I am trying to communicate with another system via the uart on pins 8 and 10. The following doesn't seem to work even though, using a scope, I can see data rolling in on pin 10. I never see any data coming out on pin 8.

Here is my test code:

include <stdio.h>

include <stdlib.h>

include <fcntl.h>

include <unistd.h>

int main(void) {
char uptty[] = "/dev/ttyS1";
int ret = 0;
char c;

puts("!!!Hello World!!!"); /* prints !!!Hello World!!! */

int fd = open(uptty, O_RDWR | O_NOCTTY | O_NONBLOCK);
if (fd < 0) {
    perror(uptty);
    exit(-1);
}

while (1) {

    // Read characters from uart.
    while (1) {
        ret = read(fd, &c, 1);
        if (ret == 1)
            write(1, &c, 1);
        else
            break;
    }
    if (ret == -1) perror("Read error");

    // Write to the uart.
    write(fd, uptty, sizeof(uptty));
    write(1, "cc\n", 3);
    usleep(500000);
}

return 0;

}

The perror() gives an "Input/output error:" if I substitute /dev/ttyS4, the error is "Resource temporarily unavailable".

Any help would be greatly appreciated. Thanks.