         else
             log(F("Invalid motor designator"));
     }
+    else if (c1 == 'd')
+    {
+        if (c2 == 's')
+          set_diff_speed(value);
+        else if (c2 == 't')
+          set_diff_turn(value);
+        else
+          log(F("Invalid drive parameter"));
+    }
     else if (c1 == 's') {
         // "sp" - pan servo, "st" - tilt servo
         

    }
}

///////////////////////////////////////////////
// MOTORS



Servo left_motor_controller;
Servo right_motor_controller;
int diff_speed = 0; // differential speed is -100..100
int diff_turn = 0;  // differential turn is -100..100

// Configure motor hardware.
void setup_motors()
