Printrboard Rev. D - RX1 and TX1 (second serial port) / Marlin I am trying to make Marlin use the RX1 and TX1 pins that are in the EXP1 connection. I am connecting those pins to the TX, RX of an Arduino.

The Configuration.h file has a SERIAL_PORT define, normally configured to be 0, but supposedly changeable. 

What should be changed in Marlin to make it used the Serial1 port (if possible at all)? I am using the latest code provided in  Printrbot's Marlin repository

From the Configuration.h file:

// SERIAL_PORT selects which serial port should be used for communication with the host.
// This allows the connection of wireless adapters (for instance) to non-default port pins.
// Serial port 0 is still used by the Arduino bootloader regardless of this setting.
#define SERIAL_PORT 0
//#define SERIAL_PORT 1

// This determines the communication speed of the printer
// This determines the communication speed of the printer
#define BAUDRATE 250000
//#define BAUDRATE 9600
I was able to resolve this issue: 


I moved to Marlin 1.1.9 (the final release of the 1.1.x versions).
I made a fork, for those who would like to see the changes. Also summarized below.
The RX1 and TX1 pins are well identified in the question. I connected those directly to the HC-06 bluetooth module (using a voltage divider for the RX pin in the bluetooth, as it is a 3.3 V logic module).


Changes in code: 

Marlin/Conditionals.LCD.h: commented out: 

// this line conflicted with the bluetooth/serial1 options
// #define USE_MARLINSERIAL !(defined(__AVR__) &amp;&amp; defined(USBCON))


The rest are the configuration options already provided by Marlin in Marlin.Configuration.h: 

define SERIAL_PORT 1
define BAUDRATE 115200  // Note: this baudrate MUST match the one set in the bluetooth module. That's done using the AT commands. There is plenty of information on how to do that. 
define BLUETOOTH


As a result of these changes I am able to print with my PrintrBot Simple 2014 (Printrboard Rev. D) with Bluetooth over a distance of ~3.5mts, behind a wall. I haven't tested it exhaustely, but it does print.