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Grundlagen FlexRay BasicsV 1.1 35
Figure 2.31 shows that the optional Symbol Window follows the dynamic segment. It can be
used to send one Symbol per Cycle.
The length gdSymbolWindow of the Symbol Windows can be defined between 0 and 142
Macroticks. The Media Access Test Symbol (MTS) is used to check the Bus Guardian and is
the only Symbol currently used. The length cdCAS of the Media Access Test Symbols is 30
low bits, just like the one of the Collision Avoidance Symbol. The Collision Avoidance
Symbol isn’t transmitted in the Symbol Window, but during the Startup oft he cluster.
2.11.6 Network Idle Time
Every communication cycle ends with the Network Idle Time. In this segment, the bus is
silent, there is no communication.
The Network Idle Time is used to synchronize the ECUS, and is defined by the parameter
gdNIT (2 to 805 Macroticks). As seen in Figure 2.31, the Network Idle is divided in two parts.
During Channel Idle (CI) the synchronization calculation is performed, and is adjusted in the
Offset Correction Segment.
2.12 Frame format
A frame consists of three parts, header, payload, and trailer, see Figure 2.32.
Figure 2.32 Structure of a Frames
2.12.1 Header
The first 40 bits of a frame are called header. It contains information which is necessary for
the protocol.
• Status Bits – at the start of a frame, the Reserved Bit, Payload Preamble Indicator,
Null Frame Indicator, Sync Frame Indicator and Startup Frame Indicator are
transmitted. The bits describe the type of payload transmitted. A detailed description is
available in [FRPS05].
Bäurle 12.10.2012