Debian – Problem using Beaglebone Black UART with C-Program

Stack Overflow,

I desperately need help getting a BeagleBone Black UART to work from a C program.
The code that I am attempting to use is listed below. The code came from Chapter 8 of Derek Molloy’s “Beaglebone” book.

The code compiles without any warnings or errors and seems to run. The program passes the sanity checks that are built into the C code below. The program finishes by printing “Finished sending the message, exiting.” to the console.

The fundamental reason that I think the UART4 is not sending data is that I see no activity on P9/13 when monitoring with a nice digital O’scope. I have also tried monitoring the port by way of a PC running a terminal program.

I am convinced the C-program is reading my command-line message (e.g. “testing123”) correctly. I modified to code to print the message to the console and it printed the message as expected. I also modified the code run in a loop to make it easier to monitor P9/13 with the O’scope. The O’scope shows a constant 3.3VDC on the P9/13.

I also blindly tried a pull-down resistor. The port does not provide much current. A 2k Ohm resistor loaded the port to about 0.2 Volts.

I verified that ttyO4 and ttyS4 are in /DEV, whatever that means.

I am not certain what version of Debian that I am using. I monitored the “header” output from J1 and found this:
Debian GNU/Linux 9 ebb ttyS0
BeagleBoard.org Debian Image 2019-08-03

Also, I found this:
Machine ID: 229f313330aab6c19873bdc15d4645c7
Boot ID: b74381516ae64aefa37415709164f0e4
Operating System: Debian GNU/Linux 9 (stretch)
Kernel: Linux 4.14.108-ti-r113
Architecture: arm

Do I need to enable the UART port or pins? If so, how do I do this and how do I confirm it was successfully done? Do I need to hardwire the control lines (e.g. CTS, RTS)? Do I need to edit the uEnv.txt file?

I am almost certain my board is a rev-c.

As a final note, I tried a similar C-code from the “Bad to the Bone” book with similar unfavorable results.

Any help will be greatly appreciated.

Thanks,

Norm

/* Simple send message example for communicating with the
*  UART that is connected to a desktop PC.  */

#include<stdio.h>
#include<fcntl.h>
#include<unistd.h>
#include<termios.h>
#incl`ude<string.h>

int main(int argc, char *argv[]){
   int file, count;
   if(argc!=2){
       printf("Please pass a message string to send, exiting!n");
       return -2;
   }
   if ((file = open("/dev/ttyO4", O_RDWR | O_NOCTTY | O_NDELAY))<0){
      perror("UART: Failed to open the device.n");
      return -1;
   }
   struct termios options;
   tcgetattr(file, &options);
   options.c_cflag = B115200 | CS8 | CREAD | CLOCAL;
   options.c_iflag = IGNPAR | ICRNL;
   tcflush(file, TCIFLUSH);
   tcsetattr(file, TCSANOW, &options);

   // send the string plus the null character
   if ((count = write(file, argv[1], strlen(argv[1])+1))<0){
      perror("UART: Failed to write to the output.n");
      return -1;
   }
   close(file);
   printf("Finished sending the message, exiting.n");
   return 0;