Introduction to DLD

3 Input OR Gate

A 3-input OR gate is a digital logic gate that contains 3 inputs and one output.

  • The output will be HIGH (1) when at least one input is HIGH (1)
  • The output will be LOW (0) when all inputs are LOW (0)

Example: For three inputs, there will be 3 variables (i.e., A, B, C) that may be used, and the output is also represented by a variable as “X”. The Boolean expression for a 3-input OR logic gate will be X = A+B +C.

Let’s explain the 3-input OR logic gate symbol, Boolean expression, and Truth Table

3-Input OR Gate Symbol

The 3-Input OR logic gate symbol can be represented using three input terminals (taken as inputs), which are given in the following diagram as A, B, and C. The output is represented by “X”.

 3-Input OR Gate symbol

3-Input OR Gate Using Two 2-Input OR Gates

The 3-Input OR logic gate symbol can be represented using two 2-input OR gates; each OR gate contains 2 inputs instead of 3.

  • First OR gate: Inputs A and B, and produces the output A + B.
  • Second OR gate: Inputs (A + B) from the first OR gate and C (directly) and produces the final output as X, which is equal to A+B+C.

The following diagram explains the process

3-Input OR Gate Using Two 2-Input OR Gates

3-Input OR Gate Boolean Expression

The Boolean expression of a 3-input OR gate with inputs A, B, and C and output X is:

  • Y = A + B + C

Here, the + symbol represents the OR operation. The following diagram shows the Boolean expression of a 3-input OR gate

3-Input OR Gate Boolean Expression

3-Input OR Gate Truth Table

The 3-input OR logic gate truth Table shows the output of all possible combinations of the given inputs A, B, and C. The output X becomes HIGH (1) when at least one input is HIGH (1). The output (“X”) becomes LOW (0), in only the case when all inputs are LOW (0).

The following diagram shows the truth table of a 3-input OR logic gate

OR Logic Gate Truth Table (3-Inputs)

Let’s explain the truth table of a 3-input OR logic gate

  • A = 0, B = 0, C = 0: All inputs are LOW, so the output is 0 (LOW).
  • A = 0, B = 0, C = 1: C is HIGH, so the output is 1 (HIGH).
  • A = 0, B = 1, C = 0: B is HIGH, so the output is 1 (HIGH).
  • A = 0, B = 1, C = 1: B and C are HIGH, so the output is 1 (HIGH).
  • A = 1, B = 0, C = 0: A is HIGH, so the output is 1 (HIGH).
  • A = 1, B = 0, C = 1: A and C are HIGH, so the output is 1 (HIGH).
  • A = 1, B = 1, C = 0: A and B are HIGH, so the output is 1 (HIGH).
  • A = 1, B = 1, C = 1: All inputs are HIGH, so the output is 1 (HIGH).