Forum Programmation.c Pb de lecture port série (RS232 + transceiver RS485)

Bonjour,

Sur une plateforme Compute Module 3 avec un transceiver RS232=>RS485, je dois interroger un équipement en Modbus.
Entre l'écriture et la lecture je dois piloter une sortie (GPIO 12) pour activer la lecture au l’écriture du transceiver.
Avec le code ci dessous j'obtiens une réponse incomplète et très aléatoire. Soit pour la réponse réelle 01 03 04 17 12 03 21 9F 6A j'obtiens :
03 04 17 12 03 21 9F 6A
ou
17 12 03 21 9F 6A
et quelques fois rien

On dirait que la lecture ne se déclenche pas assez rapidement après l'écriture et le pilotage du transceiver.

Est-ce un problème de configuration ou peut-être la façon de faire ?

D'avance merci

gcc -Wall -o serial serial.c -lwiringPi

Voici le code :

#include <sys/resource.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>  /* File Control Definitions          */
#include <termios.h>/* POSIX Terminal Control Definitions*/
#include <unistd.h> /* UNIX Standard Definitions         */
#include <errno.h>  /* ERROR Number Definitions          */
#include <wiringPi.h> /*pilotage du GPIO */

int main(int argc, char **argv)
{

        //Test Priorité de l'application :
        int which = PRIO_PROCESS;
        id_t pid;
        int ret;
        int priority = -20;

        pid = getpid();
        ret = getpriority(which, pid);
        ret = setpriority(which, pid, priority);
        ret = getpriority(which, pid);
        printf("Priorité = %i\n",ret);


        //Ouverture et configuration du port
        int fd;
        fd = open("/dev/ttyAMA0",O_RDWR | O_NOCTTY);
        //printf("fd = %i\n",fd);
        if (fd == -1) printf("Pb d'ouverture du périphérique\n");
        else {
                printf("Ouverture du port OK\n");
                /*---------- Setting the Attributes of the serial port using termios structure --------- */
                struct termios SerialPortSettings;      /* Create the structure                          */
                tcgetattr(fd, &SerialPortSettings);     /* Get the current attributes of the Serial port */
                /* Setting the Baud rate */
                cfsetispeed(&SerialPortSettings,B9600); /* Set Read  Speed as 9600                       */
                cfsetospeed(&SerialPortSettings,B9600); /* Set Write Speed as 9600                       */
                /* 8N1 Mode */
                SerialPortSettings.c_cflag |= PARENB;   /*EVEN.... Disables the Parity Enable bit(PARENB),So No Parity   */
                SerialPortSettings.c_cflag &= ~CSTOPB;   /* CSTOPB = 2 Stop bits,here it is cleared so 1 Stop bit */
                SerialPortSettings.c_cflag &= ~CSIZE;    /* Clears the mask for setting the data size             */
                SerialPortSettings.c_cflag |=  CS8;      /* Set the data bits = 8                                 */
                SerialPortSettings.c_cflag &= ~CRTSCTS;       /* No Hardware flow Control                         */
                SerialPortSettings.c_cflag |= CREAD | CLOCAL; /* Enable receiver,Ignore Modem Control lines       */
                SerialPortSettings.c_iflag &= ~(IXON | IXOFF | IXANY);          /* Disable XON/XOFF flow control both i/p and o/p */
                SerialPortSettings.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);  /* Non Cannonical mode                            */
                SerialPortSettings.c_oflag &= ~OPOST;/*No Output Processing*/
                /* Setting Time outs */
                SerialPortSettings.c_cc[VMIN] = 0; /* Read at least 10 characters */
                SerialPortSettings.c_cc[VTIME] = 5; /* Wait 500ms   */

                if((tcsetattr(fd,TCSANOW,&SerialPortSettings)) != 0) /* Set the attributes to the termios structure*/
                        printf("\n  ERROR ! in Setting attributes");
                else
                        printf("Configuration OK\n");

                /*------------------------------- Write data to serial port -----------------------------*/
                //Activation de l'ecriture sur le transceivers RS485 => PIN 12 :ON

                wiringPiSetup ();
                pinMode (12, OUTPUT);
                digitalWrite (12, 1);

                char write_buffer [] = "\x01\x03\x40\x00\x00\x02\xD1\xCB";
                //char write_buffer [] = "\x01\x03\x30\x20\x00\x02\xCA\xC1";    /* Buffer ForwActiv   */
                //char write_buffer [] = "\x01\x03\x30\x40\x00\x02\xCA\xDF";      /* Buffer RevActiv   */
                //char write_buffer [] = "\x01\x03\x20\x00\x00\x02\xCF\xCB";      /* Buffer Voltage   */
                int  bytes_written  = 0;        /* Value for storing the number of bytes written to the port */

                bytes_written = write(fd,write_buffer,sizeof(write_buffer));/* use write() to send data to port                                            */
                                                                     /* "fd"                   - file descriptor pointing to the opened serial port */
                                                                     /* "write_buffer"         - address of the buffer containing data              */
                                                                     /* "sizeof(write_buffer)" - No of bytes to write                               */
                //printf("\n  %s ecrit sur le périphérique",write_buffer);
                printf("\n  %d octets ecrit sur le périphérique\n", bytes_written);

                //Activation de la lecture sur le transceiver RS485 => PIN 12 :OFF
                //delay (90);   /test delai
                if (tcdrain(fd) != 0){
                        perror("tcdrain() error");
                }
                digitalWrite (12, 0);

                /*------------------------------- Read data from serial port -----------------------------*/
                tcflush(fd, TCIFLUSH);   /* Discards old data in the rx buffer            */
                char read_buffer[32];   /* Buffer to store the data received              */
                int  bytes_read = 0;    /* Number of bytes read by the read() system call */
                int i = 0;

                bytes_read = read(fd,&read_buffer,8); /* Read the data                   */
                printf("Bytes Rxed -%d\n", bytes_read); /* Print the number of bytes read */
                for(i=0;i<bytes_read;i++)        /*printing only the received characters*/
                    printf("%x",read_buffer[i]);

                close(fd);
                printf("\nfermeture du port\n");
        }

return 0;
}