address con %00000001
This equals 1

chk_sum = (address + address)
chk_sum = 2

if A = 0 && B = 0 then keydata = %10101111
keydata = 175

chk_sum = chk_sum + (keydata + keydata)
chk_sum = 2 + 175 + 175

chk_sum = 352

chk_sum var byte
byte = 0 to 255

Maybe try WORD sized variables.

Or maybe I am missing something