UNIT-3

29) What are combinational circuits?

A combinational circuit consists of logic gates whose outputs at any time are determined from the present combination of inputs. A combinational circuit performs an operation that can be specified logically by a set of Boolean functions. It consists of input variables, logic gates, and output variables.

30) Give the design procedures for the designing of a combinational circuit.

The procedure involves the following steps, From the specification of the circuit, determine the required number of inputs and outputs and assign a symbol to each. Derive the truth table that defines the required relationships between inputs and outputs. Obtain the simplified Boolean functions for each output as a function of the input variables. Draw the logic diagram and verify the correctness of the design.

31) Define half adder.

A combinational circuit that performs the addition of two bits is called a half adder. A half adder needs two binary inputs and two binary outputs. The input variables designate the augend and addend bits; the output variables produce the sum and carry

32) Define full adder?

A combinational circuit that performs the adtion of three bits is a full adder.It consists of three inputs and two outputs.

33) Define binary adder.

A binary adder is a digital circuit that produces the arithmetic sum of two binary numbers. It can be constructed with full adders constructed in cascade, with the output carry from each full adder connected to the input carry of the next full adder in the chain.

34) What is overflow?

Over flow is a problem in digital computers because the number of bits that hold the number is finite and a result that contains n + 1 bits cannot be accommodated. For this reason many computers detect the occurrence of an overflow, and when it occurs a corresponding flip flop is set that can be checked by the user. An overflow condition can be detected by observing the carry into sign bit position and the carry out of the sign bit position. If these two carries are not equal, an overflow has occurred.

35) Define magnitude comparator?

A magnitude comparator is a combinational circuit that compares two numbers, A and B, and determines their relative magnitudes. The outcome of the comparison is specified by three binary variables that indicate whether a>b, A = b, or A < B.

36) What are decoders?

A decoder is a combinational circuit that converts binary information from n input lines to a maximum of 2n unique output lines. If the n bit coded information has unused combinations, he decoder may have fewer than 2n outputs.

37) What are encoders?

An encoder is a digital circuit that performs the inverse operation of a decoder. An encoder has 2n and n output lines. The output lines generate the binary code corresponding to the input value.

38) Define priority encoder?

A priority encoder is an encoder circuit that includes the priority function. The operation of priority encoder is such that if two or more inputs are equal to 1 at the same time, the input having the highest priority will take precedence.

39) Define multiplexer?

A multiplexer is combinational circuit that selects binary information from one of many input lines and directs it to a single output line. The selection of a particular input line is controlled by a set of selection lines. Normally there are 2n input lines and n selection lines whose bit combinations determine which input is selected.

40) Define binary decoder?

A decoder which has an n- bit binary input code and a one activated output out-of -2n output code is called binary decoder. A binary decoder is used when it is necessary to activate exactly one of 2n outputs based on an n-bit input value.