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Python Script to Reset USB Modem COM Port on Raspberry Pi

Pradeep Singh | 1st Dec 2017


If you are working with the Serial devices connected to the Raspberry Pi, For example, a USB analog modem; you may have noticed that sometimes these devices become unresponsive and don’t respond to the issued commands. Most of the times you will end up rebooting your Raspberry Pi to fix the problem.

I faced similar issues with one of the applications that I was developing with Raspberry Pi and U.S. Robotics USB Voice Fax Modem. At times the modem was becoming unresponsive especially during the testing, and to deal with this situation I wrote this python script and made it part of the auto port recovery method.

This script uses a “USB_DEV_NAME” variable to identify the right device while performing the USB port reset operation. You can use the “lsusb” command on the shell prompt of your Raspberry Pi to list all the USB devices connected to your Raspberry Pi. Locate the manufacturer/device name for your device and use that to set the value for “USB_DEV_NAME” variable. Since I am using a U.S. Robotics analog modem, I used “U.S. Robotics” value to set the script variable.

Following screenshot shows the USB devices connected to my Raspberry Pi –


Following code will help you reset USB devices connected to your Raspberry Pi. If you face issues with the copy paste of the code, you can download it from my GitHub Repo Octocat –

##--- Author: Pradeep Singh
##--- Blog:
##--- Date: 1st Dec 2017
##--- Version: 1.0
##--- Python Ver: 2.7
##--- Description: This python code will reset a USB port connected to Raspberry Pi

import subprocess
import os
import fcntl

# Define "USB_DEV_NAME" variable based on the Device/Manufacturer name as shown
# in the output of the "lsusb" command on the linux shell
# For Example - 
# I am using U.S. Robotics Analog USB Modem and want to reset it
# as shown in the following output my Modem is listed as "U.S. Robotics"
# hence I will use "U.S. Robotics" in the USB_DEV_NAME variable -
# pi@RaPi3:~ $ lsusb
# Bus 001 Device 004: ID 0baf:0303 U.S. Robotics  <<<<<=========
# Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter
# Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp. 
# Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
# pi@RaPi3:~ $ 

USB_DEV_NAME = 'U.S. Robotics'

# Reset Modem
def reset_USB_Device():

	# Same as _IO('U', 20) constant in the linux kernel.
	CONST_USB_DEV_FS_RESET_CODE = ord('U') << (4*2) | 20
	usb_dev_path = ""

	# Based on 'lsusb' command, get the usb device path in the following format - 
	# /dev/bus/usb//
	proc = subprocess.Popen(['lsusb'], stdout=subprocess.PIPE)
	cmd_output = proc.communicate()[0]
	usb_device_list = cmd_output.split('\n')
	for device in usb_device_list:
		if USB_DEV_NAME in device:
			print device
			usb_dev_details = device.split()
			usb_bus = usb_dev_details[1]
			usb_dev = usb_dev_details[3][:3]
			usb_dev_path = '/dev/bus/usb/%s/%s' % (usb_bus, usb_dev)

		if usb_dev_path != "":
			print "Trying to reset USB Device: " + usb_dev_path
			device_file =, os.O_WRONLY)
			fcntl.ioctl(device_file, CONST_USB_DEV_FS_RESET_CODE, 0)
			print "USB Device reset successful."
			print "Device not found."
		print "Failed to reset the USB Device."



Following screenshot shows script execution output –



Related Articles:

USB Analog Modem with Raspberry Pi

Record Audio from Phone Line with Raspberry Pi

Play Audio File on Phone Line with Raspberry Pi

Incoming Call Details Logger with Raspberry Pi

Python Script to Autodetect Modem COM Port on Raspberry Pi


Wikipedia – Voice modem command set

USRobotics 56K USB Modem Voice Commands

USRobotics 56K USB Modem – Data and General Commands

Voice +V Commands – Developer’s Guide

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