Electronic – XOR Gate malfunctioning

digital-logic

I have an XOR gate wired (74LS136) and I don't think it is working properly. Here is the table I think it should be outputting:

|X|Y||O|
-------
|0|0||0|
|0|1||1|
|1|0||1|
|1|1||0|

1 is a High
0 is a Low

I think this is the right output for the gate, but this is not what I'm getting. This is what I am getting out of my XOR gate:

|X|Y||O|
-------
|0|0||0|
|0|1||~|
|1|0||~|
|1|1||0|

1 is a High
0 is a Low 
~ is nothing

When I probe the output pin I get neither a high nor a low, it says no current. I have tried a couple different chips from the same manufacturer but they all have the same result. Is this normal for an XOR gate? Is this chip suppose to be doing this? Am I using the wrong chip?

Best Answer

The part you're looking at has open-collector outputs. Consequently, every place its output is specified as "high", what it really means is "floating". This allows the outputs from multiple xor gates to be 'and'ed by simply tying them all together and attaching a single pull-up resistor to the lot of them.

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