BeagleBone with LinuxCNC to replace old PC?

The adapter looks interesting.

How would Mach4 help? I thought it also needs a parsllell port?

Plenty of USB solutions that will work with either Mach 3 or 4, if all you can get is a laptop with USB

Mini PCs are a good solution for LinuxCNC using Ethernet or EtherCAT
connected motion. These all have built in Ethernet AFAIK. The RPI4 is another
option.

I looked at the BBB and used it some years ago. It absolutely works.
But...
limited amount of memory so forget 100.000 lines of g-code. Crashes
very poor graphics, unusable, not to mention working with large g-code files
you need to find a cape for getting the signals out - I made my own cape

I spent a lot of time on this, then. Save yourself the trouble and
go for a PC or perhaps a RPi4.

BeagleBrainz wrote: If you want small the Odoird H2 would make a good candidate.

Odroid H2+ J4115 Celeron CPU, 2 NIC this has a 64GB SSD the size of your thumbnail and better latency than my Gigabyte Brix J1900

I think the Desktop PC is dead and replaced with Gigabyte BRIX and Intel NUC's. (The NUC I have even has a Radeon Graphics card!) and its mounted to the back of my monitor.

I think the laptop is dead and replaced by the Chromebook I am writing this on.

I want to buy another NUC to have at home.

I am looking forward to use the H2+ on my next CNC machine.