Managing Serial Ports in Linux and Windows

Differences Between Serial Ports on Linux and Windows

A serial port is an interface that allows external devices to communicate with your computer. Legacy devices such as older mice and modems traditionally use this port. While Windows refers to these as “COM ports,” Linux takes a different approach by identifying them with names like /dev/ttyS0 as part of the /dev/tty system. Recognizing this distinction is crucial for effectively managing serial devices on a Linux system.

To illustrate the difference clearly, here’s a simple comparison:

Software for Operating Serial Ports on Linux

Working with serial ports on Linux typically involves using terminal-based software. One popular option is gtkterm, a lightweight serial terminal application that’s perfect for straightforward tasks. However, depending on your needs, there are several other powerful tools available.

Popular Serial Terminal Applications

Below is a comparison table of some commonly used serial communication programs:

Why Use Dedicated Serial Terminal Software?

Serial terminal applications simplify interactions with your serial devices by providing:

• Ease of Use: Intuitive interfaces (both text-based and graphical) that streamline device communication.
• Advanced Options: Customizable settings such as baud rate, data bits, parity, and stop bits for tailored configurations.
• Logging Capabilities: The ability to record session data for troubleshooting and analysis.

For example, while gtkterm is excellent for basic tasks, minicom offers a broader range of features for users who require detailed communication control. This flexibility makes Linux an ideal platform for both simple and advanced serial communications.

What is /dev/tty?

In Linux, every hardware device is represented as a file. This design means that serial ports appear in the file system just like regular files, typically under names like /dev/ttyS0 or /dev/ttyUSB0.

Device File Examples

By writing data to these device files, Linux communicates with the corresponding hardware. This file-based approach simplifies device management and interaction.

Visual Comparison of Serial Port Identification

      [PC]
         │
         ├─ Windows: COM Port (COM1, COM2, etc.)
         │
         └─ Linux: Device Files
                ├─ /dev/ttyS0 (Built-in Serial Port)
                └─ /dev/ttyUSB0 (USB-connected Serial Device)

This diagram highlights the contrast between the Windows naming convention (COM ports) and the Linux method of using device files for serial communication.

Permission Error: “cannot open /dev/ttyS0”

When you try to run a serial communication program without proper access rights, you might encounter an error like this:

cannot open /dev/ttyS0: Permission denied

Why Does This Error Occur?

In Linux, hardware devices—including serial ports—are represented as files. Accessing these files requires the correct permissions. This error indicates that your current user does not have the privileges needed to interact with the /dev/ttyS0 device file.

How to Fix the Issue

To resolve this, you need to adjust the permissions of the serial port. A common temporary solution is to grant read and write permissions to all users with the following command:

sudo chmod 666 /dev/ttyS0

Breakdown of the Command

Considerations and Best Practices

• Security Concerns:
  Granting full read and write access (666) to everyone can pose security risks. In a production environment, it’s often better to restrict access using user groups.
• Permanent Solutions:
  The permission change with chmod is temporary and will revert after a reboot. For a more permanent solution, consider one of the following approaches:
  1. Add Your User to the Appropriate Group:
     Many Linux distributions assign serial port devices to a specific group (e.g., dialout or uucp). You can add your user to that group with:
     sudo usermod -a -G dialout yourusername
  2. Configure Udev Rules:
     For a more robust and automatic solution, create a udev rule that sets the desired permissions each time the device is detected.

Checking the Serial Port Status

To ensure that your serial ports are properly recognized and configured, you can use the setserial utility. This command displays detailed information about the status of your serial ports.

Steps to Verify Serial Ports

1. Install setserial (if not already installed):Open a terminal and run:
   sudo apt install setserial
2. Check the Status of Serial Ports:Once installed, execute the following command to display information for ports /dev/ttyS0 through /dev/ttyS3:
   setserial -g /dev/ttyS[0123]
   This command queries the specified serial port devices and outputs their configuration details.

What to Look For

When you run setserial, you might see output resembling this:

Note: The values for the I/O address, UART type, and IRQ might vary depending on your system’s configuration.

Additional Tips

• Understanding UART Types:
  The UART (Universal Asynchronous Receiver/Transmitter) type indicates the chip responsible for handling serial communication. Common types include 16550, 16550A, etc.
• Troubleshooting:
  If a port is not recognized or shows as “not present,” double-check that the hardware is properly connected and that the appropriate drivers are installed.
• Automation:
  For users managing multiple devices, consider creating a small script that runs setserial at startup and logs the output for easier troubleshooting.

How to Maintain Permissions After Reboot

When you adjust serial port permissions manually using chmod, the changes are temporary and will revert after a reboot. To avoid reapplying these settings every time your system starts, you can automate the process.

Method 1: Using /etc/rc.local

One common approach is to add the permission commands to the /etc/rc.local file, which is executed during the boot process.

Steps:

1. Open the /etc/rc.local File:
   sudo nano /etc/rc.local
2. Add the Following Commands Before the exit 0 Line:
   chmod 666 /dev/ttyS0
chmod 666 /dev/ttyS1
chmod 666 /dev/ttyS2
3. Save the File and Exit:Press Ctrl+O to save, then Ctrl+X to exit.

Note:
The /etc/rc.local file might not exist on some newer Linux distributions by default. In such cases, you may need to create it or consider an alternative method.

Method 2: Using Systemd Service

For systems that use systemd, you can create a custom service to set the permissions at boot.

Steps:

1. Create a New Service File:
   sudo nano /etc/systemd/system/serial-permissions.service
2. Add the Following Content:
   [Unit]
Description=Set Serial Port Permissions
After=network.target

[Service]
Type=oneshot
ExecStart=/bin/sh -c 'chmod 666 /dev/ttyS0 /dev/ttyS1 /dev/ttyS2'

[Install]
WantedBy=multi-user.target
3. Enable and Start the Service:
   sudo systemctl enable serial-permissions.service
sudo systemctl start serial-permissions.service

Summary of Options

Using either method ensures that your serial port permissions persist after every reboot, streamlining your workflow and reducing repetitive configuration tasks.

Using Wine to Run Windows Software on Linux

Wine is a powerful tool that lets you run Windows applications directly on your Linux system. In this example, we’ll install TeraTerm—a popular Windows serial communication program—and verify that it correctly maps Linux serial ports to Windows-style COM ports.

What is Wine?

Wine (originally an acronym for “Wine Is Not an Emulator”) enables many Windows applications to run on Linux without the overhead of a virtual machine. With Wine, you can access a range of software that might otherwise be exclusive to Windows. For more in-depth details on how Wine works and its configuration options, feel free to visit my blog post on the subject.

Installing TeraTerm via Wine

Once you have Wine installed, you can use it to run the TeraTerm installer. After installation, TeraTerm will treat the Linux serial ports as if they were native Windows COM ports. For example, Linux’s /dev/ttyS0 will appear as com1 in the Wine environment.

Verifying Serial Port Mapping

Wine automatically maps Linux serial ports to Windows COM ports. You can confirm this mapping by checking the dosdevices directory in your Wine configuration:

ls -l ~/.wine/dosdevices

You might see output similar to:

Note: Since there is no com0 in Windows, Linux devices shift by one—meaning /dev/ttyS0 becomes com1.

Verifying Communication

To ensure everything works as expected, you can perform a simple test:

1. Launch TeraTerm via Wine:
   Open TeraTerm from the Wine menu or via the terminal using a command like:
   wine "C:\Program Files\TeraTerm\TTERMPRO.EXE"
2. Select the COM Port:
   In TeraTerm, select com1 (or the appropriate COM port corresponding to your Linux device) as your communication port.
3. Cross-Test with gtkterm:
   Open gtkterm on Linux and configure it to use /dev/ttyS0. By sending data between TeraTerm and gtkterm, you can confirm that serial communication is established correctly across the two environments.

Why Combine Wine and Native Tools?

Using Wine to run Windows-based serial communication software alongside native Linux tools (like gtkterm) allows for a more flexible workflow. It’s particularly useful if:

• You’re transitioning from a Windows environment and still rely on specific Windows software.
• You need features from both platforms to troubleshoot or manage serial devices.

Verifying Communication Using Wine and gtkterm

To confirm that your serial port communication is functioning correctly, it’s a good idea to perform a cross-test using both TeraTerm (via Wine) and a native Linux tool like gtkterm. This dual approach not only verifies the port mapping but also ensures that data is transmitted seamlessly between the two environments.

Step-by-Step Verification

1. Launch TeraTerm through Wine:Open TeraTerm by running it with Wine:bashコピーするwine "C:\Program Files\TeraTerm\TTERMPRO.EXE" In TeraTerm, select the COM port corresponding to your Linux serial device (e.g., com1).
2. Start gtkterm on Linux:Open gtkterm and configure it to use the corresponding device file (e.g., /dev/ttyS0).
3. Send and Receive Data:
   • From TeraTerm:
     Send a simple message (like “Hello from Wine!”) and check if it appears in gtkterm.
   • From gtkterm:
     Similarly, send a message (e.g., “Hello from Linux!”) and verify that it is received in TeraTerm.
4. Observe the Communication Flow:Ensure that both applications can transmit and receive data without errors. This confirms that the Linux serial port (mapped as a COM port in Wine) is operating correctly.

Visual Flow Diagram

Below is a simplified diagram illustrating the data flow:

              +---------------------+
              |     TeraTerm        |  (via Wine)
              |      (COM1)         |
              +----------+----------+
                         │
        Data Transmission │
                         ▼
              +---------------------+
              |      Linux          |
              |   /dev/ttyS0        |
              +---------------------+
                         ▲
        Data Transmission │
                         │
              +---------------------+
              |      gtkterm        |
              |   (Native Linux)    |
              +---------------------+

Additional Considerations

• Troubleshooting Tips:
  • If messages are not transmitted correctly, double-check the COM port mapping in the Wine configuration (~/.wine/dosdevices).
  • Verify that both TeraTerm and gtkterm are configured with the same serial parameters (baud rate, parity, data bits, etc.).
• Benefits of Dual Testing:
  • Cross-Platform Compatibility:
    Ensures that serial communication is reliable regardless of whether you’re using a Windows or Linux application.
  • Flexibility:
    Provides the freedom to switch between tools based on your current task without sacrificing functionality.

This combined testing approach reinforces confidence in your setup, confirming that the mapping between Linux device files and Windows COM ports is accurate and that serial communication works flawlessly across both environments.