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How to use the RS485 on Acqua 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 SAMA5D3x MPU used on Acqua A5 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 class='acmetable' signals

This is the list of suitable class='acmetable' signal available on Acqua pinout useful to implements up three RS485 lines.

usart0 - /dev/ttyS1

usart0: serial@f001c000 {
    pinctrl-0 = <&pinctrl_usart0 &pinctrl_usart0_rts_cts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

usart1 - /dev/ttyS2

usart1: serial@f0020000 {
    pinctrl-0 = <&pinctrl_usart1 &pinctrl_usart1_rts_cts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

usart2 - /dev/ttyS3

usart2: serial@f8020000 {
    pinctrl-0 = <&pinctrl_usart2 &pinctrl_usart2_rts_cts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

usart3 - /dev/ttyS4

usart3: serial@f8024000 {
    pinctrl-0 = <&pinctrl_usart3 &pinctrl_usart3_rts_cts>;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

Links

• Acme Systems dts repository
• Acqua A5 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
Github repositories: https://github.com/tanzilli --- https://github.com/acmesystems
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