I'm using a Raspberry Pi Zero in device mode and a Raspberry Pi B in host mode. I'm connecting the two with a USB cable. My goal right now is just to send simple, arbitrary data back and forth between the two Pi's.
The problem is whichever Pi writes to the serial port first ends up reading what it wrote. The program I've written has the device send d\n and the host send h\n. So, if the device writes first, the the host reads correctly the d\n, then writes h\n to the serial port. But the device ends up reading d\n! The problem persists if I switch it so that the host writes first.
I've tried adding various tcflush calls to the program after writing but before reading, but it doesn't work. I've also tried sleeping for various amounts of time. I've read waiting 100 microseconds for each character written and I've slept for several seconds.
My setup requires me to not have a constant connection between both Pi's at the same time because of the Pi Zero's single data-capable usb port. So, to test, I'm actually plugging in a keyboard and running the program, then plugging in the proper cable to transfer data. I can transfer data, but not after writing because the program simply reads back what it wrote.
I'm starting to think I've fallen into a noob trap that I can't fathom. Here is the code I'm using:
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
/*
* gcc -o device_rw -DDEVICE serial_rw.c
* gcc -o host_rw serial_rw.c
*/
#define SERIAL_DEVICE "/dev/ttyGS0"
#define SERIAL_HOST "/dev/ttyACM0"
#ifdef DEVICE
#define _TTY SERIAL_DEVICE
#else
#define _TTY SERIAL_HOST
#endif
int
set_interface_attribs(int fd, int speed)
{
struct termios tty;
if (tcgetattr(fd, &tty) < 0) {
printf("Error from tcgetattr: %s\n", strerror(errno));
return -1;
}
cfsetospeed(&tty, (speed_t)speed);
cfsetispeed(&tty, (speed_t)speed);
tty.c_cflag &= ~PARENB; /* no parity bit */
tty.c_cflag &= ~CSTOPB; /* only need 1 stop bit */
tty.c_cflag &= ~CSIZE;
tty.c_cflag |= CS8; /* 8-bit characters */
tty.c_cflag &= ~CRTSCTS; /* no hardware flowcontrol */
tty.c_cflag |= (CLOCAL | CREAD); /* ignore modem controls */
tty.c_iflag |= IGNPAR | IGNCR;
tty.c_iflag &= ~(IXON | IXOFF | IXANY);
tty.c_iflag |= ICANON;
tty.c_iflag &= ~OPOST;
if (tcsetattr(fd, TCSANOW, &tty) != 0) {
printf("Error from tcsetattr: %s\n", strerror(errno));
return -1;
}
return 0;
}
void
write_serial (int fd, const char *buf, int len)
{
printf("WRITE: %s\n", buf);
write(fd, buf, len);
}
void
read_serial (int fd, char *buf, int len)
{
ssize_t nread = read(fd, buf, len);
if (nread > 0 && nread <= len) {
buf[nread] = 0;
printf(" READ: %s\n", buf);
}
}
int
main (int argc, char **argv)
{
char buf[80];
int fd = open(_TTY, O_RDWR | O_NOCTTY);
if (fd < 0) {
fprintf(stderr, "Can't open %s: %s\n", _TTY, strerror(errno));
goto exit;
}
if (set_interface_attribs(fd, B115200) < 0) {
goto exit;
}
#ifdef DEVICE
printf("device: %s\n", _TTY);
write_serial(fd, "d\n", 2);
usleep((2 + 25) * 100);
read_serial(fd, buf, 2);
#else
printf("host: %s\n", _TTY);
read_serial(fd, buf, 2);
//usleep((2 + 25) * 100);
write_serial(fd, "h\n", 2);
#endif
close(fd);
exit:
return 0;
}
