I picked up a SX2 Mini mill recently via Little Machine Shop (They call it the HiTorque Mini Mill 3900). When I received it, I noticed there was a port for a digital tachometer readout on the side of it. I though this might be useful, but felt 125$+ for it was a little excessive for what it was. Atop of this, they were out of stock.
I ended up reverse engineering the protocol, building some less expensive kits, and documented how exactly I figured out the protocol. Along the way, I also discovered how to make the mill run in reverse!
If you wondered how a reverse-engineering problem like this is approached or what tools are used, this could be an interesting read: