 An SSI UART message consists of a 3-byte UART header, an SSI message as the payload and an optional Cyclic redundancy check checksum.The use of a checksum is defined by the SSI message/command type, with lower case commands indicating the use of CRC.The header consists of a start byte (0xFE), a 2-byte (total) length of the message and a 2-byte bitwise Negation length to help identify the frame start.