PLC 1141 - Read from Serial

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PLC 1141 - Read from Serial

Postby yammi » Sun Sep 12, 2010 11:13 am

Hi

This is my first post here - I recently started to play around with PLC (1141+ - Serial) and right now I try to develop some C libs to work with the API.
I can send commands to the PLC and everything works fine but I also want to receive answers and here is where I got stuck.

When I open the device I can easily send commands - BUT when I try to read I don't get any answer (despite the ACK bit is set).
I am pretty sure my serial settings are wrong - so I assume the correct settings are those:

9600 BAUD
8N1
Canonical (meaning line-based - no need to process every byte)
Syncronous (so no Signal Handler is needed - answer will be sent right after command).

Here is what I am doing:

Open Serial at the beginning of the programm
Code: Select all
    struct termios tio;
    memset(&tio,0,sizeof(tio));
    tio.c_iflag=0;
    tio.c_oflag=0;
    tio.c_cflag = CS8|CREAD|CLOCAL; //8n1,
    tio.c_lflag=0;
    tio.c_lflag |= ICANON;

    *fd = open(SERIAL_DEVICE, O_RDWR | O_NOCTTY | O_NONBLOCK | O_SYNC );
    cfsetospeed(&tio,BAUDRATE); // Baudrate is B9600
    cfsetispeed(&tio,BAUDRATE);  // Baudrate is B9600

    tcsetattr(*fd,TCSANOW,&tio);



read and write functions
Code: Select all
int tty_read (int *fd, char *buffer, int size) {
    int ret;
    ret = read (*fd, buffer, size);

    if (ret < 0) {
        if (errno == EAGAIN) {
            printf("SERIAL EAGAIN ERROR\n");
            strerror(errno));
            return 0;
        } else {
            printf("SERIAL read error %d %s\n", errno,
            strerror(errno));
            return 0;
        }
    }
    if ( ret == 0 ) {
        printf ("read returned 0 byte\n");
        return 0;
    }
    return 1;
}

int tty_write (int *fd, char *buffer, int size){
    int ret;
    ret = write (*fd, buffer, size);
    return ret;
}



and here is the main code where I send some data to PLC and don't get any answer:

Code: Select all
    char buffer[10] = {STX,0x05,0xff,0x00,0x22,0x00,0x00,ETX}; // 0x22 means "ON" and "send ack"
    ret = tty_write(&serial_fd, buffer, sizeof(buffer));
   
    usleep(500000); // wait ...
    char read_buffer[100];
    memset(&read_buffer,0,sizeof(read_buffer));

    if  ( tty_read(&serial_fd, read_buffer, sizeof(read_buffer)) ) {
        if (read_buffer != NULL) {
            printf ("got feedback %d\n", sizeof(read_buffer));
            hexdump (read_buffer, sizeof(read_buffer)); // let me see it's hex code
        } else {
            printf ("no feedback\n");
        }
    }
    else
        printf ( "read failed\n" );


Reaction is - the Light turns on and the stdout says:
Code: Select all
SERIAL EAGAIN ERROR
SERIAL read error 11 Resource temporarily unavailable
read failed


Since I am playing around for a few days now without any clue how to solve this it would be really great if somebody has any ides :)

Bye,
yammi
yammi
Starting Member
Starting Member
 
Posts: 1
Joined: September 2010

Re: PLC 1141 - Read from Serial

Postby steve2xyz » Mon Sep 13, 2010 4:47 pm

You almost certainly don't want canonical mode. Try something like this:

Code: Select all
   struct termios sb;

   memset(&sb, '\0', sizeof(sb));
   sb.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON | ISIG);
   sb.c_lflag = 0;
   sb.c_oflag = 0;
   sb.c_cc[VMIN] = 1;
   sb.c_cflag = CLOCAL | CS8 | CREAD | B9600
   tcsetattr(fd, TCSANOW, &sb);


Code: Select all
   if (read_buffer != NULL) {

This is always going to be true. I'd just examine the number of characters returned by the read call.

Cheers, Steve
steve2xyz
Starting Member
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Posts: 34
Joined: February 2010


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