Hey guys! Ever needed to peek at the raw data flowing through your serial port on a Linux system? Whether you're debugging hardware, monitoring sensor data, or just tinkering with embedded systems, knowing how to read a serial port from the command line is an incredibly useful skill. This guide will walk you through several methods, from simple tools to more advanced techniques, ensuring you can capture and interpret that valuable serial data like a pro.

Understanding Serial Ports

Before we dive into the how-to, let's briefly cover what a serial port actually is. Serial communication is a method of transmitting data one bit at a time over a single wire (or a few wires, for ground and control signals). Think of it like a narrow pipeline where each piece of information has to wait its turn. This is in contrast to parallel communication, where multiple bits are sent simultaneously over several wires.

Why Serial Ports Matter

Even in our high-speed, parallel-processing world, serial ports remain incredibly important. You'll find them in a wide range of devices, including:

• Microcontrollers: Arduinos, ESP32s, and other microcontrollers often use serial ports for communication with computers or other devices.
• GPS Modules: Many GPS units output their location data over a serial interface.
• Industrial Equipment: PLCs, sensors, and other industrial devices frequently use serial communication protocols.
• Legacy Devices: While less common these days, many older devices still rely on serial ports.

Identifying Your Serial Port

On Linux, serial ports are typically represented as device files under the /dev directory. Common names include:

• /dev/ttyS0, /dev/ttyS1, etc.: These usually refer to the traditional serial ports on your computer (if you have any).
• /dev/ttyUSB0, /dev/ttyUSB1, etc.: These represent USB serial adapters. If you're using a USB-to-serial converter, this is likely the device you're looking for.
• /dev/ttyACM0, /dev/ttyACM1, etc.: These are often used by devices that emulate a modem, such as some Arduinos or other USB devices.

To figure out which serial port your device is using, you can try a few things:

1. Check the Documentation: The documentation for your device should tell you which serial port it uses.
2. Use dmesg: After plugging in a USB serial device, run dmesg | tail in the terminal. This will show you the most recent kernel messages, which may include information about the device and the serial port it's assigned to.
3. List /dev/tty*: Run ls /dev/tty* to see all the available serial port devices. Then, try connecting and disconnecting your device and see which one appears and disappears.

Method 1: Using cat (The Simplest Approach)

The easiest way to read data from a serial port is using the cat command. This is a very basic method and doesn't give you much control over the serial port settings, but it's great for a quick peek.

The Command

cat /dev/ttyUSB0

Replace /dev/ttyUSB0 with the actual serial port device you want to read from. Once you run this command, cat will simply output any data it receives from the serial port to your terminal. To stop it, press Ctrl+C.

Limitations

The main limitation of cat is that it uses the default serial port settings (baud rate, parity, etc.). If your device uses different settings, you won't see any meaningful data. Also, cat doesn't provide any buffering or error handling, so you might miss some data if the serial port is sending it too quickly.

Method 2: Using minicom (The Versatile Terminal)

minicom is a text-based serial communication program. It allows you to configure various serial port settings and interact with the device connected to the port. It's like a dedicated terminal emulator for serial communication.

Installation

If minicom isn't already installed on your system, you can install it using your distribution's package manager. For example, on Debian/Ubuntu:

sudo apt-get update
sudo apt-get install minicom

On Fedora/CentOS/RHEL:

sudo dnf install minicom

Configuration

Before you can use minicom, you need to configure it to use the correct serial port and settings. Run minicom -s to enter the setup menu. Here are the key settings you'll need to configure:

1. Serial port setup: Select "Serial port setup" and enter the device name (e.g., /dev/ttyUSB0).
2. Bps/Par/Bits: Select "Bps/Par/Bits" and set the baud rate, parity, and number of data bits to match the settings used by your device. Common settings include 9600 8N1 (9600 baud, 8 data bits, no parity, 1 stop bit) and 115200 8N1.
3. Hardware Flow Control: Unless your device specifically requires it, disable hardware flow control (RTS/CTS).
4. Software Flow Control: Similarly, disable software flow control (XON/XOFF) unless required.

After configuring the settings, select "Save setup as dfl" to save them as the default configuration. Then, exit the setup menu.

Using minicom

To start minicom with the default configuration, simply run minicom. You should see a terminal window where you can interact with the serial port. Any data received from the serial port will be displayed in the window, and anything you type will be sent to the serial port.

Important minicom commands:

• Ctrl+A Z: Opens the help menu, where you can find a list of all available commands.
• Ctrl+A X: Exits minicom.
• Ctrl+A E: Toggles local echo (whether what you type is displayed on the screen).

minicom is a powerful tool for interacting with serial ports, allowing you to send commands and receive data in a controlled environment. It's especially useful for debugging and configuring serial devices.

Method 3: Using screen (The Swiss Army Knife)

The screen command is a terminal multiplexer, but it can also be used to connect to serial ports. It's a versatile tool that can be used for many different purposes, making it a handy tool to have in your toolbox.

Installation

If screen isn't already installed, you can install it using your distribution's package manager. For example, on Debian/Ubuntu:

sudo apt-get update
sudo apt-get install screen

On Fedora/CentOS/RHEL:

sudo dnf install screen

Connecting to a Serial Port

To connect to a serial port using screen, use the following command:

screen /dev/ttyUSB0 115200

Replace /dev/ttyUSB0 with the serial port device and 115200 with the baud rate. This command will open a new screen session connected to the serial port. Any data received from the serial port will be displayed in the terminal, and anything you type will be sent to the serial port.

Detaching and Reattaching

One of the great features of screen is that you can detach from a session and reattach to it later. To detach from the screen session, press Ctrl+A followed by D. This will disconnect the terminal from the session but leave the session running in the background.

To reattach to the screen session, use the following command:

screen -r

If you have multiple screen sessions running, you may need to specify the session ID. You can see a list of running sessions with screen -ls.

Exiting

To exit the screen session, simply type exit or press Ctrl+D.

screen is a great option if you need to detach from a serial port session and reattach to it later. It's also useful if you want to run a serial communication program in the background.

Method 4: Using stty and dd (The Power User's Choice)

For those who like to get down and dirty with the command line, stty and dd offer a powerful way to read from serial ports. This method gives you the most control over the serial port settings and data processing.

Configuring the Serial Port with stty

stty (set tty) is a command-line utility for configuring serial port settings. You can use it to set the baud rate, parity, data bits, and other parameters. Here's an example of how to configure a serial port:

stty -F /dev/ttyUSB0 115200 cs8 raw -parenb -cstopb clocal cread

Let's break down this command:

• -F /dev/ttyUSB0: Specifies the serial port device.
• 115200: Sets the baud rate to 115200.
• cs8: Sets the character size to 8 bits.
• raw: Disables all input and output processing.
• -parenb: Disables parity checking.
• -cstopb: Sets one stop bit.
• clocal: Disables modem control signals.
• cread: Enables the receiver.

Adjust these settings as needed to match the requirements of your device.

Reading Data with dd

dd is a command-line utility for copying data from one file to another. It can also be used to read data from a serial port. Here's an example of how to read data from a serial port using dd:

dd if=/dev/ttyUSB0 of=output.dat bs=1 count=100

Let's break down this command:

• if=/dev/ttyUSB0: Specifies the input file (the serial port device).
• of=output.dat: Specifies the output file (where the data will be written).
• bs=1: Sets the block size to 1 byte.
• count=100: Reads 100 blocks (bytes).

This command will read 100 bytes of data from the serial port and save them to the output.dat file. You can then open the file with a text editor or other program to view the data.

Combining stty and dd

To use stty and dd together, first configure the serial port with stty, then read the data with dd. For example:

stty -F /dev/ttyUSB0 115200 cs8 raw -parenb -cstopb clocal cread
dd if=/dev/ttyUSB0 of=output.dat bs=1 count=100

This will configure the serial port and then read 100 bytes of data from it.

Advantages and Disadvantages

The main advantage of using stty and dd is that it gives you complete control over the serial port settings and data processing. However, it's also more complex than the other methods and requires a good understanding of the command line.

Conclusion

Reading serial port data on Linux is a valuable skill for anyone working with embedded systems, hardware, or industrial equipment. Whether you choose the simplicity of cat, the versatility of minicom, the multitasking power of screen, or the low-level control of stty and dd, there's a method that's right for you. Experiment with these tools and find the one that best fits your needs and workflow. Now go forth and conquer those serial ports! Happy hacking!