When setting up a system with only two axes, such as a laser engraver or pen plotter, there are a couple of items that must be considered or you will run into strange errors and apparently unexplainable behavior.

Homing Configuration

Ensure you've modified GRBL's config.h file properly before uploading the GRBL code to your Arduino or homing may fail and it won't be obvious why. The default 3-axis homing mechanism is defined in config.h as:

#define HOMING_CYCLE_0 (1<<Z_AXIS)                // REQUIRED: First move Z to clear workspace.
#define HOMING_CYCLE_1 ((1<<X_AXIS)|(1<<Y_AXIS))  // OPTIONAL: Then move X,Y at the same time.

You will note here that the Z axis is homed first, which will work fine as long as you use only soft limits. The system will appear to hang for a few moments, and then home the X and Y axes properly. However, if you've also set up hardware limit switches and hard limits ($21=1), the system will hang indefinitely because while homing Z, it will never hit the non-existent Z limit switch.

Comment out the above lines in config.h and uncomment one of the two provided two-axis configurations:

// NOTE: The following are two examples to setup homing for 2-axis machines.
// #define HOMING_CYCLE_0 ((1<<X_AXIS)|(1<<Y_AXIS))  // NOT COMPATIBLE WITH COREXY: Homes both X-Y in one cycle. 

// #define HOMING_CYCLE_0 (1<<X_AXIS)  // COREXY COMPATIBLE: First home X
// #define HOMING_CYCLE_1 (1<<Y_AXIS)  // COREXY COMPATIBLE: Then home Y

The first one will work fine for non-corexy systems where you can home both X and Y and the same time. Use the second one for corexy systems where homing both X and Y at the same time is not possible.

Limit Switches

Jumper your Z limit terminals to ground for normally closed switch configurations.

If you've built your system to use normally-open limit switches, you will avoid this issue because the unconnected Z axis is normally open by default. However, if you've wired your system to use normally-closed limit switches then, unless you jumper your Z limit pins to ground, GRBL will always start up with a limit-switch fault. You'll scratch your head, wondering why your limit switches seems to be working properly but there's always a limit-switch fault reported.

For more information on limit switch configurations, refer to the limit switch page.