Question

What is the relation between the two outputs of 74ls151?Is ENABLE input active-HIGH or active-LOW? a) Identify the ENABLE pins for 74ls138 chip. Are these pins active-HIGH or active-LOW? Are the outputs of 74ls138 active-HIGH or active-LOW? c) When used as an octal decoder, this IC converts what type of input code into a corresponding activated output? d) When used as a DEMUX, which input(s) may be used as a data line and be distributed to the output?

Answer 1

74ls151 - 74ls151 is a 8-input Multiplexer. It has 8 inputs, 3 selection inputs, 1 enable input and 2 outputs.

The relation between two outputs is that they are complement to each other. It means one output is the multiplexer output and the other output is the complement of the multiplexer output.

The Enable input is active-LOW.

74ls138 - 74ls138 is a 3 to 8 line decoder. It can also be used for De-multiplexing applications.

(a) There are three ENABLE pins of 74ls138 chip. The pin numbers are 4,5 and 6. Out of these three ENABLE pins, the pins 4 and 5 are active-LOW and the pin 6 is active-HIGH.

(b) The outputs of 74ls138 are active-LOW.

(c) When used as an octal decoder, this IC converts 3-input binary code into its corresponding octal output.

For example, if the 3-input binary is '000', then the first octal output () will be '0' and other octal outputs () are '1'. It gives the octal output as '0' because the octal equivalent of binary '000' is '0'.

Similarly, if the 3-input binary is '001', then the second octal output () will be '0' and other octal outputs () are '1'. It gives the octal output as '1' because the octal equivalent of binary '001' is '1' and so on.

(d) When 74ls138 is used as a DEMUX, then any one of the active-LOW ENABLE input can be used as a data line and distributed to the output. Other ENABLE inputs are connected to their appropriate active-HIGH or active-LOW state.