with all versions of Windows OS, including Windows 10 (both x32 and x64). What’s great is that all internal drivers of the app are digitally signed.

✔ The ease of use. The developers of the program have provided the app with an intuitive GUI which makes the process of COM port testing as simple and efficient as possible.

A loopback test is a common way to troubleshoot RS232, RS422, and RS485 communications. It releases you of the need to connect to additional hardware in order to identify issues with a serial port, cable, or application generating requests. With the help of a loopback test, you can simulate a complete communications circuit. All that is required to do is temporarily connect the proper pins to allow signals to be sent and received on the same COM interface.

Simply follow these steps to check serial ports:

“Control Panel > Device Manager > Ports” Your system’s COM ports will be displayed.