As we were discussing at the closed loop stepper thread, in a closed loop system (servo or stepper) the driver knows both the commanded and the actual position and tries to compensate when they are found different. The drivers will also typically have some sort of output signal. In step & direction mode the output signal of a Clearpath’s embedded driver can be configured to report shutdown, torque or speed.
Without using this output, each driver only cares about its own axis. Having switched to single Y transmission (and having each axis properly aligned) I have no particular use of the output signal. In case I was still running dual Y though, I would be more than interested. Since GRBL has no native way to deal with such a signal, I would probably hook up the two Y output signals in a logical OR circuit in order to cut down the common Enable signal (i.e. shutdown all motors) should something went wrong with either Y drive.
Or if I wanted something fancier, send them to the GPIO of the Raspberry Pi and in case something went wrong, shutdown the motors, play an anthem, send a tweet and pay the rent.
Which reminds me that at some point I need to research bCNC’s top killer (and least documented) feature: python scripting. Which should compare to gcode macros as a scooter compares to a Ferrari. (macros as parametric and conditional, not UGS snippet “macros”).