Q) What happens when a 74S151 multiplier E* is not asserted
My answer: Multiplexer gets disabled and there is no output at all

Q) What happens when input E* was lifted momentarily?
My ans: The multiplexer produced an infinitely high output at t=0. This is because the multiplexer is disabled and outputs a logic high.

My answers in bold are marked wrong. Can some kind soul please explain? Thanks.

 

Hello,

Take a look at the function table:

Greetings,
Bertus

 

There is an output with the enable in either state. Check the truth table. It is not a 3-state IC with a high impedance state.

There is no such thing as an "infinitely high output". "Lifting" the enable sounds as if it is being de-asserted. When it is high, Z and Z* follow the truth table.

 

Q) What happens when a 74S151 multiplier E* is not asserted
My answer: Multiplexer gets disabled and there is no output at all

The Y output (which is active high) will go low and the W output (active low) will go high.

Q) What happens when input E* was lifted momentarily?
My ans: The multiplexer produced an infinitely high output at t=0. This is because the multiplexer is disabled and outputs a logic high.

My answers in bold are marked wrong. Can some kind soul please explain? Thanks.

In the second case, if what is meant by the input E* being "lifted", it meant momentarily disabled the same thing occurs as above. But if that moment is very brief the propogation delay time (built into the device with buffers) will prevent any transient. I may be wrong on this latter point.

Good luck!

 

Ok. when the output is lifted, it involved the candidate pulling out the wire from the multiplexer for a brief moment. that is what I meant.

Dear me I recorded my observations but this is what I get

 

Be aware that opening up a control input usually results in the input level oscillating right at the logic level trip points. The result is unpredictable, except that the logic won't work correctly.

 

Hitmen, did the question have a circuit using a 74151? And if it did was there a pull-up or pull-down resistor connected to E*. In the latter case if you pull out the wire the E* line goes to whatever voltage it is pulled up or down to. And in that case you look at the truth table as posted by Bertus.

 

Ok thanks.