Realtime isn't about latency but determinism and the standard usb driver stack is non-deterministic. I believe there are realtime capable stacks for usb devices but it needs to be supported by both devices. And I don't think most mcus support it.
If you don't care about the smoothness of tour motion or use a buffered approach, usb works fine. On Fri 19. Feb 2021 at 0.36, justin White <[email protected]> wrote: > It's not that simple. USB2 has higher latencies, true, but if you are OK >> with them, then the USB2 interface is as capable of real-time communication >> as any other interface. There have been attempts to mitigate these >> latencies with various results. (Search research papers online.) USB3 >> should be much better in this regard. >> > > I dont see how that's true when the default USB2 polling rate is 8ms. You > can change the mouse driver to get 1ms but I don't know that the RT patches > touch USB. So whats the secret sauce to getting RT USB if the polling > interval remains at 8ms on a shared bus? > > USB3 isn't really even worth talking about yes its peer to peer with much > better latency but an rpi4 having a USB3 port doesn't do any good when MCUs > really do not. Most MCUs bake usb2 hardware right onto the chip while usb3 > is generally a PCIE lane attached to a PHY which I have yet to see on any > dev boards. > > So what's the secret sauce to RT USBx? And .......why, when any MCU > contains fast UARTs, spi and sometimes ethernet. > > On Thu, Feb 18, 2021 at 11:23 AM cern via Machinekit < > [email protected]> wrote: > >> Hello, >> >> Feb 18, 2021, 14:32 by [email protected]: >> >> > USB itself isn’t realtime capable so it can’t be used for any realtime >> tasks. >> > >> It's not that simple. USB2 has higher latencies, true, but if you are OK >> with them, then the USB2 interface is as capable of real-time communication >> as any other interface. There have been attempts to mitigate these >> latencies with various results. (Search research papers online.) USB3 >> should be much better in this regard. >> >> Cern. >> >> > >> > On Thu 18. Feb 2021 at 11.18, > [email protected]> <> >> [email protected]> > wrote: >> > >> >> Be interesting to see more details on the Linuxcnc\USB\MESA setup. >> >> As far as I was aware there was no RT support for USB. >> >> I'm not even sure if MESA themselves recommend their USB boards for >> use with Linuxcnc. >> >> >> >> On Thursday, February 18, 2021 at 7:32:57 AM UTC+11 >> >> [email protected]>> wrote: >> >> >> >>> Great, thanks for finding it! >> >>> >> >>> I've seen lots of people purchasing 3DP controller boards to upgrade >> 3DP/Lasers/CNC's(consumer stuff) and many of them also use an rPi with >> OctoPrint or CNCjs, GridBot etc. >> >>> I like this approach in that all of my macines(4 3DPs, 2 laser >> cutters and 1 CNC) all have consumer grade controllers in them running on >> different uCPUs. Some AVR328, STM32, LPC17xx and one a BBB/CRAMPS. >> >>> But would love to be able to have LinuxCNC/Machinekit on an rPi doing >> the control, UI, setup. >> >>> On Wednesday, February 17, 2021 at 11:02:18 AM UTC-8 >>> >> [email protected] <>>>> wrote: >> >>> >> >>>> Hi, >> >>>> the code is available in >>>> >> https://github.com/scottalford75/Remora>>>> - looks like he though that >> spiPRU is not a great name and changed it to something more flashy. >> >>>> >> >>>> Looks interesting from programming point of view, but I wonder how >> many people would buy Smoothieboard and RPi both to run 3D printer with >> LinuxCNC on it? Most will stay with Smoothie, I think. >> >>>> >> >>>> Cern. >> >>>> >> >>>> Dne úterý 16. února 2021 v 19:18:20 UTC+1 uživatel >>>> >> [email protected] <>>>>> napsal: >> >>>> >> >>>>> I was about to start on setting up a Xylotex BareBoneCNC with a >> BBB/Machinekit and thought I'd look at what's going on with Raspberry Pi in >> CNC world. I found a video of a guy driving a Mesa board over USB with >> LinuxCNC and it looked impressive(doesn't take much). >> >>>>> >> >>>>> Got to wondering if a standard 3DP controller like SKR, >> Smoothieboard, etc couldn't run firmware similar to the MESA board and only >> found mention of this spiPRU but no details, code, anything. >> >>>>> >> >>>>> https://github.com/scottalford75/LinuxCNC-on-RPi >> >>>>> >> >>>>> I searched for spiPRU here and in general inter-web lands and found >> nothing but the mention of it in the above git repo. If anyone knew about >> it I figured they would be here. >> >>>>> Thx >> >>>>> >> >> >> >> >> >> >> >> -- >> >> website: >> http://www.machinekit.io>> blog: >> >> http://blog.machinekit.io>> github: >> https://github.com/machinekit >> >> --- >> >> You received this message because you are subscribed to the Google >> Groups "Machinekit" group. >> >> To unsubscribe from this group and stop receiving emails from it, >> send an email to >> [email protected]>> . >> >> To view this discussion on the web visit >> >> https://groups.google.com/d/msgid/machinekit/cc241b97-47bb-48d0-b3b4-e595dc809b55n%40googlegroups.com >> < >> https://groups.google.com/d/msgid/machinekit/cc241b97-47bb-48d0-b3b4-e595dc809b55n%40googlegroups.com?utm_medium=email&utm_source=footer>>> >> . >> >> >> > -- >> > Terv, >> > Joonas Forsell >> > >> > >> > >> > -- >> > website: > http://www.machinekit.io> blog: > >> http://blog.machinekit.io> github: > https://github.com/machinekit >> > --- >> > You received this message because you are subscribed to the Google >> Groups "Machinekit" group. >> > To unsubscribe from this group and stop receiving emails from it, send >> an email to > [email protected]> . >> > To view this discussion on the web visit > >> https://groups.google.com/d/msgid/machinekit/CA%2B%2BSnhq%3D-rmGD2z8ewbQ9ryBoyWgwymJTbmWp6NKnGPyhmsrbw%40mail.gmail.com >> < >> https://groups.google.com/d/msgid/machinekit/CA%2B%2BSnhq%3D-rmGD2z8ewbQ9ryBoyWgwymJTbmWp6NKnGPyhmsrbw%40mail.gmail.com?utm_medium=email&utm_source=footer>> >> . >> > > > >> >> -- >> website: http://www.machinekit.io blog: http://blog.machinekit.io >> github: https://github.com/machinekit >> --- >> You received this message because you are subscribed to the Google Groups >> "Machinekit" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/machinekit/MTpwssk--3-2%40tuta.io. >> > -- Terv, Joonas Forsell -- website: http://www.machinekit.io blog: http://blog.machinekit.io github: https://github.com/machinekit --- You received this message because you are subscribed to the Google Groups "Machinekit" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/machinekit/CA%2B%2BSnhrgbwYbXtseCcC6hZmb%3D1-7HcBJSmS2bzHhEs%2BnSuUbpg%40mail.gmail.com.
