The need to convert data from one format to another and reduce the number of transmission lines lead to the development of encoders. To date, encoders are integral parts of many communication systems. However, once the data is coded into its equivalent code, a decoder is needed at the receiving end to retrieve the original data.
Functioning opposite to the encoder, a decoder converts ‘n’ inputs into ‘2n’ outputs. For instance, a 2 to 4 decoder converts a 2-bit input into a 4-bit combinational code. Depending upon the input combination, one of the output pins is enabled at high.
To further explain its working, let’s start with the simplest decoder design—a 2-4 line decoder.
2-4 Decoder
As discussed above, for every input combination, one of the output pins goes high. Some designs often use the enable pin to activate the circuit; however, it’s not a mandatory circuit element.
Simply put, the output of a decoder is a minterm of input lines. The input combination appears as I1I0— for Y0to be high, the input appears as minterm m0 corresponding to 00. For Y1 to be high, the input combination appears as minterm m1, corresponding to input 01 and the sequence continues.
The Boolean function for each output appears as.
Y0 = A1’A0’
Y1 = A1’A0
Y2 = A1A0’
Y3 = A1A0
The Boolean functions lead to the easy implementation of circuit diagrams. In case of a 2 to 4 decoder, the circuit is a combination of four three-input AND gates with two inverters for input complements.
Whether it’s a 2 to 4 or a higher-order decoder, the basic design logic remains same. A 3-8 decoder outputs eight minterms for 3-bit input combinations, while a 4-16 design produces sixteen minterms for a 4-bit input combination.
Application of Decoders
Due to their combinational operation, decoders are used in circuits where the output responds to a specific input combination. One application of decoder involves the input of counters into the system; the resulting sequential output is used as a sequencing signal for selective switching of electronic devices.
Furthermore, the combination of encoders and decoders are used for coded communication in RF technology for the transmission of critical data.
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