My problem is how to read a string from PC and return with result without data being lost

TRANSMIT
main:
stuff the buffer start first char

ISR
if buffer empty exit
else
send one char decrement char count

RECEIVE
main
initialize by setting both char counts to zero

ISR
read char to buffer, increment read char count

main loop
if read char count > than used char count use char increment used char count


This will get you started, nect step when this works will be circular bufgfers

Erik