ARINC 429 is the industry standard that is used on virtually all commercial aircraft. It is a specification that defines a local area network for transfer of digital data between avionics system elements. The specification describes how an avionics system transmits information over a single twisted and shielded pair of wires (the databus) to all other system elements having need of that information (up to as many as 20 receivers). Bi-directional data flow on a given databus is not permitted.
There are two speeds (approximately 12.5 and 100 thousand bits per second). Words are 32 bits long including a label, parity bit and other fields. The label is an eight bit field that identifies the type of information contained in the word. The 429 specification defines the units, ranges, resolutions, refresh rates, number of significant bits, pad bits, etc. for the words transmitted by the different avionics system elements.
What is Williamsburg Protocol?
Williamsburg is a bit-oriented protocol used to transfer files between systems over ARINC 429 full duplex links. Each LRU has a transmit connection and a receive connection, and implements a control protocol to manage the data transfer. The original block transfer protocol was AIM (ISO Alphabet No. 5). The Williamsburg protocol, introduced in ARINC 429-12, has replaced the AIM protocol (now discontinued).
Files are transferred in blocks known as Link Data Units (LDU) each containing from 3 to 255 words. A system initiates a transfer by sending a Request To Send (RTS). Following the receipt of a Clear To Send (CTS), the source begins the LDU sequence with a Start Of Transmission (SOT) followed by up to 253 data words. The LDU transfer is terminated with an End Of Transmission (EOT) containing a 16-bit CRC value calculated from the contents of the LDU. On successful verification of the CRC value the sink system sends an Acknowledgment (ACK) message. The source may then repeat the process until the entire file is transferred.