title: K-map ( karnaugh map ) 8,4,-2,-1 to binary code conversion
body: <p>I'm taking computer science courses and some digital design knowledge is required, so I'm taking digital design 101.</p>
  
  <p><img src="https://i.stack.imgur.com/FNmlW.png" alt="http://i.imgur.com/jQRbLsc.png"></p>
  
  <p>Image above is representing the conversion process of 8,4,-2,-1 to binary using K-map (Karnaugh map).</p>
  
  <p>I have no idea why 0001, 0011, 0010, 1100, 1101, 1110 are marked as 'X'.</p>
  
  <p>For 0001, 0011, 0010, they could be expressed as 8,4,-2,-1 as 0111, 0110, 0101.
  And for 1100, 1101, 1110,
  1110 can still be expressed as 1100 in 8,4,-2,-1 form as 1100.
  rests cannot be expressed in 8,4,-2,-1 since 1100 is the biggest amount of number in 8,4,-2,-1 binary form (I think).</p>
  
  <p>Is there something I'm missing?</p>
  
  <p>I understand the excess-3 to binary code conversion provided from my textbook example ( m10-m15 are marked as 'X' since excess-3 were used to express only 0-9.)</p>