The XMODEM protcol was developed by Ward Christensen as a simple error cheking scheme suitable for file transfer operations. Messages are sent in blocks of 128 characters surrounded by control characters. The beginning of each block is signaled by a Start Of Header (SOH) character. This is followed by a block number character in ASCII, followed by the same block number with each bit inverted, known as the 1's compliment. The 128-character piece of the file is sent, followed by a checksum that is the remainder of the sum of all the 128 bytes in the message divided by 255.