View Source Connecting to a Nerves Target

There are multiple ways to connect to your Nerves target device and different target may support different connection methods.

The default features are different depending on Nerves targets. For example, some Nerves targets support a UART serial console by default; others, HDMI and USB keyboard instead.

What features does Nerves support for my device?

Refer to the documentation of nerves_system_<target> projects for their supported features. As an example, when your target is rpi0, visit https://hexdocs.pm/nerves_system_rpi0.

USB to TTL serial cable (UART)

A target device can be accessed via a serial connection with a USB to TTL serial cable, which is connected between the host USB port and a couple of header pins on the target.

This connection method allows you to interact with the console of the target device using a terminal emulator program on your development host. It is useful for debugging networking or the boot process and for advanced development workflows.

First of all, locate the documentation of the Nerve system that corresponds to your target device, and find out how your Nerves system supports the IEx terminal feature.

As an example, as of this writing, the documentation of nerves_system_rpi0 (a Nerves system for Raspberry Pi Zero) says the system supports one UART port named ttyAMA0 available for IEx terminal. It is /dev/ttyAMA0 in the file system.

It is configured here in the nerves_system_rpi0 source code.

On the Raspberry Pi Zero, the UART that is known as UART0 in the hardware descriptions is routed to pins 8 and 10.

On Linux on the Raspberry Pi Zero, UART0 is exposed as the device file /dev/ttyAMA0.

Enabling USB serial console

Depending on your target's default settings, you may need to modify your Nerves configuration as described in the Using a USB Serial Console FAQ topic.

Get a USB-to-TTL serial cable

We've had good luck with this cable if you haven't already found one.

You may need to install to your host machine the driver software for the cable. If you use the above-mentioned cable, Adafruits provides this guide.

Connect the leads

Image credit: https://pinout.xyz

Tips

Most likely you don't need the power line since your purpose here is the serial data communication.

TX (transmit) and RX (receive) are relative terms. What is TX for one is RX for the other.

For visual learners, Adafruit's Raspberry Pi Lesson has some helpful images.

Run a terminal emulation program

The USB-to-TTL serial cable converts the text into a standard serial USB port. There are multiple open source terminal emulator programs out there that support the serial console.

• picocom
• bootterm
• screen
• tio

As an example, on a macOS host machine, you can open a terminal and try these commands.

List TTY devices available

ls /dev/tty*

Start communication with the Raspberry Pi using picocom

picocom -b 115200 /dev/ttyUSB0

Replace ttyUSB0 with the TTY device that has the USB-to-TTL serial cable. They usually have the letters "USB" somewhere in the name.

You should be at an iex(1)> prompt. If not, try pressing Enter a few times.

Linux USB gadget mode

Linux USB gadget mode also supplies a virtual serial connection for some Nerves targets.

Troubleshooting

First boot shows error messages

First boot shows error messages due to the file system not being formatted. Seems like something is wrong even though it isn't. This is visible if you attach to the UART and watch the messages the very first time that you boot off a MicroSD card.

Toolshed's exit not working in the serial console

It works, but Erlang doesn't automatically restart the shell. You should be able to type CTRL-G to get the Erlang job menu.

"could not find a PTY" Error when running screen command

Unplug the USB connector and re-plug it.

HDMI cable

On some Raspberry Pi family of targets such as rpi3 and rpi4, the IEx console is displayed on the screen attached to the HDMI port by default. You can simply connect your target device to a monitor or TV.

For troubleshooting start-up issues and for more advanced development workflows, it's desirable to connect from your development host to the target using a UART serial cable.

Here is how to override the default, for rpi3 as an example:

1. Look in the README of your target's system's documentation for a UART port name. For example, nerves_system_rpi3
2. Locate your project's erlinit configuration which is normally in your project's config/target.exs file
3. Add a ctty option with the UART port name as a value

 config :nerves,
   erlinit: [
+    ctty: "ttyAMA0",
     hostname_pattern: "nerves-%s"
   ]

USB data cable

Some Nerves targets can operate in Linux USB gadget mode, which means a network connection can be made with a USB cable between your host and target. The USB cable provides both power and network connectivity. This can be a convenient way to work with your target device.

Use correct USB port

Make sure to plug the USB cable into the USB OTG port. For example, the Raspberry Pi Zero has two USB ports. The OTG one is the "middle" one. The other one is power-only.

Use correct USB cable

Make sure your USB cable supports data transfer. Generally there are two types of USB cables:

• charging only
• charging and data transfer

Test the connection

Once the target is powered up, test the connection from your host:

ping nerves.local

Make the network connection

To make a connection via the Linux USB gadget mode virtual Ethernet interface:

ssh nerves.local

You should find yourself at the iex(hello_nerves@nerves.local)1> prompt.

To end your ssh connection type exit, or you can use the ssh command <enter>~.

nerves.local is an mDNS address

Most examples in this page are done with a macOS host, which has mDNS enabled by default. Linux and Windows hosts may have to enable mDNS networking.

Gadget-mode virtual serial connection

USB gadget mode also supplies a virtual serial connection. Use it with any terminal emulator like screen or picocom:

picocom -b 115200 /dev/ttyUSB0

Windows Device Manager / Network adapters has no USB Ethernet/RNDIS Gadget device?

It might be caused by this, so install the optional USB Ethernet/RNDIS Gadget driver to fix it.

Wireless and wired Ethernet connections

The config/config.exs generated in a new Nerves project will set up connections for USB and Ethernet by default.

The nerves_pack dependency simplifies the network setup and configuration process. At runtime, nerves_pack will detect all available interfaces that have not been configured and apply defaults for usb* and eth* interfaces.

• For eth* interfaces, the device attempts to connect to the network with DHCP using ipv4 addressing.
• For usb* interfaces, it uses vintage_net_direct to run a simple DHCP server on the device and assign the host an IP address over a USB cable.

If you want to use some other network configuration, such as wired or wireless Ethernet, refer to the nerves_pack documentation and the underlying vintage_net documentation as needed.