Serial connection with putty on Linux

This article has been last updated on 2022-12-09

This tutorial is useful for connecting to APU2/3/4/6 router using a serial cable to reconfigure BIOS or reinstall the operating system.

Serial connection on Linux can be established using a number of different programs. This tutorial uses putty (yes, putty is available for Linux), but you can use screen, tmux, or other.

Putty should be available on most Linux distributions in default repositories. Use apt install putty or similar command for your distro. 

Basic information

  1. APU routers operate on 115200 bits per second (or baud rate). 
  2. You must run putty as root, otherwise, you won't have permission to open the connection.
  3. Run ls -l /dev/tty[SU]* to see the serial devices detected on your machine. 

    On the screenshot above you can see that I have 4 regular serial ports and 1 USB-serial cable. 

The connection is slightly different depending on which cable you are using. 

Classical serial cable

It's becoming rare for computers to have a serial port, so you will likely use the USB cable. If so, skip to the next section.

If you have a classical serial cable or an RJ45 serial cable, your Linux device will most likely be /dev/ttyS0 

Run sudo putty and enter the device name and baud rate, as seen on the screenshot.

USB to DB9F serial cable

Most users (and most of our customers) will have a USB-to-serial cable. The drivers for this cable are almost surely present on your system so you don't have to install anything. Connect the cable and run ls -l  /dev/ttyUSB* to see if it's detected. The device will most likely be named /dev/ttyUSB0 

If it's detected, run sudo putty and enter the same information as on the screenshot above except for the /dev/ttyUSB0 device.

APU6 connection

APU6 has a built-in USB-to-serial adapter on the motherboard. This means that you don't need the converter on the cable - it's already in the device. You can use any micro-USB cable

Follow this procedure to connect:

  1. Connect the USB cable to the motherboard and your computer. Don't power on the router yet
  2. Run ls -l  /dev/ttyUSB* to see if the serial device on the board is detected.
  3. Connect with putty as for the cable above.
  4. Power on the device.


Using serial connection

If everything went well, you should see an output similar to the screenshot below. 

If you see blank window, press Enter. 

This connection can be used to reconfigure BIOS, fix your operating system or install a different OS.