Trying to send text out through serial port

[RogerInHawaii]

I'm made modifications to the Marlin firmware for my Prusa i3 MK3. I added some new GCodes that are processed within the Marlin firmware at the process_commands() method. I know that it's getting to my new code. But when it's there I want it to send some text out through the TX pin on the J19 socket of the Einsy Rambo board. I THINK that I'm calling the correct MarlinSerial methods, like:

In setup():

MarlinSerial::begin(115200);

In process_commands()

 where I'm processing my new GCode:

       MarlinSerial::println("Let us test a whole bunch of text going out through the serial port");
       MarlinSerial::flush(); // Ensure all outgoing data is sent

But nothing is happening at the TX pin on the J19 socket. So far all I'm doing is using a small scope to check the voltages on that pin but all I see is random noise. and very low voltages, like millivolts, not the 5 volts I'm expecting.

I'm thinking I might not be setting things up properly in the Marlin firmware to access that pin properly.

What is the APPROPRIATE things I should be doing in the Firmware to successfully send text out through that TX pin?