ISRO CSE 2014 | Question: 26

Question

The output of a tristate buffer when the enable input in $0$ is

• A. Always $0$
• B. Always $1$
• C. Retains the last value when enable input was high
• D. Disconnected state

Answer 1

When the control input or enable input (E) is not active, the output is "Z", which means that no electrical current flows through. Hence it is in disconnected state .

Thus, even if x is 0 or 1, that current does not flow through .

hence, truth table is 

E(Enable)	X(input)	y(output)
0	0,1	Z(disconnected)
1	0	0
1	1	1

Answer 2

Answer : Disconnected State 

Reference : Click Me

Answer 3

Buffer gives output , which it gets as input (with some delay)

Now, in tri state buffer

Enable input is 1 , it's circuit is closed state. Means when it gives input 0, get output as 0.

When enable input is 0, circuit is in open state. Means retains the last value when enable input was high

So, Ans C)

Comments

Here they have not mentioned whether it is with active high or with active low. As in active low if enable input is zero it will produce the input as output. So according to the answers it is active high and this case for enable input as zero will resulting high impedance i.e Z . So answer is D - disconnected state.

For reference:- http://www.deinfo.uepg.br/~ljsenger/tutoriais/tri-state-buffer.pdf