Sometimes a Pi just won't boot. Or it'll boot, but it'll do weird things. Or you don't have an HDMI display, and you can't log into your Pi via SSH. Or maybe you're like me, and someone 'accidentally' cut your Raspberry Pi in half, and you want to see what it's doing since it won't boot anymore.

The Raspberry Pi can output information over a 'serial console', technically known as a UART (Universal Asynchronous Receiver/Transmitter). Many devices—including things like storage controller cards, which in a sense run their own internal operating system on an SoC—have a 'UART header', which is typically three or four pins that can connect over the RS-232 standard (though many do not operate at 12v like a traditional serial port! Use a USB-to-TTL adapter like the one I mention below).

Simply Embedded has a great overview of UART if you want to learn more.

If you want to access the Pi's serial console, here's what you need to do:

1. Buy a USB to serial adapter. I bought the Adafruit 954 USB-to-TTL Serial Cable.
2. Pop the Pi's microSD card into another computer, edit the config.txt file inside the boot volume, and add the following line at the bottom: enable_uart=1.
3. Save that change, eject the microSD card, and stick the card back into the Pi.

4. Plug the USB to serial adapter into the pins as pictured below on the Pi (Black to GND, White to GPIO 14/pin 8 (UART TX), and Green to GPIO 15/pin 10 (UART RX)):

   

5. Open a terminal window on your Mac, and run ls /dev | grep usb

6. Note the tty.usbserial- and cu.usbserial- numbers in there. That's the device you'll connect to on your Mac.

There are a number of ways to interact with the serial console on a Mac (and most are the same as on Linux, with sometimes minor usage differences), but the two I've used in the past are minicom and screen.

Use minicom

1. Install minicom (brew install minicom) so you can emulate a terminal connected over serial.
2. Run minicom -b 115200 -D /dev/tty.usbserial-0001
3. Boot the Pi.
4. Within a few seconds, you should see data in your session.

Note: In metacom, the Meta key is mapped to 'Esc' by default, at least on macOS. So press 'Esc-Z' to get help.

Use screen

1. Run screen /dev/tty.usbserial-0001 115200
2. Boot the Pi.
3. Within a few seconds, you should see data in your session.

Note: To exit the screen session, press Ctrl-A, then Ctrl-K, and confirm you want to exit.

See also: Adafruit's guide to using a serial console on the Pi.

I've done this process a few times in the past, but I've grown tired of looking back at old notes to remember specifically which pins to use on the GPIO, since I don't do it that often and the pins are unlabeled on most Pis. Hope this helped!