Serial cable
Last updated
Last updated
A serial cable is required for professional development. This is necessary to connect to the microprocessor's console port, which will output logs on bootup, allow access to the U-Boot console, and the Elixir IEx console. It is also helpful for troubleshooting other devices with serial ports, like microcontrollers, cellular modems, etc.
Serial cables come in many different varieties, and it doesn't hurt to have different types to suit the different projects you may work on. The two most important things to consider when selecting a serial cable are the electrical interface and connector type. Most development kits will expose a 3.3V TTL interface via a 0.1" pitch header, which is designed to connect to the FTDI TTL-232R-3V3 cable. FTDI's drivers are also included with modern versions of Linux and Windows, so the cables work out of the box. On Linux the serial port will show up as /dev/ttyUSB0.
Applications like picocom or screen can be used to connect to a serial port from the terminal.
⚠️ Trap
"I can buy a serial cable for much less on Amazon."
An off-brand cable may not come with drivers, may not support your operating system, may not follow the FTDI pinout, and may not be the right voltage. A serial cable will get used frequently and will be with you a long time. Get the quality cable.
Another popular connector for the console port is Tag Connect's 6-pin footprint. This tends to be found on prototype and production PCBs, and saves space. The connectors can be ordered with or without "legs". The legs are the plastic clips that hold the connector onto the PCB. The legged version is intended for prototyping, whereas the no legs version is designed to be mounted in a fixture. The no legs version also has a smaller footprint. If you need to use a no legs cable without a fixture, check out the GRIP-6.
Tag Connect TC2030-FTDI-TTL-232R-3V3
Sometimes it may be necessary to use an FTDI cable with a serial port header that doesn't match the FTDI pinout. In this case, 0.1" jumper wires can be used to connect the pins from the serial cable to the correct pins on the header.
In this example, jumper wires are connected between the FTDI cable and a Raspberry Pi:
⚠️ Trap
"The Nerves docs say I can connect to the hardware with USB gadget mode."
The official Nerves systems expose the IEx console via a USB gadget mode network interface. When a USB cable is connected between the computer and dev kit, an SSH connection can be used to get to the Nerves IEx console on the dev kit.
Although this is great for trying out Nerves, it's not a good method for professional development. Standing up or modifying a Nerves system often involves needing to read the logs while the system boots, before the SSH daemon comes up. It can also be necessary to access U-Boot before the kernel starts. The best way to do this is by connecting a serial cable. Not to mention your product typically shouldn't expose its console to end users when they plug in a USB cable.
