send and receive data using usb to rs232 interface in Linux C - c

Question

I am writing c code to send and receive characters to and from an Ergometer using FTDI USB to RS232 Cable in Linux. The write function is working properly but the read function has failed. I am not getting any data back from the Ergometer. I have posted my code here:
#include <stdio.h> // standard input / output functions
#include <stdlib.h>
#include <string.h> // string function definitions
#include <unistd.h> // UNIX standard function definitions
#include <fcntl.h> // File control definitions
#include <errno.h> // Error number definitions
#include <termios.h> // POSIX terminal control definitions
#include <stdbool.h>
#define SOH 0x01 // Start of Header
#define ETB 0x17 // End of transmission block
#define ACK 0x06 // Acknowledged
#define NAK 0x15 // Not Acknowledged
#define GS 0x1D // Group separator
#define FP_WATT1_PROG 0x13
#define FP_CARDIO 0x2F
int main(){
/* Open File Descriptor */
int USB = open( "/dev/ttyUSB0", O_RDWR| O_NONBLOCK | O_NDELAY );
/* Error Handling */
if ( USB < 0 )
{
printf("Error beim oeffnen");
}
// Configure Port
struct termios tty;
struct termios tty_old;
memset (&tty, 0, sizeof tty);
// Error Handling
if ( tcgetattr ( USB, &tty ) != 0 ) {
printf("error beim tcgetattr");
}
// Save old tty parameters
tty_old = tty;
// Set Baud Rate of 9600 Bps
cfsetospeed (&tty, (speed_t)B9600);
cfsetispeed (&tty, (speed_t)B9600);
// 8 Bits, No Parity and 1 Stop bit settings
tty.c_cflag &= ~PARENB; // No Parity
tty.c_cflag &= ~CSTOPB; // 1 Stop Bit
tty.c_cflag &= ~CSIZE;
tty.c_cflag |= CS8; // 8 Bits
tty.c_cflag &= ~CRTSCTS; // no flow control
tty.c_cc[VMIN] = 1; // read doesn't block
tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout
tty.c_cflag |= (CLOCAL | CREAD ); // turn on READ & ignore ctrl lines
// Make raw
cfmakeraw(&tty);
// Flush Port, then apply attributes
tcflush( USB, TCIFLUSH );
if ( tcsetattr ( USB, TCSANOW, &tty ) != 0)
{
printf("from tcsetattr");
}
// WRITE
// Set the Ergometer language.
char cmd[255] = {0x60,0x01}; // Ergometer Cockpit Adresse
int n_written = write( USB, cmd, sizeof(cmd));
printf("%s\n",cmd);
// Allocate memory for read buffer
int bb = 0;
int i,x = 0;
bool cmdOK=false;
char marr[10];
do
{
if((bb = read(USB, &marr[x], sizeof( marr )-x )) > 0)
{
for (i=x;i<x+bb;i++)
{
//printf("0x%02x",marr[i]);
if ((marr[i]==ACK)||(marr[i]==NAK)) cmdOK=true;
}
x+=bb;
//printf("\n");
//printf("x=%d\n",x);
}
}while(!cmdOK);
printf("-Ausgabe---------------------\n");
for (i=0; i<x; i++)
{
printf("0x%02x",marr[i]);
}
close(USB);
// ttyUSB0 auf alten Wert setzen
tcflush(USB,TCIFLUSH);
if (tcsetattr(USB, TCSANOW, &tty) != 0) return(-1);
exit(0);
return 0;
}