Beaglebone Black RS485 Echo

rs485_BBB_echo.c

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <linux/serial.h>
#include <termios.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <poll.h>
#include <unistd.h>
#include <getopt.h>
#include <time.h>

/* RS485 ioctls: */
#define TIOCGRS485      0x542E
#define TIOCSRS485      0x542F

int main( int argc, char **argv )
{
        char buf[80];

        if( argc < 2 ) {
                printf("Usage:  %s [port name]\n", argv[0]);
                exit(0);
        }

        char *port = argv[1];

        int fd = open(port, O_RDWR);
        if (fd < 0) {
          /* Error handling. See errno. */
                fprintf( stderr, "Error opening port \"%s\" (%d): %s\n", port, errno, strerror( errno ));
                exit(-1);
        }

        struct serial_rs485 rs485conf;

	if (ioctl (fd, TIOCGRS485, &rs485conf) < 0) {
                 printf("Error: TIOCGRS485 ioctl not supported.\n");
        }

	struct termios newtio;

	bzero(&newtio, sizeof(newtio)); /* clear struct for new port settings */

	/* man termios get more info on below settings */
	newtio.c_cflag = B115200 | CS8 | CLOCAL | CREAD;
	newtio.c_iflag = 0;
	newtio.c_oflag = 0;
	newtio.c_lflag = 0;

	// block for up till 128 characters
	newtio.c_cc[VMIN] = 128;

	// 0.5 seconds read timeout
	newtio.c_cc[VTIME] = 5;

	/* now clean the modem line and activate the settings for the port */
	tcflush(fd, TCIOFLUSH);
	tcsetattr(fd, TCSANOW, &newtio);

        rs485conf.delay_rts_after_send = 0;
        rs485conf.delay_rts_before_send = 0;

	rs485conf.flags = 0;

	rs485conf.flags |= SER_RS485_ENABLED;
	
	/* If sending HI LEVEL */
	rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
	/* If recv LO LEVEL */
	rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
	
	rs485conf.flags |= SER_RS485_RX_DURING_TX;

 	if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
                 printf("Error: TIOCSRS485 ioctl not supported.\n");
  	}

	int  bytes_read = 0;    /* Number of bytes read by the read() system call */

	while(1 == 1)
	{
	    	bytes_read = read(fd, &buf, 32);
		if (bytes_read != 0)
		{
		    write(fd, buf, bytes_read);
		}
  	}

        /* Close the device when finished: */
        if (close (fd) < 0) {
                fprintf( stderr, "Error closing port (%d): %s\n", errno, strerror( errno ));
        }

        exit(0);
}