diff --git a/src/main/rx/sumh.c b/src/main/rx/sumh.c
index 7d23a10eb5..6950b5d90b 100644
--- a/src/main/rx/sumh.c
+++ b/src/main/rx/sumh.c
@@ -15,6 +15,12 @@
  * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/*
+ * References:
+ *  http://fpv-treff.de/viewtopic.php?f=18&t=1368&start=3020#p44535
+ *  http://fpv-community.de/showthread.php?29033-MultiWii-mit-Graupner-SUMD-SUMH-und-USB-Joystick-auf-ProMicro
+ */
+
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>