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Raspberry Pi Guide Part 2/3: USB Serial

By Ayaan 2-24-25 Part 2/3

So you just got your Linux install. Now what?

The goal of this step is to enable USB Serial, allowing direct communication between your PC and your Pi Zero 2W. You can then SSH into your Pi locally without any dependencies on Wifi.

USB Serial Mode allows you to connect your pi directly to your computer using the second (data) MicroUSB port. "Regular" Mode is the normal configuration where you can plug in a power source, keyboard, and HDMI and use your Pi as normal. One limitation of USB Serial Mode is that you cannot plug in a keyboard and your computer at the same time since it uses the same physical port. You need either a USB to TTL adapter and use the pins or a USB OTG adapter.

This guide is tailored for enabling USB Serial mode as an option to have. This guide will let you switch between USB Serial mode and Regular Mode. It may not suit your usecase.

This was tested on Kali Linux, which is based on Debian. Since Raspbian is also based on Debian, it should also work there, but I have not tested it.

1) Disable GUI

You can change this in raspi-config

sudo systemctl set-default multi-user.target

To reenable GUI on boot, use raspi-config.

To start the GUI in the CLI, run: 

sudo startx &

2) (optional) Disable wait for network on boot

sudo systemctl disable systemd-networkd-wait-online.service
sudo reboot

4) Edit your cmdline.txt:

Run this command to edit your cmdline.txt: 

sudo nano /boot/cmdline.txt

Then add this to your cmdline.txt right after the rootwait: 

modules-load=dwc2,g_serial

5) Edit your config.txt

Run this command to edit your config.txt:

sudo nano /boot/config.txt

Add this to the top or bottom of your config.txt and ideally remove the rest of the settings that you don't need. Make sure what you already have in your config.txt does not conflict with this!

# SETTINGS FOR USB SERIAL OR REGULAR MODE
# Please enable or disable the other to use a particular mode. 

# ENABLE FOR USB SERIAL
dtoverlay=dwc2
start_x=1

# ENABLE FOR REGULAR MODE
# dr_mode=peripheral

# Lower output resolution to 1/4th of 1080p
framebuffer_width=960
framebuffer_height=540

hdmi_force_hotplug=1
disable_overscan=1

Once you're finished, just shut down and connect your Pi to your computer via the Data port on the Pi Zero 2W. Serial Mode should be set up already, but if you're having trouble, try connecting your Pi to the network and SSHing into it.

The getty communication service for USB serial should already be running, but ensure it is with: 

sudo systemctl restart serial-getty@ttyGS0.service

check if it is running with: 

sudo systemctl status serial-getty@ttyGS0.service

It should say enabled in green.

Connecting to your Computer

It is actually pretty straightforward on a Mac. On Windows, you will need PuTTY. Use Device Manager to check the name of the device, maybe it will be listed as COM6. Then use PuTTY to connect. Below are the instructions for MacOS:

1) Plug the Micro USB data cable to the second Micro USB port.

Plug this into your Mac. Having a display is optional.

2) Run the following command on your computer to see if your computer recognizes the Pi:

ls /dev/cu.usbmodem*

Try also running this command if the above is giving issues: 

ls /dev/*.*

You should see something like this listed: 

(base) root@computer ~ % ls /dev/cu.usbmodem*              

/dev/cu.usbmodem11201

This Raspberry Pi is identified as cu.usbmodem11201.

3) To connect to your Pi, run this command.

Make sure to change the numbers to match yours.

screen /dev/cu.usbmodem2101 115200

Now it should ask you to connect. Type in kali, then kali as username and password if you did not change the default username or password.

Now you are connected to your Pi via USB Serial!

A major limitation is that you cannot directly control the Pi because the one data port is taken up. You need a USB OTG hub for this to work. In order to undo this and be able to run in "normal" mode where you can plug in the power to the power port and plug in a keyboard like normal, you must change your config.txt to match this snippet:

USB Serial Mode: 

## SETTINGS FOR USB SERIAL OR REGULAR MODE
## Please enable or disable the other to use a particular mode. 

## ENABLE FOR USB SERIAL
dtoverlay=dwc2
start_x=1

## ENABLE FOR REGULAR MODE
## dr_mode=peripheral

Regular Mode: 

## SETTINGS FOR USB SERIAL OR REGULAR MODE
## Please enable or disable the other to use a particular mode. 

## ENABLE FOR USB SERIAL
## dtoverlay=dwc2
## start_x=1

## ENABLE FOR REGULAR MODE
dr_mode=peripheral

USB Serial has an identical experience to SSHing over a network. However, there is significantly less lag and you have a lot more free memory.

And you're done!

Now you have a Raspberry Pi that you can SSH into locally!