USB to Parallel port converter

Will the USB to Parallel port converters work with LinuxCNC? I just have a cheap desktop cnc off of ebay right now and I need some software for it and my laptop doesnt have a parallel port so this is the only option for me.

Thanks for the input.

No, USB --> parallel converters will not work with LinuxCNC for step generation because
they have large latencies to access the I/O pins.

A normal hardware parallel port will change it's output pin state
typically in less than 1 uSec from when the host CPU does the port write.

A USB parallel port may take from a few uSec to a few 10s of mS from the host
write to the actual pins changing state. This random output timing makes evenly
spaced step generation pretty much impossible.

No eBay APP ID and/or Cert ID defined in Kunena configuration

Tractorpuller16 wrote: Will the USB to Parallel port converters work with LinuxCNC? I just have a cheap desktop cnc off of ebay right now and I need some software for it and my laptop doesnt have a parallel port so this is the only option for me..

Tying your (portable) laptop to your (not portable) CNC machine might not be a good idea anyway. You can run LinuxCNC on quite cheap hardware and retain the use of the laptop as a laptop.

You can run Mach3 ($175) using a USB Smoothstepper ($200) But that $375 could have bought a dedicated PC for your CNC controller.
I am using one of these: www.newegg.com/Product/Product.aspx?Item=N82E16813121710 which runs off of a 12V jack, though graphics support in Linux isn't great.
I rather suspect that one of these: www.newegg.com/Product/Product.aspx?Item=N82E16883211005 will have a parallel-port header on the motherboard, but you would need to connect to it.
Both the above need memory and a disc drive or SSD.

A second-hand business small form factor business desktop might be the better option.
as a random example.

So, here it is, 3 years later...none of those boxes are available any more, and parallel ports are a rarity.

With USB3 being so fast, is there any progress in getting a USB solution to work?

Thanks,
Steve

skelem wrote: With USB3 being so fast, is there any progress in getting a USB solution to work?

No. It isn't about speed, it is about latency The parallel port is very, very slow. But there is not very much data being transferred. Even a very busy Mesa system is probably only sending/recieving 100 bytes/mS

The problem with USB is that there is a lot of overhead and no direct access to the hardware. LinuxCNC needs data that is sent now to appear at the other end of the link immediately, and for all its massive throughput I don't think USB3 is much better for latency.

Googling for actual figures I see someone achieving 6mS latency with USB2 and 1.3mS with USB3. With a 1mS servo-thread 1.3mS is still much too long. Latency for transfer to a PCI card will be about 0.05mS and reading around th web I found numbers for the latency of the parallel port to be about 0.003mS.

Interesting discussion here:
linuxmusicians.com/viewtopic.php?t=14032
Musicians seem to be happy with 6mS latency, and unhappy with 15mS. LinuxCNC need <<1mS.

I see. But what if much of that work were offloaded to an inexpensive motion controller that handles all the critical timing? The heavy lifting would be done by dedicated hardware. This would remove the real-time linux requirement and the rapidly disappearing parallel port requirement. You could probably even run it on a raspberry pi for under $100. (If not raspberry, then on many other linux boxes.)

USB breakout boards are sold by Dynomotion and CNC4PC .

Unless you move HAL to the external device, you still need real time on the host
(or you lose the flexibility and extensibility of the HAL machine interface)

To be able to run HAL you need fast hardware, fast floating point, dynamic module loading, etc
so all you have ended up doing is moving LinuxCNC to the external device
(and LinuxCNC/Machinekit will already run on small ARM devices)

Adding another buffering layer (as USB requires) just makes the machine control model more complex

For less than the $250 bucks a Kflop costs (plus what ever breakout boards it might need) you can do a whole lot more with a couple of things from Mesa.

Do any of them work with an existing Linux box that has ethernet and USB2/3?