This article lists the SmartSet Data Protocol and explanation

The table below shows the function of each byte.



And the table below explains the function of each.



* These bytes are included only in the serial data packet.
** 01, 02 or 04 for AccuTouch controllers; 81, 82 or 84 for IntelliTouch controllers
*** Always FF for AccuTouch; 00 - FF for IntelliTouch, dependent on touch pressure

The lead-in byte, 55 hex, is a unique binary pattern in which the bits alternate (0101 0101); this unique pattern helps to identify a valid touch packet. 

The checksum byte is the sum of the previous nine bytes, plus AA hex (only the low byte of the sum is kept), and it is sent with the data (by the touchscreen controller) as the tenth byte. 

When the software driver receives the packet, it also sums the first nine bytes, pus AA hex. It then compares its sum with what is in the checksum byte sent by the controller; if the two do not match, then there was an error in the transmission, in which case the data is discarded. The extra AA hex cancels out the 55 hex lead-in byte (1010 1010 + 0101 0101 = 0).