(a) What is a combinational logic circuit and what is a sequential logic circuit? Describe the use of a flip-flop or a latch as a basic element in semiconductor memory. Prepare a truth table similar to what is shown in Figure 8.13b, for an RS flip-flop that uses two cross-coupled NOR gates.

(b) An optical encoder (a motion sensor that senses displacement in steps and produces pulses correspondingly) generates two pulse sequences that are 90° out of phase. In one direction of motion (denoted by L), one pulse sequence (denoted by A) leads the other (denoted by B) by 90°, and in the opposite direction of motion (denoted by R) this pulse sequence (A) lags the other

(B) by 90°. This situation is shown in Figure P8.31. Suppose that the two sequences are read into a 2 bit register, the high voltage level of each signal being represented by a 1 bit and the low (zero) voltage level being represented by a 0-bit. Verify that in the L direction of motion, the register value will change according to the sequence:

Explain a simple way to detect the direction of motion by checking the binary value in the register. Discuss a way to physically implement this direction- detection method.

Figure 8.13