Environmental Engineering Reference
In-Depth Information
the placement of messages and to plan for present and future content, a determi-
nistic communications channel is realized. FlexRay is one such solution to the
contention issues of event based protocols and one of the primary drivers for its
being considered for by-wire chassis functions of steering and braking. The spon-
taneity of CAN message traffic is also retained in FlexRay in its flexible dynamic
segments that can have frame lengths that vary from cycle to cycle without losing
the regulated placement of node messages. Figure 4.52 illustrates the FlexRay
protocol structure.
Time
derived
Time
derived
Cycle
y
y
- 1
Cycle
y
+ 1
NIT
Static
Dynamic
Symbol
NIT
Static
Channel A (single) or Ch 1 of dual channel (1 and 2)
Network idle time
Dynamic slot
counter with
messages
J J + 1
Frame ID
j
Frame ID
j
+ 1
j
j
+ 1
j
+ 2
j
+ 3 * * *
Dynamic
slot counter
if no
T
x
Dynamic segment messages can
have variable length
Static slot counter increments after
fixed time interval in global time
m
m
+ 1
m
+ 2
m
+ 3 * * *
Figure 4.52 FlexRay protocol structure
Figure 4.52 expands the FlexRay message formatting during one cycle of the
global time clock. In the static segment, all time slots have identical length (
m
) and
bus access is performed according to static TDMA convention. Each node on the
network, per planning and regulation, has its own time slot in the static segment,
where the slot counter increases synchronously with the pre-defined slot length in
global time.
The dynamic segment only increases its slot counter when a message is
transmitted but it does not increase until that message is terminated. For example, a
