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How to use the RS485 on Aria module

RS485 driver IC like MAX485 needs three signals to manage an RS485 bus. These lines are:

• TXD Transmit data
• RXD Receive data
• DE Driver Enable data

Some of the serial ports available on the AT91SAMG25 MPU used on Aria G25 can use the RTS line as a DE line to be used with a RS485 driver chip.

To set the serial ports to work as RS485 instead of RS232 in required to tell to the Linux serial driver to work as RS485 trought the Device Tree Binding definition as explayed below.

Suitable signals

This is the list of suitable signal available on Aria pinout useful to implements up three RS485 lines.

usart0 - /dev/ttyS1

usart0: serial@f801c000 {
    pinctrl-0 = <&pinctrl_usart0 &pinctrl_usart0_rts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

usart1 - /dev/ttyS2

usart1: serial@f8020000 {
    pinctrl-0 = <&pinctrl_usart1 &pinctrl_usart1_rts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

usart2 - /dev/ttyS3

The DE/RTS pin of usart2 (/dev/ttyS3) port is not available on Aria pinout so it can be used in RS485 mode

usart3 - /dev/ttyS4

To use the /dev/ttyS3 you have to include in your dts this file:

#include "at91sam9x5_usart3.dtsi"   

then use this definition:

usart3: serial@f8028000 {
    pinctrl-0 = <&pinctrl_usart3 &pinctrl_usart3_rts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

Links

• Acme Systems dts repository
• Aria G25 pinout
• DAISY-10 - RS485/422 optoisolated interface

Sergio Tanzilli
Systems designer, webmaster of www.acmesystems.it and founder of Acme Systems srl

Personal email: tanzilli@acmesystems.it
Web pages: https://www.acmesystems.it --- https://www.acmestudio.it
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