# Make circuit more simple

#### WBahn

Joined Mar 31, 2012
26,398
Do you know how to add four numbers together, say, 0, 1, 1, and 0?

Do you know how to write a number, say 2, as a binary number?

And, as already stated, your truth table for the decoder is NOT correct. Again, what affect does the EN input have?

#### Leite33

Joined Nov 28, 2015
57
Well i am not sure i thought is another input. Can you explain me how it really work? I think is an input enable so it can control the function of circuit. I mean when the value of En is 0 then my circuit works as decoder and etc but if the value of En is 1 then my circuit isnt work . So if En is 1 every output is 1?

#### WBahn

Joined Mar 31, 2012
26,398
If an output is disabled (i.e., not enabled), what is the most reasonable value for that output to be?

Did you not read what hp1729 said in Post #15?

Considering the truth table of the decoder, only one output will be high at any time. Enable must be high for any output to be enabled. (If ENA is low all outputs will be low.)
How about doing a little research, such as Googling: 3-to-8 decoder with enable?

#### Leite33

Joined Nov 28, 2015
57
Ok i tried to made a little search. I looked at my book and i see in encoder with enable the decoder works when the E=0 and not work when E=1. So i guess my truth table when the Enable is E=1 everything stay as it is. Whn the E=0 then somethings change. But i still cant understand the truth table

#### Leite33

Joined Nov 28, 2015
57
Truth table

I think the white part of my truth table is ok. I dont understand how will be the red. Do i need it that part after?

#### Leite33

Joined Nov 28, 2015
57
Now i study better at one decoder 2-4 and with one enable input it says:
when enable is 1 the circuit not work and everything is one at outputs despite what is b2 b1 and b0. Also it says that when Enable is 0 the circuit work and only one output can be 0
so if am i right

Is this the right truth table?

#### WBahn

Joined Mar 31, 2012
26,398
Ok i tried to made a little search. I looked at my book and i see in encoder with enable the decoder works when the E=0 and not work when E=1. So i guess my truth table when the Enable is E=1 everything stay as it is. Whn the E=0 then somethings change. But i still cant understand the truth table
You just NOW are FINALLY looking at your book!

What, exactly, does your book say? Are you sure it is not talking about ENB (or EN with a bar over it, or /EN). Think about what the name implies. EN is short for Enable. 0 is FALSE and 1 is TRUE. Does it make sense that the decoder would only work when Enable is FALSE?

#### WBahn

Joined Mar 31, 2012
26,398
Now i study better at one decoder 2-4 and with one enable input it says:
when enable is 1 the circuit not work and everything is one at outputs despite what is b2 b1 and b0. Also it says that when Enable is 0 the circuit work and only one output can be 0
so if am i right
View attachment 96006
Is this the right truth table?
Only if all of the signals are active-LO.

#### Leite33

Joined Nov 28, 2015
57
So i got the adder truth table what about the decoder 3-8 truth table where am i wrong?

#### WBahn

Joined Mar 31, 2012
26,398

#### WBahn

Joined Mar 31, 2012
26,398
Thank you.

So what pins are your three outputs?

You have two outputs from the right-most adder. Are those your only outputs?

You have two carry-out signals from the left-most adders. Are neither of those used?

You have a total of four output signals altogether, but you only get three output total.