thanks Andy you are always a great help,
Here's some explanation:
I'll have a RTC module using the timer mode as u told me befor, and i want to read its contents through the PC by serial communication so i read about the null modem technique, i understand it perfect but the problem here if i wanted to writ any thing to the MC through the serial connection how it will understand that i write or read while the null modem technique doesn't show that, i ho pe to here from u again
thanks all