Arduino serial communication hacking
You are going about this the wrong way. It is doubtful that you can even be sure of the serial interface connection and protocol used. Making a total blind guess is almost pointless.

The way to do this the proper way is to leave things connected the normal way and then investigate what is going on. Using an oscilloscope you can determine essential information as follows:


Voltage levels used for signalling.
Timing of pulses even to the extent of determining baud rate.
Understanding the signalling to see if it is actually async serial of some other serial protocol.
Determine if the signalling is normal polarity or inverted.
Observe when signalling happens such as all the time, periodically or just under command.
Figure out signal direction on various wires.


Only after you have collected such information would it be reasonable to connect to your MCU.