[sdiy] an XOR question

[Scott Gravenhorst]

Nils Pipenbrinck <np at inverse-entertainment.de> wrote:
>Schierl, Dan wrote:
>
>> Hi list--
>> I have an application I'm working on in which I need to perform an 
>> exclusive OR on 4 inputs. Since I don't think anyone makes a 4-input 
>> XOR, I tried to rig something up using some CMOS chips. My first 
>> attempt was a little too simplistic (i.e. wrong), which I didn't 
>> realize until I'd already wired it into my application. I think my 
>> second attempt works (haven't tried it yet, tho) but i think it might 
>> be overkill.
>
>Hi Dan,
>
>Well, there is no definition how a multi-input xor should behave.
>
>How do you want yours to perform? Should the output be high if one, and 
>only one of the inputs is high, or should it be high when any number but 
>all inputs are high? These are the two possible multi-input xors I could 
>think of.  Also, what are you trying to archive? Reduce the costs, the 
>pcb space or part count?
>
>If you just want to reduce part count you can use the sledgehammer 
>method and use a 16 channel multiplexer such as the 4067. All you have 
>to do is to hardwire the inputs and use the address inputs as your 4 
>inputs. You can realize any 4 input to 1 output logic that way.
>
>  Nils

Come on, you can't call that "the sledgehammer" method.  My own suggestion
of "EPROM" should claim that title.  The 4067 is simple, cheap and
programmable.  EPROM wins only when you need more than 4 bits.

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