linux/C/serial port : VTIME and VMIN timeouts of termios structure do not work

Question

I am developing a code which communicates with a serial port (in fact with a usb rs485 adapter but hey) in linux/C. And no matter how much I change the VTIME and VMIN parameters of the termios structure in all directions, I always recover a number with read() bytes lower than VMIN (read() is blocking when the bytes number is 0), and VTIME does not seem respected to me. Here is the code (I just put the essentials, when there is "(...)" it is because I omitted things of no interest to the problem):

(...)
#include <fcntl.h> // Contains file controls like O_RDWR
#include <errno.h> // Error integer and strerror() function
#include <termios.h> // Contains POSIX terminal control definitions
#include <unistd.h> // write(), read(), close()
 
  const int longueur_bufemission = 30;
   unsigned char _bufEmission[longueur_bufemission];   
   unsigned char _bufReception[50];
   int serial_port;
   struct termios tty ={0};
 
  serial_port = open("/dev/ttyS4", O_RDWR);
  // Check for errors
   if (serial_port < 0) {
        printf("Error %i from open: %s\n", errno, strerror(errno));
    }
 
    if(tcgetattr(serial_port, &tty) != 0) {
        printf("Error %i from tcgetattr: %s\n", errno, strerror(errno));
    } 
 
    tty.c_cflag &= ~PARENB; // Clear parity bit, disabling parity (most common)
    //tty.c_cflag |= PARENB;  // Set parity bit, enabling parity
 
    tty.c_cflag &= ~CSTOPB; // Clear stop field, only one stop bit used in communication (most common)
    //tty.c_cflag |= CSTOPB;  // Set stop field, two stop bits used in communication
 
    tty.c_cflag &= ~CSIZE; // Clear all the size bits, then use one of the statements below
    tty.c_cflag |= CS8; // 8 bits per byte (most common)
 
    tty.c_cflag &= ~CRTSCTS; // Disable RTS/CTS hardware flow control (most common)
    //tty.c_cflag |= CRTSCTS;  // Enable RTS/CTS hardware flow control
 
    tty.c_cflag |= CREAD | CLOCAL; // Turn on READ & ignore ctrl lines (CLOCAL = 1)
    //tty.c_cflag &= ~(CREAD | CLOCAL);
 
    tty.c_lflag &= ~ICANON;
 
    tty.c_lflag &= ~ECHO; // Disable echo
    tty.c_lflag &= ~ECHOE; // Disable erasure
    tty.c_lflag &= ~ECHONL; // Disable new-line echo
 
    tty.c_lflag &= ~ISIG; // Disable interpretation of INTR, QUIT and SUSP
 
    tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl
 
    tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL); // Disable any special handling of received bytes
    tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g. newline chars)
    tty.c_oflag &= ~ONLCR; // Prevent conversion of newline to carriage return/line feed    
 
    tty.c_cc[VTIME] = 10;    // Wait for up to 1s (10 deciseconds), returning as soon as any data is received.
    tty.c_cc[VMIN] = 1;
 
        cfsetispeed(&tty, B9600);
    cfsetospeed(&tty, B9600);
 
        if (tcsetattr(serial_port, TCSANOW, &tty) != 0) {   
        printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
    }
 
       (...)
 
       write(serial_port,_bufEmission, index+4);
 
 
       num_bytes = read(serial_port, _bufReceptionx, sizeof(_bufReceptionx));
 
       (...)

I expect to find a way to apply correctly the timeouts VTIME and VMIN.