When deploying a self-compiled kernel it is not unusual that the system does not come up to the point where something can be seen through the HDMI display, or an embedded system does not have a display connector at all.

In this case, a terminal connection through a serial connection can be made. Some embedded boards have designated connectors for that while others use GPIO pins.

There are cheap Serial-to-USB adapters, e.g., based on the PL-2303HX chipset. They should work without extra drivers on Linux and MacOS. On my Mac the device appears as /dev/tty.usbserial-120, so i can connect my terminal to it through screen /dev/tty.usbserial-120 115200

If such a USB dongle is not available, one might also connect through a Raspberry Pi SBC. The cabling for connecting a Seeed i.MX6ULL NPI board is as shown in the photo:

According to the Raspberry Pi pinout diagram, there are several options for Ground (GND, e.g. pin 6), whereas serial TX is GPIO 14 (pin 8, connected to RX on the debugged board) and RX is GPIO 15 (pin 10, connected to TX on the debugged board). 5V (at pins 2 and 4) can also be connected, in which case the debugged board can be "back-powered" from the Raspberry Pi. From the Raspberry Pi terminal, i can then connect to the debugged SBC with screen /dev/ttyAMA0 115200.