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Does a 8-3 multiplexor exist?

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jonnybgood

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Hi guys,
I am asked to design an 8-3 multiplexor from dual 4-2 demux 74HC139 IC's and using glue logic. Do you think this is possible as i've searched every where without any hints that an 8-3 multiplexor ic exists :s.
 

Do you really mean a multiplexor or do you mean an encoder.
A binary 8 to 3 line encoder is very simple.
If you mean a multiplexor, what is being multiplexed? Do you mean eight separate inputs, each going to one of three outputs with the output for each input decided by three address select bits. If that is the case, the routing logic is straight forward but preventing a collision (more than one input connected to a single output) is significantly more complicated.

Brian.
 
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    jonnybgood

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The question in my assignment is clear he asks for a multiplexor 8-3 and doesn't specify how much address inputs. I assume the 8 inputs are independent and are mapped to the three output accordingly.
 

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The reason I asked was that the easiest solution doesn't use 74HC139 ICs, all you have to do is gate the each input to each output. With no limit to address lines you can use one per gate to simply enable the signal through.
I suggest you look at the data sheet for the 74HC151 which is an 8 to 1 multiplexer. Simply use 3 of them with their inputs linked and take one output from each. If you need to address them individually, you could use the '139 with outputs connected to the '151 enable inputs. From the data sheets you can see the gates inside them and possibly remove redundant ones.

Brian.
 
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    jonnybgood

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No, i'm sure it's a multiplexor but the thing is that I can't include any other ics than the 139, the other is the glue logic which i assume are only discreet components.
 

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The question in my assignment is clear he asks for a multiplexor 8-3 and doesn't specify how much address inputs. I assume the 8 inputs are independent and are mapped to the three output accordingly.
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I think it's far from being clear. I don't know what a 8-3 multiplexer is. Priority encoder would be a clear specification in contrast. Or a 8-1 multiplexer, which implies three address inputs.
 
Then there must be a mistake in my assignment. When I clear it out with my lecturer I will leave some feedback.

thanks

Brandon
 

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