Here’s a simple way of creating an ASCII table through type casting:
32..255 | ForEach-Object { 'Code {0} equals character {1}' -f $_, [Char]$_ }
Likewise, to get a list of letters, try this:
PS> [Char[]](65..90) A B (...) X Y Z PS> -join [Char[]](65..90) ABCDEFGHIJKLMNOPQRSTUVWXYZ PS> [Char[]](65..90) -join ',' A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z