diff --git a/g2core/canonical_machine.h b/g2core/canonical_machine.h
index a557d608..742b21f0 100644
--- a/g2core/canonical_machine.h
+++ b/g2core/canonical_machine.h
@@ -192,6 +192,7 @@ typedef enum {                          // these are in order to optimized CASE
     NEXT_ACTION_MARLIN_RESET_LINE_NUMBERS,     // M110
     NEXT_ACTION_MARLIN_REPORT_VERSION,         // M115
     NEXT_ACTION_MARLIN_DISPLAY_ON_SCREEN,      // M117
+    NEXT_ACTION_MARLIN_SET_MT,                 // M84 (with S), M85
 #endif
 } cmNextAction;
 
diff --git a/g2core/gcode_parser.cpp b/g2core/gcode_parser.cpp
index cccb9d45..cfeac90a 100644
--- a/g2core/gcode_parser.cpp
+++ b/g2core/gcode_parser.cpp
@@ -743,7 +743,9 @@ stat_t _parse_gcode_block(char *buf, char *active_comment)
 
                 case 117: return STAT_OK;  //SET_NON_MODAL (next_action, NEXT_ACTION_MARLIN_DISPLAY_ON_SCREEN);
 
+                case 18: // compatibility alias for M84
                 case 84: SET_NON_MODAL (next_action, NEXT_ACTION_MARLIN_DISABLE_MOTORS);
+                case 85: SET_NON_MODAL (next_action, NEXT_ACTION_MARLIN_SET_MT);
 
                 case 110: SET_NON_MODAL (next_action, NEXT_ACTION_MARLIN_RESET_LINE_NUMBERS);
                 case 111: return STAT_OK; // ignore M111, and don't process contents of the line further
@@ -941,11 +943,26 @@ stat_t _execute_gcode_block(char *active_comment)
             }
             break;
         }
-        case NEXT_ACTION_MARLIN_DISABLE_MOTORS:    {      // M84
-            ritorno(marlin_disable_motors());
+        case NEXT_ACTION_MARLIN_DISABLE_MOTORS:    {      // M84 and M18
+            if (gf.S_word) {
+                ritorno(marlin_set_motor_timeout(gv.S_word));
+                gf.S_word = false;
+            } else {
+                ritorno(marlin_disable_motors());
+            }
+
             break;
         }
-        case NEXT_ACTION_MARLIN_DISPLAY_ON_SCREEN: {      // M184
+        case NEXT_ACTION_MARLIN_SET_MT:            {      // M85
+            if (gf.S_word) {
+                ritorno(marlin_set_motor_timeout(gv.S_word));
+                gf.S_word = false;
+            } else {
+                // this means nothing, but it's not an error
+                return status;
+            }
+        }
+        case NEXT_ACTION_MARLIN_DISPLAY_ON_SCREEN: {      // M117
             // ignore for now
             return status;
         }
diff --git a/g2core/marlin_compatibility.cpp b/g2core/marlin_compatibility.cpp
index e80ce843..e1625f19 100644
--- a/g2core/marlin_compatibility.cpp
+++ b/g2core/marlin_compatibility.cpp
@@ -31,6 +31,7 @@
 #include "temperature.h"     // for temperature controls
 #include "json_parser.h"
 #include "planner.h"
+#include "stepper.h" // for MOTOR_TIMEOUT_SECONDS_MIN/MOTOR_TIMEOUT_SECONDS_MAX
 #include "MotateTimers.h"    // for char definitions
 #include "MotateUniqueID.h"  // for Motate::UUID
 
@@ -484,7 +485,7 @@ stat_t marlin_set_fan_speed(const uint8_t fan, float speed)
 
 
 /*
- * marlin_disable_motors() - M84
+ * marlin_disable_motors() - M84 (without S)
  *
  */
 
@@ -500,6 +501,33 @@ stat_t marlin_disable_motors()
     return (STAT_OK);
 }
 
+/*
+ * marlin_set_motor_timeout() - M84 (with S), M85 Sxxx
+ *
+ */
+
+stat_t marlin_set_motor_timeout(float s) // M18 Sxxx, M84 Sxxx, M85 Sxxx
+{
+    if (s < MOTOR_TIMEOUT_SECONDS_MIN) {
+        return (STAT_INPUT_LESS_THAN_MIN_VALUE);
+    }
+    if (s > MOTOR_TIMEOUT_SECONDS_MAX) {
+        return (STAT_INPUT_EXCEEDS_MAX_VALUE);
+    }
+
+    char buffer[128];
+    char *str = buffer;
+
+    // TODO: support other fans, or remapping output
+    str_concat(str, "{mt:");
+    str += floattoa(str, s, 1);
+    str_concat(str, "}");
+
+    cm_json_command(buffer);
+
+    return (STAT_OK);
+}
+
 
 /*
  * marlin_report_version() - M115
diff --git a/g2core/marlin_compatibility.h b/g2core/marlin_compatibility.h
index d1673ff0..1f6d50c6 100644
--- a/g2core/marlin_compatibility.h
+++ b/g2core/marlin_compatibility.h
@@ -48,6 +48,7 @@ stat_t cm_marlin_set_extruder_mode(const uint8_t mode); // M82, M82
 stat_t marlin_set_fan_speed(const uint8_t fan, float speed); // M106, M107
 
 stat_t marlin_disable_motors(); // M84
+stat_t marlin_set_motor_timeout(float s); // M18 Sxxx, M84 Sxxx, M85 Sxxx
 
 stat_t marlin_report_version(); // M115
 #endif  // End of include guard: MARLIN_COMPAT_H_ONCE