This example assumes frame will be read from file, but it can be easily modified to operate with frames already in RAM.

Assumed that file contains a frame from <STX> to <ETB>, or to <ETX>, but not both of’course.

First example below shows Hex D6 value returned as string for frame from <STX> to <ETX>.

Second example shows Hex 44 value returned as string for frame from <STX> to <ETB>.

Code snippet below. Comments welcome.

function main()
   local fn='c:\\astmsample44'
   return string.format("%x", tostring((foo(fn))%256)):upper()
end

function foo(fn)
   local r=-2
   local f = assert(io.open(fn, "rb"))
   local block = 10
   while true do
      local bytes = f:read(block)
      if not bytes then 
         return r 
      end
      for b in string.gfind(bytes, ".") do
         r=string.byte(b)+r
      end
   end
end