COM port on a laptop: how to connect legacy equipment via USB in 2026

Modern laptops have long lost the classic COM port (RS-232) - the same 9-pin connector that has been used for decades to connect industrial equipment, old printers, program microcontrollers or configure network devices. However, many professionals are still faced with the need to work with legacy systems, where USB or Ethernet don't fit. In this article we will look at how to return functionality COM port on a laptop using adapters, what nuances are important to consider when choosing a device, and how to avoid common mistakes when setting up.

The problem is complicated by the fact that not all adapters are equally reliable: cheap Chinese models often emulate signals incorrectly DTR/DSR, due to which the equipment may not be detected or malfunction. We tested 12 popular adapters (from FTDI, Prolific, Silicon Labs and other chips) and identified key selection criteria. Also in the article are current drivers for Windows 10/11 And Linux, connection diagrams for typical scenarios (for example, firmware Cisco or work with PLC-controllers), and instructions for diagnosing problems through Control Panel → Device Manager.

What is a COM port and why is it still needed?

COM port (or serial port RS-232) is the interface for asynchronous data transfer, which appeared back in the 1960s. Despite the antiquity of the technology, it is still in demand in:

• 🔧 Industrial automation: connection PLC-controllers (Siemens S7-200, Allen-Bradley), sensors, relays.
• 🖨️ Old printers and cash registers: cash registers Athol, Shtrikh-M, dot matrix printers Epson LX-300.
• 📡 Network equipment: setting Cisco, MikroTik, Huawei via console cable.
• 🤖 Robotics and microcontrollers: firmware Arduino, STM32, debugging via UART.
• 🚗 Automotive diagnostics: connection to ECU through K-Line (for example, for VAG-COM).

The main advantage of the COM port is reliability of data transmission over long distances (up to 15 meters without amplifiers) and resistance to interference, which is critical for industrial conditions. However, modern laptops are not physically equipped with such a connector, so the only way out is to use USB-to-COM adapters.

Types of USB-to-COM adapters: which one to choose

All adapters are divided into two categories: with real COM port emulation (based on chips FTDI, Silicon Labs CP210x) and with simplified emulation (cheap Chinese chips, for example, CH340). The difference is critical when working with equipment sensitive to flow control signals (RTS/CTS, DTR/DSR).

For critical tasks (for example, firmware Cisco IOS or work with Siemens S7) we recommend adapters for FTDI FT232R or Silicon Labs CP2104. If you need a budget option for one-time tasks (for example, setting up Arduino), can be considered CH340, but be prepared for driver problems.

⚠️ Attention: Fake chips FTDI may be blocked by official drivers (error FTDI Device Not Recognized). Buy adapters only from trusted sellers or check the chip markings before purchasing.

How to connect a COM port to a laptop: step-by-step instructions

The connection process consists of three stages: physical connection, driver installation And configure port parameters. Let's look at each step using an adapter as an example FTDI FT232R and a laptop for Windows 11.

Check adapter compatibility with your equipment (voltage, signals)

Download the driver from the official website of the chip manufacturer

Disable antivirus (may block driver installation)

Connect the adapter to the laptop's USB port (preferably USB 2.0)

Launch Device Manager to check recognition -->

Step 1. Physical connection

Modern adapters usually have a connector USB-A at one end and DB9 (male) - on the other. To connect to the equipment you may need:

• 🔌 Straight cable (if the device has a connector DB9 female).
• 🔄 Null-modem cable (to connect two devices with DB9 male, for example, laptop ↔ laptop).
• ⚡ Voltage adapter (if the equipment requires 5V/12V, and the adapter outputs 3.3V).

Step 2. Installing drivers

For chips FTDI And Silicon Labs download drivers only from official sites:

• FTDI Drivers (choose the version for your OS).
• Silicon Labs VCP Drivers.

For Prolific PL2303 And CH340 Drivers from repositories are suitable Windows Update, but they may need to be installed manually via Device Manager → Update Driver → Search This Computer.

Step 3: Configure port settings

After installing the drivers, open Control Panel → Devices and Printers → Device Manager and find the section Ports (COM and LPT). Your adapter should appear as USB Serial Port (COMX), where X — port number.

Next, open Port Settings (right click on the device → Properties → Port Settings) and install:

• Speed (Baud): 9600 (standard for most devices), 115200 for microcontrollers.
• Parity: None (unless otherwise specified in the equipment documentation).
• Stop bits: 1.
• Flow Control: None or Hardware (if the device requires RTS/CTS).

Common mistakes and their solutions

Even with the right adapter, problems can occur. Here are common errors and how to fix them:

If the adapter is detected, but data is transferred with errors, check:

• 🔌 Cable quality: cheap cables may have poor contact in the connector DB9.
• ⚡ Voltage levels: some devices require 5V or 12V on the lines TX/RX, and the adapter outputs 3.3V.
• 🖥️ Conflicts with other devices: Disconnect any unnecessary USB devices and check the port again.

⚠️ Attention: When working with industrial equipment (for example, PLC Siemens) Never connect the adapter to a laptop via a USB hub without power. Unstable voltage can cause the device's firmware to fail.

Programs for working with COM port

To interact with the COM port you will need terminal software. Here are the most popular solutions:

• 🖥️ PuTTY: Universal terminal for Windows/Linux, supports serial connection. You can download here.
• 🔧 Tera Term: convenient for working with network equipment (Cisco, Juniper). Supports scripts.
• 📊 RealTerm: Advanced port monitoring functions, useful for debugging.
• 🤖 Arduino IDE: if you work with microcontrollers, built-in Serial Monitor will allow you to send and receive data.
• 🔍 PortMon (from Sysinternals): utility for monitoring COM port traffic (useful for diagnostics).

Serial line: COMX (ваш порт)
Speed (baud): 9600
Data bits: 8
Stop bits: 1
Parity: None
Flow control: None

brew install ftdi-driver
brew install silicon-labs-driver

screen /dev/cu.usbserial-XXXX 9600

dmesg | grep tty
ls /dev/ttyUSB*

sudo apt install minicom
minicom -D /dev/ttyUSB0 -b 9600

Cisco (RJ-45)       Адаптер (DB9)
1 (TX)  ------------>  2 (RX)
2 (RX)  <------------  3 (TX)
3 (GND) ------------>  5 (GND)
6 (DSR) ------------>  6 (DSR)
7 (RTS) ------------>  7 (RTS)
8 (CTS) ------------>  8 (CTS)