Use `dd` to read/write from a serial port (eg, `/dev/ttyUSB0`)

Using dd to Read and Write Data from a Serial Port in Linux

Introduction

In this post, we explore the powerful yet often underappreciated Linux command dd for handling I/O operations, particularly focused on its application to serial ports such as /dev/ttyUSB0. Serial ports are crucial for low-level communication between computers and various devices like modems, sensors, and microcontrollers.

Q&A on Using dd for Serial Port Operations

Q1: What is the dd command in Linux? A: The dd command in Linux stands for 'data duplicator'. It is used primarily for copying and converting data. This command can copy data from one file or block device to another, handling differing block sizes and managing conversion settings, making it remarkably flexible.

Q2: How can dd be used with serial ports? A: Although primarily used for data copying, dd can be applied to read from or write to serial ports by specifying the serial port device (like /dev/ttyUSB0) as the input or output file. This can be useful for direct data transmission to and from serial devices.

Q3: Can you give an example of how to set up dd for reading from a serial port? A: Sure! To read from a serial port using dd, you first need to configure the serial port settings (like baud rate and parity) using the stty command. Here's a simple setup:

stty -F /dev/ttyUSB0 cs8 9600 ignbrk -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke noflsh -ixon -crtscts
dd if=/dev/ttyUSB0 of=output_file bs=1 count=100

This configuration sets up /dev/ttyUSB0 for 9600 baud with 8 data bits and no parity, and reads 100 bytes from the serial port into output_file.

Q4: What about writing data to a serial port using dd? A: Writing to a serial port is similar but in reverse. You would use:

echo -n "This is a test" > input_file
dd if=input_file of=/dev/ttyUSB0 bs=1

This example writes the string "This is a test" from input_file to the serial port /dev/ttyUSB0.

Further Explorations and Examples

Here are a few simple examples and explanations to deepen your understanding:

Reading from serial port continuously:

stty -F /dev/ttyUSB0 speed 9600 cs8 -cstopb -parenb
dd if=/dev/ttyUSB0 of=logfile.txt &

This sets up the port at 9600 baud, with 8 data bits, no parity, 1 stop bit and continuously dumps the read data into logfile.txt.

Sending a file to a printer or another serial device:

stty -F /dev/ttyUSB0 19200
dd if=document.txt of=/dev/ttyUSB0

This example configures the serial port to 19200 baud and sends the contents of document.txt to the device connected via the serial port.

Executable Script Example

Below is a simple bash script demonstrating the read and write capabilities of dd with serial ports:

#!/bin/bash

# Configure /dev/ttyUSB0
stty -F /dev/ttyUSB0 cs8 9600

# Read from serial port
dd if=/dev/ttyUSB0 of=received_data.txt bs=1 count=50 &

# Write to serial port
echo -n "Hello, Serial World!" | dd of=/dev/ttyUSB0 bs=1

# Cleanup
wait
echo "Data transmission complete."

This script sets up the serial port for 9600 baud, reads 50 bytes from the serial port into received_data.txt, writes a string to the serial port, and completes the operations cleanly.

Conclusion

The dd command offers flexibility not only in copying and converting data between files but also in facilitating direct interaction with hardware through facilities like serial ports. Whether you're setting up communication with a network device, interfacing with a microcontroller, or managing a modem, understanding how to harness dd in these scenarios can significantly streamline your processes. Thus, diving into tools like dd can unlock new possibilities for both developers and system administrators in the Linux environment.

Further Reading

For more information on using dd and managing serial ports in Linux, consider the following resources:

• Linux dd Command Tutorial for Beginners: https://www.howtoforge.com/tutorial/linux-dd-command/ This tutorial provides a beginner-friendly guide to the dd command, illustrating various uses and options.

• Comprehensive Guide to Serial Port Communication in Linux: https://www.cyberciti.biz/faq/find-out-linux-serial-ports-with-setserial/ An article detailing how to manage serial ports in Linux, including finding and configuring them.

• Working with the Serial Console: https://www.freebsd.org/doc/handbook/serialconsole-setup.html Though focused on FreeBSD, this resource offers valuable insights applicable to Linux regarding serial console setups.

• Understanding and Using Systemd: https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files Useful for managing services that interact with serial ports, this guide explains systemd units in Linux.

• Advanced Bash-Scripting Guide: https://tldp.org/LDP/abs/html/ An extensive resource for those looking to write more complex scripts in bash, which might involve using dd for device management and data handling.

These links will help expand your knowledge on the dd command, serial port configurations, and related scripting in Linux environments.