LinuxCNC Support Forum :: Topic: Why not USB? (again) (1/8)

grawp	I've just read, that USB's Interrupt transfer mode can guarantee latency of 125uS. So why can't it be used for CNC control?
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Re:Why not USB? (again) 19 Sep 2011 12:33 #13314

cmorley OFFLINE Moderator Posts: 1489 Thank you received: 40 Karma: 89	If that is true then it is within our timing limits. Do enough USB devices support transfer mode? The biggest obstacle is just finding someone who will build a working system. reference: wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Emc2HardwareDesign
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Re:Why not USB? (again) 19 Sep 2011 12:53 #13316

BigJohnT OFFLINE Administrator Posts: 4960 Thank you received: 87 Karma: 134	Do you have a link to this info? John
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Re:Why not USB? (again) 19 Sep 2011 14:23 #13318

PCW OFFLINE Moderator Posts: 1668 Thank you received: 87 Karma: 116	Yes, you can set uFrame times of USB 2.0 to 125 uSec but I dont think that you can actually guarantee 125 uSec latency especially if the USB system is not totally dedicated to motion control. (it would be almost impossible to share the USB system with non real time tasks)
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Re:Why not USB? (again) 20 Sep 2011 14:24 #13335

grawp	BigJohnT wrote: Do you have a link to this info? John www.embeddedsys.com/subpages/resources/i...InterruptsAndUSB.pdf
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Re:Why not USB? (again) 20 Sep 2011 16:25 #13336

BigJohnT
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It seems to say that maximum latency is 125 usec to 4 seconds on high speed and says the smallest possible interrupt is 125 usec... I don't see anything that says it guarantees a steady 125 usec latency.

Depending on many factors the host processor may not be able to transfer the interrupt data at
the requested interval. OS design, driver design, application software, CPU speed, and bus
bandwidth may all limit the host’s ability to meet the obligation to poll for interrupt transfer data
within the required interval.

John

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