Klipper is in a sense a parsing regime, based on USB (up to 64byte transfers); Source
Message Blocks¶
All data sent from host to micro-controller and vice-versa are contained in “message blocks”. A message block has a two byte header and a three byte trailer. The format of a message block is:
<1 byte length><1 byte sequence><n-byte content><2 byte crc><1 byte sync>
The length byte contains the number of bytes in the message block including the header and trailer bytes (thus the minimum message length is 5 bytes). The maximum message block length is currently 64 bytes. The sequence byte contains a 4 bit sequence number in the low-order bits and the high-order bits always contain 0x10 (the high-order bits are reserved for future use). The content bytes contain arbitrary data and its format is described in the following section. The crc bytes contain a 16bit CCITT CRC of the message block including the header bytes but excluding the trailer bytes. The sync byte is 0x7e.
The format of the message block is inspired by HDLC message frames. Like in HDLC, the message block may optionally contain an additional sync character at the start of the block. Unlike in HDLC, a sync character is not exclusive to the framing and may be present in the message block content.