Characteristics of Encoder
List below :
- It has maximum of 2n input lines and ‘n’ output lines.
- It encodes 2n input lines with ‘n’ bits.
- It performs the reverse operation of Decoder.
- At any time, only one of inputs can be ‘1’ in order to get the respective binary code at the output and rest of the inputs are zero(0).
- It is used in input system like Keyboards
4 to 2 Encoder:
Let 4 to 2 Encoder has four inputs D, D1, D2 & D3 and two outputs X & Y. The block diagram of 4 to 2 Encoder is shown in the following figure-
At any time, only one of these 4 inputs can be ‘1’ in order to get the respective binary code at the output. The Truth table of 4 to 2 encoder is shown below.
From Truth table, we can write the Boolean functions for each output as
We can implement the above two Boolean functions by using two input OR gates. The circuit diagram of 4 to 2 encoder is shown in the following figure-
The above circuit diagram contains two OR gates. These OR gates encode the four inputs with two bits.
Octal to Binary Encoder or 8 to 3 encoder:
Octal to binary Encoder has eight inputs, D to D7 and three outputs X, Y & Z. Octal to binary encoder is nothing but 8 to 3 encoder. The block diagram of octal to binary Encoder is shown in the following figure.
At any time, only one of these eight inputs can be ‘1’ in order to get the respective binary code. The Truth table of octal to binary encoder is shown below.
We can implement the above Boolean functions by using four input OR gates. The circuit diagram of octal to binary encoder is shown in the following figure-
The above circuit diagram contains three 4-input OR gates. These OR gates encode the eight inputs with three bits.
Drawbacks of Encoder
- There is an ambiguity, when all inputs of encoder are equal to zero.
- If more than one input is active High, then the encoder produces an output, which may not be the correct code.
So, to overcome these difficulties, we should assign priorities to each input of encoder. Then, the output of encoder will be the code corresponding to the active High inputs, which has higher priority.
Uses of Encoders:
- Encoders are very common electronic circuits used in all digital systems.
- Encoders are used to translate the decimal values to the binary in order to perform the binary functions such as addition, subtraction, multiplication, etc.
- Other applications especially for Priority Encoders may include detecting interrupts in microprocessor applications.