Usually, headless embedded systems can be managed conveniently using ssh. The consumer focused devices have web interfaces. SSH is perfect, for when the system is running, but if the boot process has problems, there is no screen to read the boot messages that the kernel prints out. For this purpose these devices usually log to a good old serial port, either exposed as an RS232 (+-12V) or internally with TTL level (usually 5 or 3.3V). Most desktop and notebook computers these days don’t have an RS232 port any more. But there are USB to Serial converters, which are a bit pricey but available in most stores. And then you need a serial nullmodem cable. I threw my old one away many years ago, thinking the serial age was over. But then I bought one again exactly to debug the boot process of an alix. But sometimes, the embedded system is in a location where it’s inconvenient to span a cable. That’s when an RS232 bluetooth adapter comes in handy.