UART Usage

Description

AIO-3576JD4 use UART3 for RS485, which is /dev/ttyS3 in system. And
UART8 for RS232, which is /dev/ttyS8 in system.

And GPIO3_A3 is used for direction control of RS485, pull it up will
change RS485 to send mode, pull it down will change RS485 to receive
mode.

Users can use /sys/class/gpio subsystem to control this GPIO.

Interfaces: