> The usual technical solution for exact synchronization where commands must > go over a shared media is to time-tag the commands with the time when the > command is to be executed then send the commend in advance of that time. > On a fast bus like CAN you only have to send the command a few > milliseconds in advance. But on a slow channel such as a radio link to a > rover on Mars, they send commands hours or a day in advance.
In CANopen over a CAN bus standard method a sync message is used for this purpose. Periodic PDOs are sent anywhere within communication period and started to be used then sync message is received, usually COB-ID 0x80 but may be configured to other. > On CAN you can take advantage of the fact that all devices read the bus at > the same time. Each reader decides what information it wants to read and > ignore the rest so a time-sync heartbeat could be implemented if the nodes > all needed to be time synchonized. So on CAN if it were needed I'd invent > a protocol that did something like this: "motor A, On the mark, you are to > move to X", "motor B, On the mark, you are to move to Y", "MARK". Sync is used for this purpose. The fact that all devices read the bus at the same time may have some advantages, no crossing of messages may have some advantages then things should be agreed upon. > For a robot on Mars, I would send a table of instructions where the first > column is the time to execute and the second column is the instruction > itself. I would send the table then have the rover read it back to me to > verify. Then at some later time, the wheels would move and so on. Read something in the CANopen about receive FIFO but are uncertain about possibility to verify if properly received, guess simplest would be if receiver make complain if expected message not received in time so it could be resent and in such case some missing messages could be expect without loss of any functionality. It will also work for coordinated motion but add some delay but if path is known beforehand this is not a problem. FIFO will work for CNC path but EDM will be worse. Nnicklas Karlsson _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
