I didn't plan to be suggesting anything when I initially posted, I just wanted to know what direction to go.
but when the idea of rolling a new g-code interpreter came up, I felt I needed to suggest better options.
I also suspect that the big CNC machines all have proprietary stuff inside, there's not a one of them that allows any other controller to be used so compatibility with anything is a non-issue for them.
I don't know what odrive had in mind to sync multiple boards to be able to do a coordinated move across boards, but I'd like to see it be something that could actually work with other boards (for example, use odrive for the big motors to move a gantry and then pthat to do Z axis, 4th axis, extruder or similar)
I think it's fair to say that the thing that issues the commands should take into account the speed needed for each axis so that coordinated motion will still happen across boards. It's just a matter of having a way to tell all boards to start at the same time.
the pthat command structure already supports defining coordinated moves within one board. We could extend that command set to include a 'delayed start' command
Define a GPIO on each board to be part of a bus that has a pull-up resister on it. Have every board (possibly including the master computer) pull the pin low under normal conditions.
Once they get a 'delayed start', they would switch from pulling the pin low to it being an input (possibly an interrupt). Once all boards stop pulling it low, the line will go high and all boards would start at the same time (and after a very short window, pull it low again)
the same command could be used in the middle of a sequence of commands to make sure that all the boards are in sync (modern clocks are pretty accurate, but we want to deal with what drift does happen)