The easiest way would be using a monostable circuit with pulse width of 0.5 sec and ANDing its inverted output with the data stream .Yet, monostable circuits wouldn't be very accurate at the pulse width .If you need higher accuracy, you would try going for a ucontroller with the data connected at the interrupt pin and an input pin .When data xmission starts, the interrupt will be triggered that would initiate a counter that would count N times where N*Tclock=0.5 sec during which data isn't allowed to propagate to output then after the .5 sec, data is echoed from input to output .