C++ libserial skipping 0x09

Question

I am attempting to read data over serial from a Pixhawk flight controller which communicates via the Mavlink protocol. It sends 17 bytes, the first three being 0xFE, 0x09 followed by a counter that increments every message. I have confirmed this with GtkTerm.

However when I run the following code, 0x09 (the second byte) is always skipped so only 16 bytes of each 17 byte message is received.

Any ideas? Thanks, James.

LibSerial::SerialStream pixhawkSerial;

pixhawkSerial.Open("/dev/ttyACM0");

pixhawkSerial.SetBaudRate( LibSerial::SerialStreamBuf::BAUD_57600 ) ;

pixhawkSerial.SetCharSize( LibSerial::SerialStreamBuf::CHAR_SIZE_8 );

pixhawkSerial.SetNumOfStopBits(1);

pixhawkSerial.SetParity( LibSerial::SerialStreamBuf::PARITY_NONE ) ;

pixhawkSerial.SetFlowControl( LibSerial::SerialStreamBuf::FLOW_CONTROL_NONE );

char next_byte [100];
int i = 0;
while (i<100){
    if( pixhawkSerial.rdbuf()->in_avail() > 0 ){
        pixhawkSerial >> next_byte[i];
        i++;
    }
    else cout << "No data" << endl;
}

Your code always reads 100 bytes. What makes you think that only 16 are read? — M.M, May 23 '15 at 12:12
Sorry, I should have been more clear. I do receive 100 bytes of data, however each Mavlink packet consists of 17 bytes. And out of those 17 bytes sent the code only receives 16, despite confirming the all 17 arrive using GtkTerm — , May 24 '15 at 12:50
what's the offset into the entire packet of the missing byte? — M.M, May 25 '15 at 00:15
so there is meant to be 17 bytes of payload then 83 bytes of packing? seems odd — M.M, May 26 '15 at 03:28
nah, there doesn't seem to be any packing, i receive a stream of mavlink packets, but the 2nd byte is always missing, i increased the number of bytes to 100 so I could check whether it was skipping consistently or not, really would have liked to include pictures but I wasn't able to — , May 27 '15 at 09:44

score 1 · Accepted Answer · answered May 23 '15 at 11:14

Wasn't able to get libserial to work, however I gave temios a go and it worked without issues.

Attached is the working code.

int fd;
struct termios oldAtt, newAtt;
fd = open("/dev/ttyACM0", O_RDWR  | O_NOCTTY | O_NDELAY);

tcgetattr(fd, &oldAtt);
memset(&newAtt, 0, sizeof(newAtt));
newAtt.c_cflag = CRTSCTS | CS8 | CLOCAL | CREAD;
newAtt.c_iflag = IGNPAR;
newAtt.c_ispeed = B57600;
newAtt.c_oflag = 0;
newAtt.c_ospeed = B57600;
newAtt.c_lflag = 0;

newAtt.c_cc[VTIME] = 0;
newAtt.c_cc[VMIN] = 1;

tcflush(fd, TCIOFLUSH);
tcsetattr(fd, TCSANOW, &newAtt);

char rBuffer;
char next_byte [100];
int i=0;
int dataReceived;

while (i<100) {
    dataReceived = read(fd,&rBuffer,1);
    if (dataReceived>0){
        next_byte[i] = rBuffer;
        i++;
    }
}

tcsetattr(fd,TCSANOW,&oldAtt);
close(fd);

C++ libserial skipping 0x09

1 Answers1