I think you should really go for using the Tuner's serial port too, instead of the primary empeg serial port. Just two pins on the tuner connector (RX,TX), plus (exising GND).

The RX (to tuner/BT-module) pin requires a resistor ladder to step down the voltage from 5V to 3V logic. the TX (from tuner/BT to empeg) can connect without modification.

The resistor ladder on RX should consist of a setup like this:

Code:
RX(empeg side) ------> 10Kohm ---+---> 20Kohm -------> GND

Connect the RX (BT module) to the (+)point in between the 10K/20K resistors. No MAX rs232 chip required.

The two resistors function as a voltage divider, converting the 5V RX output from the empeg into (20/(10+20) * 5.0V == 3.3V) suitable voltage to not fry the BT-module.

EDIT: Oh, I keep forgetting you have the Arduino in-between right now..

-ml



Edited by mlord (25/11/2017 00:26)