[SOLVED] UART access with Ubuntu

rstuartrstuart New Member Posts: 2
edited July 2018 in UART

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) {

while (1) {

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

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

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.


  • ccaldeccalde Administrator, Moderator, Emutex Posts: 327 admin

    Hi @rstuart ,

    First, which are you using the UP custom kernel for Ubuntu to get access to the header pins?

    Then, you could try a easy test connecting RX to TX and sending an echo data to the ttyS port.

    Also, you can test it using https://github.com/cbrake/linux-serial-test


  • rstuartrstuart New Member Posts: 2

    Thanks for the help ccale. By consulting the https://github.com/cbrake/linux-serial-test code I was able to see that I was setting the termios structure incorrectly. (Unfortunately, the code snip-it I included in the original post did not show this part.) This was just the pointer I needed. Thanks again.

  • ccaldeccalde Administrator, Moderator, Emutex Posts: 327 admin

    Hi @rstuart ,

    So, I will redefine your discussion as SOLVED if it is ok for you.


Sign In or Register to comment.