i am asked to complete a timing diagram of a 74LS151 and the output is low when disabled. But i don't know how to start this any hints?

 

Get the data sheet - http://www.datasheetcatalog.com/datasheets_pdf/7/4/1/5/74151.shtml That explains how the multiplexer works.

 

yeah i have it, but what confuses me is "output is low when disabled"

 

Take a look at the E^ line. When it is low, it provides a partial enable for all the decoder AND gates. If high, all the AND gates are broken, so the output will be low.

 

All that it says is that the output will conduct Ground (will be driven LOW) when the Enable input is HIGH.

In other words, when 1 comes in the Enable terminal, the output will stay to 0, no matter what the other input pins will be.

Another word ordering is that the MUX will work only when Enable is LOW, otherwise the output will be LOW.

...I 'm feeling quite talkative today...