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how to enable 74LS138 decoder???

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tarun0xe

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hello friends, in the given image the decoder 74LS138 is enabled by E3,E1-bar and E2-bar.now since A3 bit is connected to E1-bar enable input,it must be at logic 0 to activate E1-bar.but soon it is taken as 0,it makes the output of above NAND gate to be 1 which disables the E2-bar enable input and thus disable the decoder too.
firstly,please correct me if i'm wrong at any of the mentioned arguments then tell what to do with this A3 bit....
the other bits according to me,should have following logic:
A7=1,A6=1,A5=1,A4=1,A3=1(at input of NAND gate) and 0 at input of E1-bar, A2=1,A1=0,A0=1.
thanks in advance.... :)
 

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