I was just reading a few weeks ago in the book "Sapiens" that the early explorers set up an experiment where they would observe an astronomical event from both England and the South Pacific. Something about either time or position.
I think it was Cook who was exploring at that point. I'll have to dig through to see exactly what it was. Still quite something to plan on observing something that will take you a year or more before you are even there to do the observing. John Dammeyer > -----Original Message----- > From: Chris Albertson [mailto:albertson.ch...@gmail.com] > Sent: August-19-20 2:09 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] OT: Synchronised motion using RS485/CAN bus motors > > I think the question was intended to be more theoretical and asks about > "exactly" synchronizing commands. The LinuxCNC/SPI solution is not > that. SPI works only because it is so fast that the error in > synchronization is tiny and goes unnoticed. > > Here is a harder problem. Let's say I am in North America and by buddy > lives in Europe and we want to each run clocks and we want them to stay in > phase at a high level of accuracy. To make matters worse assume this is > the mid-1800s and the radio is not yet invented. They actually solved this > problem. The solution was "mutually observed events" and we use this same > solution today to keep widely dispersed machines in sync. In the old > days, they would observe one of Jupiter's moons from both America and > Europe and assume they both say the moon transit the planet at the same > time. Orchestras use a conductor waving a stick who is "mutually > observed" by all musicians. Same with a CAN bus, you could, if needed > use a high priority "clock tick" message that all nodes see at the same > time. > > But in real-life. We accept "close enough" and just us a SPI signal that > is fast enough that no one notices the error. > > On Tue, Aug 18, 2020 at 8:08 PM Frank Tkalcevic <fr...@franksworkshop.com.au> > wrote: > > > > You subject line says RS485/CAN which are dramatically different from the > > SPI based synchronous clocked serial interfaces. Even RS485 and CAN are > > dramatically different. > > > > Thanks for the replies... > > > > The question was around slower RS485/CAN. I'm seeing a lot of actuators > > (motor/gearbox/driver combinations) that are driven by CAN bus (MIT > > cheetah). > > > > Brute speed seems to be a common solution, which I'm guessing protocols > > like > > EtherCAT rely on. > > > > Given the CAN bus speed limits - 1MHz, it doesn't seem possible to > > send/receive messages to many motors at a typical LinuxCNC 1kHz rate. Is > > there some kind of "smarts" that let these control systems work smoothly at > > lower update rates? > > > > > > > > > > > > > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
