diff --git a/ROMFS/px4fmu_common/init.d/rcS b/ROMFS/px4fmu_common/init.d/rcS
index 6f38640ede..33f3fbd4e0 100644
--- a/ROMFS/px4fmu_common/init.d/rcS
+++ b/ROMFS/px4fmu_common/init.d/rcS
@@ -15,7 +15,7 @@ set +e
 #
 # UART mapping on FMUv2/3/4:
 #
-# UART1			/dev/ttyS0		IO debug
+# UART1			/dev/ttyS0		IO debug (except v4, there ttyS0 is the wifi)
 # USART2		/dev/ttyS1		TELEM1 (flow control)
 # USART3		/dev/ttyS2		TELEM2 (flow control)
 # UART4