c code rs_2323

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>

#define DEVICE "/dev/ttyUSB0" // serial device path
#define BAUDRATE B115200 // baud rate
#define DEVICE_ADDR 1 // device address
#define FUNCTION_CODE 3 // function code
#define START_ADDR_HI 0x01 // starting address (high byte)
#define START_ADDR_LO 0x91 // starting address (low byte)
#define DATA_LEN_HI 0x00 // number of registers to read (high byte)
#define DATA_LEN_LO 0x01 // number of registers to read (low byte)

// compute the CRC for the message
unsigned short compute_crc(unsigned char *data, int len)
{
    unsigned short crc = 0xFFFF;
    int i, j;

    for (i = 0; i < len; i++) {
        crc ^= data[i];
        for (j = 0; j < 8; j++) {
            if (crc & 0x0001) {
                crc >>= 1;
                crc ^= 0xA001;
            } else {
                crc >>= 1;
            }
        }
    }

    return crc;
}

int main()
{
    int fd, bytes_sent, bytes_recv;
    struct termios options;
    unsigned char send_data[] = { DEVICE_ADDR, FUNCTION_CODE, START_ADDR_HI, START_ADDR_LO, DATA_LEN_HI, DATA_LEN_LO };
    unsigned short crc;
    unsigned char crc_hi, crc_lo;
    unsigned char recv_data[1024];

    // compute CRC for the message
    crc = compute_crc(send_data, sizeof(send_data));
    crc_hi = crc >> 8;
    crc_lo = crc & 0xFF;

    // append CRC to the message
    send_data[sizeof(send_data)] = crc_lo;
    send_data[sizeof(send_data) + 1] = crc_hi;

    // open serial device
    if ((fd = open(DEVICE, O_RDWR | O_NOCTTY)) == -1) {
        perror("open");
        exit(1);
    }

    // set serial port options
    tcgetattr(fd, &options);
    cfsetispeed(&options, BAUDRATE);
    cfsetospeed(&options, BAUDRATE);
    options.c_cflag |= (CLOCAL | CREAD);
    options.c_cflag &= ~PARENB;
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    options.c_cc[VMIN] = 1;
    options.c_cc[VTIME] = 0;
    tcsetattr(fd, TCSANOW, &options);

    // send data
    if ((bytes_sent = write(fd, send_data, sizeof(send_data) + 2)) == -1) {
        perror("write");
        exit(1);
    }