74LS74 D type flip/flop
This device contains two independent positive-edge-triggered D flip-flops with complementary outputs. The information on the D input is accepted by the flip-flops on the positive going edge of the clock pulse. The triggering occurs at a voltage level and is not directly related to the transition time of the rising edge of the clock. The data on the D input may be changed while the clock is LOW or HIGH without affecting the outputs as long as the data setup and hold times are not violated.
H = HIGH Logic Level
X = Either LOW or HIGH Logic Level
L = LOW Logic Level
UP Arrow= Positive-going Transition
Q0 = The output logic level of Q before the indicated input conditions were