Thank You for your suggestion.

system is very good and working perfectly at that stage.

there are two cards with 8051 controller 4k ROM and 128 Bytes RAM.

And the two devices exchange data with seperate clkin,clkout, datain and data out pins.

clkin is connected to external interrupt zero and data pin is read in ISR EXT.0 when ever interrupt gets and the interrupt in both the cards configured to falling edge.

Data out and Clkout are just gets out data.

problem is when i am trying to read data in one device some times it coming correctly (data is checked by tranferred out on serial port)

when data is not appearing on serial port if i probe the data input pin data appears in serial port.

i hope this info is sufficient.

regards
sudheer