The decoder used in the microprocessor-based system is 74LS138 and 74LS139. The 74LS138 is a 3-to-8 decoder and the 74LS139 is a dual 2-to-4 decoder. The 74LS138 decoder consists of 3-input lines, 8-output lines (logic low), and three enables or ground. Of the three enables, two are logic low and one is a logic high enable. The pin configuration of a 3-to-8 decoder (74LS138) is shown in the below figure.
The truth table of the decoder is given in the below table. The 74LS139 decoder consists of two numbers of 2-to-4 decoders packed in a single IC package. Each decoder has two input pins, four output lines and a logic low enable. The pin configuration of 74LS139 is shown in Fig. 3.13. The truth table of the 2-to-4 decoder is given in Table-3.9. In the 74LS139 each decoder can work independently.