Why there aren't many USB CNC boards?

[VAGOS]

Hi there!!!

I would like to know why there are't many usb CNC boards in the market?

I am thinking before buying a CNC, to buy a cnc board and some small (cheap) steper motors to just get some first hand experience...

So I searched and found this board its usb:

CNC USB controller

But mostly I have found serial port boards:like this

CNC TB6560 3 Axis Stepper Motor Driver Controller Board | eBay

Can someone please tell me why this happens? Is thre a reason?

Best, Regards
Vagos

[ger21]

First, the Planet CNC board is a controller, and will not directly run your motors. You still need to get motor drives.

The Ebay board is a motor driver board, and is parallel port, not serial.
Parallel port boards are very common, because they work with many software control programs like Mach3, and are inexpensive.

There are a few other USB options available, but tend to be quite a bit more expensive than using a parallel port control.

[VAGOS]

Thank you ger21 for your reply!

So from what I understand there are two board options to drive the motors :

1)Get a Stepper Motor Driver Controller Board like this:

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2)Or get a Breakout board with separate motor drivers like this:

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Am I right?
Also the second option I assume it can control bigger steper motors?

If I take a parallel to usb cable adapter like this:



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,and connect the parrallel port board to my pc with usb will it work?
...or I would have problems?

Best, Regrads
Vangelis

[aarongough]

Hey VAGOS,
USB -> Parallel port adapters won't work. Here is the reason why:

When a computer is talking to a printer via a Parallel port it is transmitting data, and it doesn't really matter how long it takes to get that data there, your print job will just take longer.

So a usb to parallel adapter basically takes the printer data 1 bit at a time and transfers it to the chip on the other end of the adapter cable. That chip then reconstructs that data and sends it out over the parallel port.

This won't work for a CNC machine because when you're talking to a CNC you're not transmitting data, instead you are turning on and off individual pins of the parallel port simultaneously in order to be able to tell the motors where to go. It's very important that you can talk to more than one motor at the same time!

I realize this is a little technical, the short answer is that it won't work.

If you have a desktop computer however the PCI based parallel port cards will often work fine. The most reliable method though is to get a motherboard that has the parallel port directly on it...

[Al_The_Man]

The other method that uses true closed loop USB CNC is the Dynomotion system.
In this case the PC is used just for the HMI via USB.
Al.

[VAGOS]

Thank's aarongough really well explained!

For start I am thinking to buy a usb control board so I dont't have to go back and work to my old computer witch has parallel port...

Also after searching and reading on the internet I found, that you run the risk of toasting your computer if you don't use a break board (for parallel port boards) to isolate your computer, if something happens...

Is there the same risk if I am using a usb control board do I have to worry destroying my computer if I make something wrong?

[aarongough]

Originally Posted by VAGOS Thank's aarongough really well explained! For start I am thinking to buy a usb control board so I dont't have to go back and work to my old computer witch has parallel port... Also after searching and reading on the internet I found, that you run the risk of toasting your computer if you don't use a break board (for parallel port boards) to isolate your computer, if something happens... Is there the same risk if I am using a usb control board do I have to worry destroying my computer if I make something wrong?

I think that there's always some level of risk when connecting your computer to homebrew electronics, but if the USB controller board is separate from the motor driver boards then I imagine that risk would be relatively small.

Another thing to think about is that it is generally considered bad practice to do other tasks on the computer while it is running your CNC machine, that is because if you do something like open a resource-intensive web-page then the computer may be too busy doing that to run the CNC properly. This may result in malformed parts or even a damaged machine.

So keep in mind that you will likely be swapping back and forth between computers anyway, which may mean that it would be easier just to use a computer with a parallel port...

I use and highly recommend the Geckodrive G540 driver board. It connects directly to your parallel port and is a *very* high performance stepper driver.

Low-end boards like the TB6560 board you linked earlier will give you all sorts of problems if you're trying to run fast step rates, so I tend to stay away from them...

[mactec54]

VAGOS

The Breakout Board needs to be Opto Isolated to protect the computer no matter what you use USB or Parallel

[aarongough]

Originally Posted by mactec54 VAGOS The Breakout Board needs to be Opto Isolated to protect the computer no matter what you use USB or Parallel

I thoroughly agree.

[Al_The_Man]

Originally Posted by aarongough I thoroughly agree.

There are two methods, complete isolation or bonding of all systems to Earth Ground at a central point, if both if carried out properly either can be effective, I have used the equipotential (common) bonding method for many years with no ill effects.
In any case, in the majority of systems that use a spindle fed from the AC supply, e.g. router or VFD etc, the motor AC (neutral) will NOT be isolated from your PC P.S or mother board (-ve) as both are made common to Earth Ground.
Not bonding systems together has been the cause of many noise and switch pre-tripping problems IMO.
Al.

[wcarrothers1]

I run a few machines. One is a 4x8 I built with gecko servos, and can run paralle or Smooth stepper. I can switch between the 2 drive types at will with a selector switch. Most of the time I run in parallel mode. Only perhaps minor improvements in performance with SS if any. But for the most part I don't like how it lags movement between me spinning my pendant dials and actual start or stop of movement on the machine. Parallel mode movement is 1 to 1.

I will also admit to owning a G100 (Grex) back in the day as well. How ever compaired to the SS that was a joke. SS is an easy drop in..

my new machine 5x10 uses analog Moog brushless servo drives. And because of that I had only a few controller choices. I'm currently using a Kanalog + Kflop controller on that machine. Dynomotion stuff is just briliant by the way. Little more complex to drop in then the SS. But has a ton of great IO and once you understand a little it really isn't bad at all. I really like it. Dial pendant movement is also very nice and precice...

If I did any update to my 4x8 machine it would be to convert it from parallel to Kflop control.

Support from Dynomotion is also top notch...

I would rate the chances of toasting your parallel port or machine as pretty low unless you do something completely stupid. Really you are just routing those in and out signals to where they should go.

b.

[MechanoMan]

USB is a dicey way to do it, because USB was not intended for realtime control like this and you can't guarantee the transfer of data on-time. So, you can buffer. How much buffering? Who knows? There's no set specification on how many data points go into a curve. This may come down to the decisions Mach3/EMC2 makes, or even the way the G-code was written.

A milling program may have a set way to handle G02/G03 circles, but then a CAM program decides to describe a circle with a million G01 moves, which requires like 20K for a 1/2 sec buffer.

Many attempts to talk through USB have involved a microcontroller. The problem here is that the controller has a limited PWM resolution here, and for multiple axes and high pulse rates like in a fast router this can be a SERIOUS resolution problem, as well as basic instruction speed. A router cutting air at 1500IPM on 200 steps/rot at 10x microstep Geckodrive on a CNCRouterParts router would need 2.8M steps/sec. The basic core on a common microcontroller is only like 20M-40M instructions/sec, so keeping up with 3 or 4 axes is unrealistic.

So, it often comes down to a faster FPGA solution. FPGAs are expensive beasts. The thing is, once you get into expensive beasts, we start wondering "why screw around with the problematic USB link?"