Hello FvM,
Other connections are for SPI for Other board to interface which is currently unsued.
Tx pin is pull up using 10kohm with vcc=3.3, also if GND not connected then that interrupt should be trigger always not only when the transmitting,and i have checked it its connected to the other cpu GND.
The problem is when i interface MAX232 with MAx3232 and transmitting the data from MAX232 at that time DOUT pin of MAX3232 is fluctuate which is also equals to number of data send means if i sending 8 char then i get 8 time receive interrupt. Dout fluctuate means when there is no transmission then Dout is -3.3v approximate i forgot the voltage, but when transmitting from MAX232 it fluctuate to GND.