Can someone explain active-low outputs to me?

[Kryst]

I have a DMUX with 4 active low outputs.

Im assuming the truth table for the SELECT lines are:

0 0
0 1
1 0
1 1

From my understanding, the outputs are Active-low aswell. If apparently you get an output of 1, it is run through a NAND gate, which typically would demote the 1 to a 0.

Why would a decoder only set output to zero?

Isnt a decoder something that can decode binary input? If every output is zero, what makes it a unique output?

 

[Matthias99]

'Active Low' outputs probably means that the output lines are '1' by default, except for the the one corresponding to the input, which will be set to '0'. If you provided a part number, though, you could look it up on the manufacturer's website and be sure.