wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

FlexRay 44-5
Host
subsystem
Controller
host
interface
FlexRay controller
Bus guardian
Protocol
engine
Bus
driver
Communication channels
FIGURE 44.5
Node architecture.
44.3.2 Node Architecture
A typical node architecture consists of a host CPU subsystem, a network controller, and bus drivers
for the separate channels; see Figure 44.5. Interfacing between the host and the network subsystem is
facilitated via a controller host interface employing some kind of dual-ported memory. This choice of an
interface allows separating control from data flow, creating some kind of temporal firewalls restricting
error propagation to defined areas. Optional bus-guardians may be installed either at the nodes or at
central, active star couplers. The bus-guardians oversee and inhibit bus access whenever a node is not
allowed to transmit in order to avoid so-called babbling idiot failures; situations where a node playing
havoc disrupts the entire communication.
44.3.3 Star Couplers
Both passive and active star couplers are supported by the FlexRay protocol. Whereas passive star couplers
are simply circuits that relay the signal received on one branch to all other branches; active star
couplers provide logic and circuitry for power-down and wakeup, error detection and isolation, as well
as signal reshaping. Typically, an active star coupler listens on all branches for traffic and when a signal
is detected it is relayed—in an error free case—to all other branches. Whenever an error occurs or before
a cluster is synchronized, collisions of data frames or symbols may occur. In this case, an active starcoupler
relays the superimposed signal to the other branches.
44.4 System Design Considerations
Guaranteed bandwidth and reliable operation are some of the benefits of time-triggered communication
systems. On the other hand, such systems may become less flexible (especially when frequent system
changes occur) and require higher efforts at design time.
44.4.1 Configuration
Next to topology considerations, one has to configure which nodes act as coldstarters and participate
at the clock synchronization (sending of sync frames). At least two or more nodes should be assigned
therefore; in fact, more of these nodes enhance the availability of the cluster but increase, however, the
likeliness for the establishment of cliques (cliques are groups of nodes that are able to communicate with
each other, however, not with other nodes), cf. [MHS08].
Furthermore, the physical distribution of these nodes as well as the distribution of their slots impairs
the overall robustness of the cluster. In fact, having these nodes and slots evenly dispersed over the
cluster and schedule, respectively, will likely improve the overall availability. Furthermore, configuring
more nodes as the minimum to transmit the specified wakeup patterns and perform coldstart attempts
will improve the startup mechanism. When one of these so configured nodes, however, is faulty, a higher
© 2011 by Taylor and Francis Group, LLC