Connecting to Serial Devices via USB on Linux

This tutorial will guide you through connecting to serial devices using a USB-to-serial adapter on a Linux system. Serial communication is commonly used for interfacing with embedded systems, microcontrollers, and various hardware components. This guide covers device recognition, configuration, and connection using terminal-based tools.

Understanding USB-to-Serial Adapters

USB-to-serial adapters allow you to connect devices that use a serial interface (typically RS-232) to your computer via a USB port. When you plug in a USB-to-serial adapter, the Linux kernel typically recognizes it and creates a device file, commonly found under /dev/ttyUSB* or /dev/ttyACM*. The specific device file name (e.g., /dev/ttyUSB0, /dev/ttyUSB1) can vary.

Identifying Your Serial Device

The first step is to identify which device file represents your serial adapter. Several methods can help:

dmesg Command: The dmesg command displays kernel messages. After plugging in your USB-to-serial adapter, use:
```
dmesg | grep ttyUSB
```
or
```
dmesg | grep ttyACM
```
This will show you messages related to the USB device and indicate which /dev/ttyUSB* or /dev/ttyACM* device file was created. Look for lines indicating a "ttyUSB" or "ttyACM" device being attached.
ls /dev/ttyUSB* and ls /dev/ttyACM*: List the available USB serial devices:
```
ls /dev/ttyUSB*
ls /dev/ttyACM*
```
This will show you all devices matching those patterns.

Setting Permissions

After identifying your serial device (e.g., /dev/ttyUSB0), you might encounter permission issues. By default, these devices are often owned by the root user, preventing regular users from accessing them. To allow a user to access the device, add them to the dialout group:

sudo usermod -a -G dialout <username>

Replace <username> with your actual username. Important: After running this command, you’ll need to log out and log back in for the group membership change to take effect. Alternatively, reboot your system.

Connecting with Minicom

minicom is a popular terminal program for serial communication.

Installation: If minicom is not already installed, use your distribution’s package manager. For example, on Debian/Ubuntu:
```
sudo apt update
sudo apt install minicom
```
Configuration:
- Run minicom -s to enter the configuration menu.
- Select "Serial port setup".
- Change the "Serial Device" to the device file you identified earlier (e.g., /dev/ttyUSB0).
- Verify other settings like "Bps" (Baud rate), "Bits", "Parity", and "Flow control" are appropriate for your serial device. Common settings include 9600 bps, 8 data bits, no parity, and no flow control (8N1).
- Select "Save setup as dfl" to save these settings as the default.
- Exit the configuration menu.
Connecting: Run minicom to start the serial connection. You should now be able to communicate with the device connected to the serial adapter.

Alternative Terminal Programs

While minicom is a solid choice, other terminal programs can also be used:

screen: screen is a versatile terminal multiplexer that can also be used for serial connections. Example: screen /dev/ttyUSB0 9600
gtkterm and moserial: These are graphical terminal programs that provide a user-friendly interface for serial communication. They can help you identify the correct device file if you are unsure.

Troubleshooting

"Cannot open /dev/ttyUSB0: No such file or directory": Double-check that the device file exists and that you have the correct permissions. Use ls /dev/ttyUSB* to verify the device file name.
No communication: Verify the baud rate, data bits, parity, and flow control settings match the settings of the device you are trying to connect to.
Permission denied: Ensure you have added your user to the dialout group and logged out/logged back in or rebooted.