Hello friends..
I have already placed 104 cap between vcc and ground of both ics as Mr. Mehmood suggested.I'm also refreshing the data periodically as Mr.Alfredo said. One thing i forgot to tell you is that both serial to paralled converter and latch ics are not in the same board containing controller.I'm using shielded cable to provice connections between them and one end of shield is connected to ground. Another thing is , i have used optocouplers for input section only. the data required for serial transmission is taken from the internal ram not directly from the reading of optos. Moreover there is no opto in o/p secion. O/p goes from port1 to serial to parallel converter to latch. I have also tried to pull up the serial inputs of s/p ic using 4.7k resistors but still not succeded.
I don't have oscilloscope.