FlexRay
The FlexRay Protocol was developed by the FlexRay consortium which was started when BMW and DaimerChrysler worked together to create a new network scheme.
FlexRay
Introduction
The FlexRay protocl is a time triggered approach to network communications. TDMA(Time Division Multiple Access) method was chosen for the FlexRay protocol because the resulting message transmission delays can lead to problems for safety systems.
FlexRay is an option for upgrading existing network systems using CAN in the automotive industry as well as other industrial control applications.
The FlexRay protocol has been designed to carry information at a rate of 10Mbits/s over each of its two channels while CAN has a date rate of 1Mbit/s. The high bit rate of FlexRay systems makes it suitable as the basis of a network backbone even where CAN is already in use.
Network Topology
The FlexRay protocol defines a two channel network, channel A and channel B. A node can be attached to one or both of these channels. If a node is attached to a single channel it does not matter if it is channel A or channel B.
The FlexRay protocol allows for various bus topologies. These can be a point to point connection, passive star, linear passive bus, active star network, cascaded active stars, hybrid topologies and dual channel topologies. The FlexRay protocol will support hybrid topologies as long as limits of each topology which makes up the hybrid topology(i.e. the star and bus topologies) are not exceeded.
FlexRay Hardware
Each node has a communication controller, a host, a power supply unit and two bus drivers, one for each channel.
The host handles the applications of the system while the FlexRay protocol is handled by the communications controller. The bus driver is used to read and write data to the physical medium over which the data is transmitted. In sleep mode it also has the ability to start the wakeup procedure if it detects a wakeup symbol. The communications controller will mainly handle the framing of data and the checking of received data to ensure it was uncorrupted before passing it to the host. The host and communications controller share information such as control information and payload data from the host, while the communication controller relays status information and data received. The host interface to the bus driver allows it to change the operation of the bus driver as well as read status and error flags.
Media Access Control
The FlexRay protocol uses a TDMA approach and also allows for a node to send frames in a dynamic way. To do this the protocol defines a recurring cycle called the communications cycle.
This cycle has the same format and is of the same time length each time it occurs and in the case of FlexRay is divided into four sections : the static segment, dynamic segment*, the symbol window, and the network idle time**.
FlexRay vs CAN vs Fieldbus
Ref)
https://ieeexplore.ieee.org/abstract/document/4676987
💾 PDU (Protocol Data Unit)
PDU는 하위 레벨의 신호(Signal)들을 적절하게 배치한 상위 레벨의 통신 데이터 컨테이너, 즉 메세지로 정의할 수 있다.
FlexRay 프레임은 복수의 PDU를 포함할 수 있다. 프레임의 레이아웃은 사이클에 따라 변화할 수 있기 때문에 동일한 PDU를 다양한 프레임으로 맵핑할 수 있다. PDU 레이어는 PDU 객체들을 도입하고 버스와 사용자 인터페이스 사이에 각각 위치시킨다.
PDU의 주요한 특성은 업데이트 비트(Update Bit)에 의해 정의되며 네트워크 상의 프레임 발생과는 분리되어 있다. 업데이트 비트 값은 사전에 정의된 신호를 통해 시각화되거나 평가될 수 있다. 간단한 분석 또는 시뮬레이션 기능을 위해 만일 수신된 PDU가 계속 업데이트되지 않는다면 이 PDU를 무시하는 것이 기본 기능이다.
FlexRay 프로토콜이 주기적으로 전송될 프레임들을 정의함에도 불구하고 본래의 PDU는 이러한 특성을 가지고 있지 않다. PDU가 업데이트되지 않는 경우에 수신기는 일반적으로 PDU를 인식하지 않는다. 수신기를 주기적으로 트리거하기 위해서는 PDU는 반드시 주기적으로 업데이트되어야 한다.
