Environmental Engineering Reference
In-Depth Information
message being transmitted by node 7 has its static segment in slot 7 for identifi-
cation and places its messages into dynamic slot
j
= 7. The dynamic slot counter
increases up to
j
= 7 after a pre-defined time elapses for each dynamic segment if
no messages are sent prior to node 7. When node 7 transmits, its message may have
arbitrary length within the dynamic slot.
Because of the pre-defined positioning of node messages with time slots, it is
possible to determine whether a particular node is responding properly. If for some
reason node 7 sends its message out of its assigned slot, then an error is detected.
This functionality is that of a bus guardian and in FlexRay the bus guardian controls
access to the physical bus. The bus guardian receives the pre-defined schedule from
the bus host during initialization and from that point on the bus guardian activates
its bus watchdog that contains an independent time reference. Bus drivers and bus
guardian collaborate and support each other in the identification of faults. Diag-
nostics of bus faults are then passed onto the bus host.
4.6.5 Competing future communications protocols
It is clear that modern personal transportation vehicles, hybrid vehicles in parti-
cular, will rely on multiple communications buses per vehicle. This is because the
need for communications varies according to systems supported and the speed at
which information must be shared. Body modules for powered seats, windows,
mirrors and the like are slow functions and low speed communications suffices
(Class A). With the introduction of x-by-wire functionality there is pressing
demand for real time control in chassis, powertrain and safety related systems.
In some circles there is a belief that as many as eight different communications
networks will be required in the future vehicle [43]. These networks include Class
A, B and C, as already defined plus a dedicated network for diagnostics, airbag
deployment within SRS, mobile media, x-by-wire and wireless. Since each of these
categories would require their own protocol, there can be multiple protocols in use
per vehicle. These protocols are now defined:
Class A networks for general purpose communications. Many proprietary
schemes have disappeared, as are all such closed architecture and OEM spe-
cific protocols. For class A the leading contender is LIN, the smart connector
protocol.
●
Class B networks cover the majority of non-critical system communications
such as battery monitoring and transmission shifting. Network traffic is event
driven with some periodic traffic (time stamped data). For class B the standard
remains CAN.
●
Class C networks are reserved for faster, higher bandwidth functions such
as engine management, timing signals, fuel injection etc. Message speed is
125-500 kb/s over predominantly twisted pair copper wire. ISO 11898 is the
leading CAN protocol for class C networks.
●
Emissions diagnostics now use ISO 14230 standard on the data link connector
between the engine controller and the external diagnostic connector (used for
●
