Question

1) The figure shown below shows how four 74xLS138s (3 to 8 Decoder) can be arranged to function as one-of-32 decoder. The decoders are labeled Zo to Z Answer the following: a. Which output will be activated for A 4As A2 Ai Ao 11101 b. What range of input codes will activate Z2? (MSB 123 :1 2 3 123 11 23 74ALS138 74ALS138 74ALS138 74ALS138 012345671 10123'4 5 6 71 10123A 5 6 7| |0123-4 5 6 7 Oni Os

Answer 1

Solution:-

74 x LS138s is a decoder

A0, A1, A2 are adress inputs on selection inputs

E1, E2 - Enable low inputs

E3 - Enable high input

O0 to O7 - active low outputs

E1, E2 are enable low inputs means that even if one of these two input is high the olp (output) is set to default value (that is in this case high as output are active low).

Don't bother about all these let's see internal circuit of 74 x 138s and arrive at above conclusions and proceed to further question.

74 x LS138s internal equivalent CKt diagram.