"17" ascii ?
"1"  = 41hex   (4th column, 1st row)  = 0100  0001bin
"7"  = 47hex   (4th column, 7th row)  = 0100  0111bin

17   
76543210
00010001  = 16 + 1  = 2^4  + 2^0

-17
00010001
11101110   (invert all digits --> this is one s complement)
+      1       (add 1 to get two s compl.)
-----------
11101111 


0100  1011  1111    (form groups of 4 digits, starting at right)
4     B     F

3       F               
0011    1111