1. Introduction

There are several ways to create a virtual serial port in Linux to test and debug serial communication protocols. In this tutorial, we’ll first discuss the socat utility to create a virtual serial port. After that, we’ll look at the tty0tty kernel module update for creating a virtual port.

2. Virtual Serial Ports

Virtual serial ports are a logical representation of physical serial ports. These ports don’t connect with the serial port. A virtual serial port allows us to emulate a physical serial port, thereby removing the requirements of cable and physical communication interface.

Additionally, a virtual port allows software packages to communicate with internal memory using the null modem emulator. The null modem emulator is a virtual driver for Linux that enables virtual serial port communication.

3. Create a Virtual Serial Port Using socat Utility

The first method to create a virtual serial port is using the socat utility. This utility pipes the port traffic to executables using standard input and output. Here’s the syntax for socat:

socat [OPTIONS] <ADDRESS1> <ADDRESS2>

The two addresses can be:

  • stdio: standard input/output
  • udp:host:port: UDP connection to host and port
  • tcp:host:port: TCP connection to host and port
  • file:<filename>: file or FIFO (named pipe)
  • pty: pseudo-terminal
  • pty,link=<filename>: pseudo-terminal with a symbolic link
  • pty,rawer,link=<filename>: pseudo-terminal with a raw data transfer and a symbolic link

3.1. Install socat Utility

Although the socat utility is usually available by default, we can install it on our machine. To install it, we’ll use the apt-get package or the yum package:

# Debian-based Linux
$ sudo apt-get install -y socat
# RHEL-based Linux
$ sudo yum install -y socat

This installs socat on the Linux machine.

3.2. Create Simple Virtual Serial Port

For instance, to create two pseudo terminals with the debug mode on, we’d type socat with the -d option and pty,rawer in place of the address:

$ socat -d -d pty,rawer,echo=0 pty,rawer,echo=0
2023/05/04 09:32:35 socat[606] N PTY is /dev/pts/2
2023/05/04 09:32:35 socat[606] N PTY is /dev/pts/3
2023/05/04 09:32:35 socat[606] N starting data transfer loop with FDs [5,5] and [7,7]

The -d -d option after the socat command prints fatal, error, warning, and notice messages. In addition, the pty,rawer,echo=0 arguments create two pseudo terminals under /dev/pts. These terminals act as an end-point for serial connection. To connect these two, we can use any serial communication tool, such as minicom. For example, to connect /dev/pts/2 at a baud rate of 9600, we’d run:

$ minicom -D /dev/pts/2 -b 9600

This connects /dev/pts/2 with a baud rate of 9600.

3.3. Create Serial Port With Baud Rate

Alternatively, we can create a virtual port with a custom baud rate by adding the baud argument to the socat command:

$ socat -d -d pty,rawer,echo=0,link=/tmp/ttyV0, baud=115200 pty,rawer,echo=0,link=/tmp/ttyV1,baud=115200

This creates a pair of virtual rates with custom names of /tmp/ttyV0 and /tmp/ttyV1.

4. Create a Virtual Serial Port Using tty0tty Kernel Module

The Linux modem emulator, tty0tty, creates a virtual serial port driver for Linux. Additionally, we can use these ports to establish communication between serial applications. In other words, it provides a pair of virtual TTY devices that we can use as a null-modem cable replacement.

4.1. Install tty0tty on Ubuntu

To install tty0tty on Ubuntu, we’ll use the apt-get command:

$ sudo apt-get install tty0tty

We’ll follow the instructions on the screen to complete the installation.

On the other hand, we can install it from the website. First, we’ll download the package from SourceForge. Next, we’ll clone this GitHub repository. After that, we’ll extract the package using the tar command:

$ tar xf tty0tty-1.2.tgz

Next, we’ll build the kernel modules:

$ cd tty0tty-1.2/module
$ make

After that, we’ll copy the new kernel module in the current kernel module directory:

$ sudo cp tty0tty.ko /lib/modules/$(uname -r)/kernel/drivers/misc/

Lastly, we’ll load the module using the depmod command:

$ sudo depmod

The command analyzes the dependent kernel modules and the associated map file consisting of information related to these dependency modules. This ensures that all the required modules are loaded before loading the tty0tty module.

4.2. Create a Virtual Serial Port

Now, to create and use the serial port, we use the modprobe command:

$ sudo modprobe tty0tty

After that, we’ll give permission to the new serial port using chmod:

$ sudo chmod 666 /dev/tnt*

We can now access the serial ports as /dev/tnt0 and /dev/tnt1. To connect these two devices, we’ll use a serial communication tool such as minicom:

$ minicom /dev/tnt0 -b 9600

In addition, we can change the baud rate of the ports created using tty0tty:

$ sudo modprobe tty0tty baud_base=115200

This should create the port with the baud rate of 115200.

4.3. Make Module Update Persistent

To make this kernel module change persistent, we’ll edit the /etc/modules or /etc/modules.conf. For this step, we’ll use the nano text editor:

$ nano /etc/modules

After that, we’ll add the module name at the end of the file:

tty0tty

Lastly, we’ll press Ctrl + S to save and Ctrl + X to exit the nano editor.

However, it’s crucial to remember that this change will be overwritten by the kernel updates. To elaborate, if we update the kernel, we’ll have to build the tty0tty module once again by following all the steps.

5. Conclusion

To conclude, by using virtual serial ports, we can test serial communication protocols without accessing physical hardware. In this article, we’ve discussed two ways to create virtual ports, socat utility, and tty0tty kernel module. The socat module offers more flexibility and advantage than tty0tty, making it a powerful tool for advanced users.

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