Chinese CNC 3020 / 6040 - USB variant

Hello there,

so, reading through some of the basics of "LinuxCNC", from what I gathered, this all started using CNCs which are controlled via LPT parallel port, using a real-time linux kernel, to get around having an extra control box which buffers g-code commands and can execute them safely in real-time.

Now I browsed some ebay offerings for Chinese 3020 CNC routers.
Some today do come with USB (and a demo version of Mach3).
For this to work at all reliably, I would assume they must have a command buffering "on board" already with this setup?

So my actual question is - does LinuxCNC support those thingies with USB instead of parallel port?
*If* it has a command buffer / real-time command issuing built-in, that would nicely eliminate the hardware restrictions of the PCs I can use to run the software. And it costs like 15..20 bucks more than with parallel port...

Thanks in advance!

No Linuxcnc does not support motion control via USB. The whole idea of using a command buffer violates one of the main tenets of Linuxcnc.

PS it didn't start with lpt interfaces. It started with proprietary servo interfaces. Step/dir over lpt came later.

Now I'm confused!
Violates "tenets"?
The mere existence of the page listing hardware combos that do not screw up the real-time constraint tells me that having a command buffer is a good thing, not a bad one.
What am I missing?
What's actually a *virtue* of this setup?

One virtue is that complex real time motion control is located in the host. This allows things like
spindle synchronized motion (threading and rigid tapping) and real time reaction to other external
data (say for plasma torch height control or adaptive feed) etc to all be available and identical at
the control level regardless of the external hardware used.

It also means that down to the lowest level, almost any possible motion control strategy is achievable
(and fully open and extensible and not dependent on closed source external devices)

Correct me if I'm wrong, but I believe the Chinese 3020/6040 USB models do not actually talk via USB. They still have the parallel port connector to interface with the PC for motion control.

The USB port is provided only to tap off a 5V supply from the PC to the control box. It's no different to those cheap, generic parallel port breakout boards you can get on eBay.

The 'demo' of Mach3 that is provided with those machines is usually a cracked copy of the software. When I bought a 3020 several years back, I just chucked mine in the bin when it arrived.

Ah! Makes sense.

Can you tell how relevant it would be for some chinese CNC 3020 thing with at most 800W (or less if I can get away with it)? If it can't even do those advanced things because it's too weakly or inaccurately built, it may never be relevant ...

Just so that there's some idea about that...
My vision of what I'm going to do with the CNC router is:
1) mill PCBs, to try out a mildly complex* schematic on a weekend (I'm aware of Seeed Studio as a more pro option)
Especially larger, lower density PCBs which are unduly expensive even with chinese PCB makers.
2) front panels: prototypes from PVC or so, or if the machine can do it, thin aluminium: make LCD "windows", slits for fader potentiometers, drill holes etc.
3) perhaps plot labeling onto panels, with an ink pen or what have you
4) make some not too complex plastic parts, like angled distance pieces of a certain size, to hold PCBs in some case

There are also arduino-based motion controllers, not looked into it, but if they are open source, it would be possible, in theory, to modify this stuff to one's needs, right? Although this probably comes with a dozen caveats

Don't get me wrong, I'm absolutely open to going the route as it has been explained here.
Just mapping uncharted (to me) territory, to be able to make good decisions.
I haven't bought anything yet, but the fingers are itching to get going finally

I mean... I'd want this thing in another room than where I'm residing anyway, so it "needs" an extra computer.
I have a couple old PCs in the cellar: Duron 600, Athlon XP 2000, I think even an AMD 64
But I doubt it's gear that is in your list.
If I could get that to work, though, I'd tell my father on the next visit: "see, I told you I shouldn't throw those out prematurely!" But let's see, next couple evenings or so...

* too error-prone for perfboard + spaghetti wires & no DRC