From 357e31270aa97cf9cae6cf8da335666af1fcaec6 Mon Sep 17 00:00:00 2001
From: Gabe Rosenhouse <gabe@missionst.com>
Date: Wed, 19 Feb 2014 00:51:43 -0800
Subject: [PATCH] Prevent G29 without first homing in X and Y

If position is unknown, then G29 can be dangerous.
---
 Marlin/Marlin_main.cpp | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Marlin/Marlin_main.cpp b/Marlin/Marlin_main.cpp
index f98d530de..444d69828 100644
--- a/Marlin/Marlin_main.cpp
+++ b/Marlin/Marlin_main.cpp
@@ -1409,6 +1409,15 @@ void process_commands()
             #error "You must have a Z_MIN endstop in order to enable Auto Bed Leveling feature!!! Z_MIN_PIN must point to a valid hardware pin."
             #endif
 
+            // Prevent user from running a G29 without first homing in X and Y
+            if (! (axis_known_position[X_AXIS] && axis_known_position[Y_AXIS]) )
+            {
+                LCD_MESSAGEPGM(MSG_POSITION_UNKNOWN);
+                SERIAL_ECHO_START;
+                SERIAL_ECHOLNPGM(MSG_POSITION_UNKNOWN);
+                break; // abort G29, since we don't know where we are
+            }
+
             st_synchronize();
             // make sure the bed_level_rotation_matrix is identity or the planner will get it incorectly
             //vector_3 corrected_position = plan_get_position_mm();