USB to Parallel port converter
- Tractorpuller16
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18 Mar 2013 01:29 #31518
by Tractorpuller16
USB to Parallel port converter was created by Tractorpuller16
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.
Thanks for the input.
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- PCW
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18 Mar 2013 03:19 - 18 Mar 2013 03:24 #31521
by PCW
Replied by PCW on topic USB to Parallel port converter
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.
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.
Last edit: 18 Mar 2013 03:24 by PCW.
The following user(s) said Thank You: Tractorpuller16
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- andypugh
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19 Mar 2013 00:52 #31553
by andypugh
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.
www.ebay.com/itm/230947604821 as a random example.
Replied by andypugh on topic 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..
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.
www.ebay.com/itm/230947604821 as a random example.
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- skelem
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28 Sep 2016 05:32 #81022
by skelem
Replied by skelem on topic USB to Parallel port converter
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
With USB3 being so fast, is there any progress in getting a USB solution to work?
Thanks,
Steve
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- andypugh
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28 Sep 2016 08:53 #81028
by andypugh
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.
Replied by andypugh on topic USB to Parallel port converter
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.
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- andypugh
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28 Sep 2016 08:57 #81029
by andypugh
Replied by andypugh on topic USB to Parallel port converter
Interesting discussion here:
linuxmusicians.com/viewtopic.php?t=14032
Musicians seem to be happy with 6mS latency, and unhappy with 15mS. LinuxCNC need <<1mS.
linuxmusicians.com/viewtopic.php?t=14032
Musicians seem to be happy with 6mS latency, and unhappy with 15mS. LinuxCNC need <<1mS.
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- skelem
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28 Sep 2016 15:53 #81042
by skelem
Replied by skelem on topic USB to Parallel port converter
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 .
USB breakout boards are sold by Dynomotion and CNC4PC .
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- PCW
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28 Sep 2016 16:12 - 28 Sep 2016 16:19 #81044
by PCW
Replied by PCW on topic USB to Parallel port converter
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
(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
Last edit: 28 Sep 2016 16:19 by PCW.
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- Todd Zuercher
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28 Sep 2016 16:21 #81045
by Todd Zuercher
Replied by Todd Zuercher on topic USB to Parallel port converter
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.
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- skelem
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28 Sep 2016 16:30 #81046
by skelem
Replied by skelem on topic USB to Parallel port converter
Do any of them work with an existing Linux box that has ethernet and USB2/3?
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