Configuring Homing

This guide covers the practical steps to enable and configure the homing cycle on your machine. For a theoretical explanation of how homing works, see Homing Explained.

Step 1: Enable Homing

By default, homing is disabled. To enable it:

1. Send $22=1 in the console.

Note: Once enabled, grblHAL will likely lock the machine upon startup (Alarm 1) until you perform a homing cycle ($H).

Step 2: Set Homing Direction ($23)

This is the most critical step. You must tell grblHAL which direction to move to find the switches.

Default Behavior:

• Z axis moves UP (Positive)
• X axis moves RIGHT (Positive)
• Y axis moves BACK (Positive)

If your switches are in the opposite location (e.g., front-left), you must invert the search direction using $23.

To find the correct value:

1. Determine where your switches are physically located.
2. Toggle the bits for the axes you need to invert.
3. Use the table below:

Value ($23=)	X Direction	Y Direction	Z Direction	Typical Setup For
0 (Default)	+ (Right)	+ (Back)	+ (Up)	Standard CNC (Back-Right-Top)
1	- (Left)	+ (Back)	+ (Up)
3	- (Left)	- (Front)	+ (Up)	Front-Left Origin (Common on Lasers)
7	- (Left)	- (Front)	- (Down)

Test carefully:

• Be ready to hit E-Stop!
• Send $H
• If an axis moves the wrong way, reset, change $23, and try again.

Step 3: Configure Switch Type ($5)

Are your limit switches Normally Open (NO) or Normally Closed (NC)?

• $5=0 (Default): Normally Open (NO). Signal is HIGH when triggered.
• $5=...: Invert specific axes where switches are Normally Closed (NC) or logically inverted.

Unlike 8-bit Grbl, grblHAL uses a bitmask for $5, meaning you can invert limit pins per axis.

Axis	Bit Value	Description
X	1	Invert X limit input
Y	2	Invert Y limit input
Z	4	Invert Z limit input
A	8	Invert A limit input
B	16	Invert B limit input
C	32	Invert C limit input

Example: To invert X and Y but keep Z normal: Add 1 + 2 = $5=3.

Recommendation: Use Normally Closed (NC) switches if possible. They are safer because a broken wire will trigger the alarm immediately, rather than failing silently.

Step 4: Fine Tuning

Homing Search Rate ($25)

• Speed at which the machine "seeks" the switch initially.
• Default: 500 mm/min
• Tune: Make it fast enough to be convenient, but slow enough to not crash hard into the switch.

Homing Locate Rate ($24)

• Speed for the second, precise touch-off.
• Default: 25 mm/min
• Tune: Keep this slow for maximum repeatability.

Homing Pull-off ($27)

• Distance the machine backs away from the switch after triggering it.
• Default: 1.0 mm
• Tune: If your switch stays triggered (Alarm 8), increase this to 2.0 or 5.0 mm. It must back off enough to clear the internal switch mechanism.

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Troubleshooting Homing

Machine moves wrong way

• Adjust $23 (Homing Direction Invert).

Alarm 8: Homing Fail

• The switch did not clear after pull-off. Increase $27 (Pull-off distance).
• Or, interference/noise on the switch line.

Alarm 9: Homing Fail

• The switch was not found within the expected distance ($130, $131, $132 max travel settings).
• Ensure your Max Travel settings are accurate or slightly larger than physical travel.

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Next Step: With homing set, let's squeeze the last bit of precision out of your mechanics: Backlash Compensation