Convert String Containing Upper ASCII Characters To Byte Array?

How to correctly convert string containing upper ASCII characters to byte array besides looping through each character and filling the array using AscW(chr)?I know I can do something like:

Code:
Dim bSourceData As Byte() = System.Text.ASCIIEncoding.GetBytes(sourcedata)

But this only works for lower ascii characters (0-127).Instead of ASCIIEncoding I can specify encoding myself (System.Text.Encoding.Getencoding(encoding).GetBytes(sourcedata)). But I have to specify correct encoding in this case, otherwise most of special characters are converted to "?". What should I do if I don't know the encoding?