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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?

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    \$\begingroup\$ I'm not used to being able to measure "nothing"! Though, ironically, "floating" turns out to be the closest thing to "nothing". Just wondering what you were using to measure the output with. \$\endgroup\$ – gbarry Feb 23 '12 at 0:51
  • \$\begingroup\$ @gbarry I thought the same thing. "No Current" would imply to me that he is doing a current measurement. But there should be no current also when he measured low. Even if it was voltage measurement I am sure it would still read some voltage value. \$\endgroup\$ – Kellenjb Feb 23 '12 at 2:02
  • \$\begingroup\$ it was not a voltage meter, it was a digital high/low probe, i didnt know about floating so i described it as no current/nothing \$\endgroup\$ – John Feb 23 '12 at 3:37
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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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The problem you're having is that this particular XOR gate has an open-collector output. All you need to do is attach a pull-up resistor on the output, and you should be able to use it the way you're expecting; on the other hand, you could also just go find a different kind of XOR gate; you'd want to look for the ones that are just 74[numbers] - if there's letters in there, it might be something fancy.

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  • \$\begingroup\$ Drat, supercat beat me to it - that's what I get for never having actually logged in to electronics.se :( \$\endgroup\$ – Tacroy Feb 23 '12 at 0:32
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    \$\begingroup\$ The letter family typically does not specify that - things like Schmitt triggers and open collector outputs are normally encoded in the type. For instance, 7486 is a quad xor, 74136 is the open collector variant. 74186 has only one gate, and 74386 differs from 7486 in pinout (outputs toward middle rather than down). More at logic families and 7400 series. \$\endgroup\$ – Yann Vernier Feb 23 '12 at 6:18
  • \$\begingroup\$ It's strange because I thought that 'LS' were TTL gates... \$\endgroup\$ – clabacchio Feb 23 '12 at 8:23
  • \$\begingroup\$ @YannVernier Oh, right - it's been a while since I used those chips, and I just had a vague memory of the letters meaning that strange magic I didn't understand was going on :) \$\endgroup\$ – Tacroy Feb 23 '12 at 16:47

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