VS 2010 Signed And Unsigned Integer NOT: For Hex File Checksum Computation?

I have been trying to compute the checksum for a line of hex code. This is to create a hex file which will be loaded onto an embedded micro.Some of you guys may not know how this is done so very briefly:

a line of hex bytes:

: 10 01 00 00 21 46 01 36 01 21 47 01 36 00 7E FE 09 D2 19 01 40

The underlined bytes are taken and summed. (in this case the decimal 960). and the bold is the checksum Only the 8LSB (or single least significant byte) is needed to compute the checksum... which in this case, 960 = 03C0 in hex, so only C0 is needed.Two's complement is then taken:

step 1) logical not of C0 which is
step 2) add one (integer)
BIN...........DEC...HEX

[code]....

So here is the code I have been using: (quick mention that in the string passed, the hex bytes are all grouped together, there are no space characters)

Public Sub CreateLine(ByVal Data As String)
'Dim BDataArray(DataLength) As Byte
Dim IDataArray(Data.Length) As Integer

[code]....

where i get into trouble is where i commented 'i get into trouble'. My code works, but it's rather crude Now, in that line i first convert the hex value (string) "SChecksum" into an integer. I want to NOT this, so I get my step 2 from my explanation. However, the results appears to be a signed logical NOT. which gives me the wrong answer. My hashed togther version simply subtracts 256 before the not, in order to give me the correct answer.

Basically, I would love for someone to chime in and point out the flaws in my function, and push me towards figuring out the 'proper' way of doing this. Also I can't quite wrap my head around what would happen in the case a string were passed to my function that is over 16 bits, but I imagine I would have to subtract a larger value than 256, before doing the NOT in this case?(whilst we're on the subject of functions, I've used a 'sub', in which case should I used a sub, and which case should I use a function?).