How to use the RS485 interface

This article shows how to configure the serial port ttyS3 on the FOX Board LX to operate as RS485 half duplex interface

RS485 basic hardware interface

The schematic below is the simplest RS485 interface suitable for FOX Board LX. This interface is included in the FOXZB add-on board (see @h1='42').


Simple half duplex RS485 interface for the FOX Board LX

The only serial port enabled to work as RS485 is the /dev/ttyS3.

To use correctly the RS485 interface chip we need to configure the serial port driver in the Phrozen SDK to manage the RTS3 signals (Request To Send) to enable the TX section (DE pin) only when data are transmitted on TXD3 line (Transmit Data) as shown in this image.

To enable the RS485 support use make kernelconfig command and enable: 

Drivers for built-in interfaces --->
  [*] RS-485 support

then generate the new flash image and transfer it to the FOX Board LX (@h1='703').

Example of C code to transmit data

Here is a basic example on how to transmit a sequence of chars (in this case 0x55 0x00 0x55) over the RS485 line. @source='rs485tx.c'

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