EADIN_Lite Network Protocol
Authored by Eliot Aretskin-Hariton NASA Glenn Research Center, Cleveland, Ohio, 44135 [email protected]
This code was created to support the Distributed Engine Control task as part of the Fixed Wing Aeronautics project. The purpose of this research was to enable multiple microcontrollers to speak with eacho ther per the protocol specified in the preliminary release of EADIN BUS. EADIN BUS is a candidate for distributed control systems on aircraft engines and is being worked on by the Distributed Engine Control Working Group (DECWG) http://www.decwg.org/. The primary use of this code was to assist in the modelling of small local networks which contain 16 or fewer nodes. Ultimately, we expect this network to be implemented in an FPGA or ASIC as opposed to it's current implementation on a microcontroller.
This communication protocol uses a master node which distributes information between nodes through a call and response system. The RS-485 network is simplex and thus does not allow multiple nodes to talk at the same time. No time synchronization between nodes is required for this network. These factors enable the master to request information from sensors and command actuators, one at a time. In the current implementation, no information is passed from individual nodes without first going through the master node.
While other communication protocols do exist like ModbusMaster and simple-modbus, the speed of these communication protocols on the RS-485 network was not sufficient for our needs which required message send to reply receipt times of 1 millisecond. Additionally, the other protocols did not implement the same message system as specified by the preliminary documents regarding the EADIN protocol.
The EADIN protocal as implemented by this code has the following structure: Total Size: 18 bytes
- Preamble: 3 bytes
- 2 bytes start of message (0x00, 0x01)
- 1 byte a sync byte (0x55)
- Headder: 5 bytes
- 1 byte request type (ENQ/ACK/NAK = 0x05/0x06/0x15)
- 1 byte node_ID of desination
- 1 byte node_ID of sender
- 1 byte unique message number
- 1 byte extra space to be used in future development
- Data: X bytes (8 bytes Default)
- 8 bytes DATA_L (can be modified)
- Footer: 2 bytes
- 2 bytes CRCFast (a 16 bit CRC, Default)
References: Ross N. Williams, A painless guide to CRC Error Detection Algorithms availalbe for download at: http://www.ross.net/crc/download/crc_v3.txt