Write to UART fails - Monitor works

Hi,
pretty new to this so bare with me. In my project I want to use Arduino UNO as sensor data collector and transfer it to a Beaglebone (BBB) via serial (UART). I can verify that the hardware is properly setup as I can write from the Arduino monitor to the BBB without any hick-ups. However, when I use Serial.println-command in the Arduino sketch, it appears in the Arduino monitor but not on the BBB.
This is what puzzles me. The data has obviously been sent, but it only appears in the monitor - BBB is blank. Any ideas what may cause this or am I missing something obvious? I thought the data sent onto the serial port is sent on the serial UART and mirrored to the USB/monitor or is that not the case? Do I need to use the software serial write-function to specifically write to the BBB on for instance pin2/3 when a PC is connected to the UNO or what am I doing wrong?

Further info, I'm using the Seeed Grove Arduino shield and the sketch is so far extremely simple/ping type:

void setup() {
Serial.begin(115200);
}

void loop() {
Serial.println("UNO");
delay(1000);
}

There is only one serial port on the Uno, and it is normally the programming port, or is connected to the serial monitor for programmed input/output (pins 0 and 1).

You can use one of the software serial libraries to send data via other pins. Be sure to read the instructions and use the correct pins and baud rates.

You MUST use a logic level converter to safely connect a 5V output (Uno) to a 3.3V input (BBB), or you will damage one or both boards.

Hi, thanks for reply and heads up. Sorry if I was not clear enough but the boiled down question was as follows: if I want to output serial data to another peripherial such as the BBB, do I need to specifically state which pins to write to as serial.println by default utilizes pins 0,1 which are reserved for PC communication (my understanding)?

From the info I got from jremington, I conclude that the answer is yes and the way forward is to use software serial on pins 2,3 (and be sure to verify voltage settings and baudrate - the capes/shields I'm using have built in logic level converters). By doing this I can still use the Arduino monitor on PC which utilizes pins 0,1 in parallel for injecting data and/or monitoring/bug-fixing.

Thanks again for the support and making things clearer (now I don't need to ponder that question mark any longer).

crisp77:
The data has obviously been sent, but it only appears in the monitor - BBB is blank

I suspect you have the Tx and Rx from the BBB wired incorrectly.

...R

Sample Example of using Software Serial Port for data exchange between 2 UNOs.
uart-1xz.png

TX Codes:

#include<SoftwareSerial.h>
SoftwareSerial InterMind (10, 2)); //SRX = DPin-10, STX = DPin-2

void setup()
{
    Serial.begin(9600);
    InterMind.begin(9600);
}

void loop()
{
    InterMind.println("Hello");
    delay(1000);

}

RX Codes:

#include<SoftwareSerial.h>
SoftwareSerial InterMind (10, 2)); //SRX = DPin-10, STX = DPin-2

void setup()
{
    Serial.begin(9600);
    InterMind.begin(9600);
}

void loop()
{
    
    byte n = InterMind.available();
    if(n != 0)
    {
       char x = InterMind.read();
       Serial.print(x);
    }
}

uart-1xz.png