MSP430FR5962: Priming the UART transmit interrupt

"The question is why is transmitting the '\0' character invisible on the UART tx?"

Yes, that is the question. Thanks

Can you describe what you mean by this? *How* is it "invisible"?

Please be detailed.

I don't see it on the teraterm window. There is nothing in the firmware that treats the '\0' character differently.

int WDtoBMS_UART_puts(char *str)
{
    int err;
    char c = '\0';

    firstChar = 1;

    while (*(str) != 0)
    {
        err = ring_buffer_put(_rbTxIdx, str);
        if (err == 0){
            str++;
        }
    }

    if (firstChar){
        firstChar = 0;
        while (WDtoBMS_UARTBusy);
        WDtoBMS_UART_TXBUF = c;
    }

    return err;
}

static inline void WDtoBMS_UART_TxISRHandler(){
    char c;

    if (ring_buffer_get(_rbTxIdx, &c) == 0){
        WDtoBMS_UART_TXBUF = c;
    }

    return;
}

Right, it is not treated any differently. What does your terminal software do when it gets a nul character? If you want it to be seen, you need to send a printable character

ok thanks. I don't know what the terminal software is doing with the null character. I open microcom from the teraterm window. Or I use a custom shell script from teraterm. The shell script is not doing anything special for the '\0' character.

A null character has a history of  "doing nothing" going back to teletypewriters. (ASR33, etc.)  Sent (several times) after sending a carriage return because the terminal needed time to move the print head. And lacking anything resembling a microchip to buffer data in, when it received a character, it printed it.

Ignoring null characters has carried on.