Environmental Engineering Reference
In-Depth Information
scan testing). High speed CAN is the only legally acceptable protocol for on-
board-diagnostics (OBD II and soon OBD III).
Mobile media or 'PC-on-wheels' for the mobile office will continue to rely
on dual networks, one for low speed (250 kb/s) and one for high speed (>100
Mb/s). The low speed channel is for telematics, navigation aides and infor-
mation services such as audio. High speed channels for real time audio and
video will likely require fibre-optic media at 10 Mb/s, 25 Mb/s and possibly 50
Mb/s rates. The leading protocol is AutoPC for information and telematics.
D2B is already being used and MOST is the top contender in this area. The
integrated data bus (IDB) consortium continues to favour FireWire as part of
their IDB-1394 initiative.
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Wireless protocols (802.11b, 802.11g and soon 802.11a) are necessary for cell
phone and palm pilots (PDAs) but could eventually include cameras and
pagers. By far the leading protocol is Bluetooth, but here IEEE 802.11 appears
a strong contender. The new protocol on the block is ultra-wide band (UWB)
communications that was approved in the US by the Federal Communications
Commission in February 2002. Essentially a 'white noise' communications
scheme, UWB transmits information using a large number of frequencies
simultaneously, each at very low power.
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Safety bus is used for airbag deployment where multiple SRS are employed in
the same vehicle and must be coordinated. The safety bus contains information
on vehicle acceleration and jerk along with occupant sensing, seat belt pre-
tensioners (such as pyrotechnic types) and other sensors. The leading con-
tenders are Delphi's Safe-by-Wire and the Bosch-Siemens BST protocol.
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Drive-by-Wire is a protocol proposed for drive (electronic throttle control),
steer and brake by wire functions. These are high speed, real time, control
functions that demand deterministic message transmission over a commu-
nications channel. The leading contenders are TTCAN (1-2 Mb/s) and Flex-
Ray (10 Mb/s). The most important concern in x-by-wire systems is fault
tolerance. Generally, dual bus architecture for redundancy is required along
with dual microprocessors, bus watchdog and proven reliability.
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4.6.6 Diagnostic test codes (DTC)
Vehicle emission legislation has led to a requirement to identify and archive
emissions system related faults in the vehicles powertrain controller for later
retrieval. DTC are standardized codes available on the vehicle data link connector
and accessible using any of the available scan tools developed for this purpose. For
example, the most common mistake made by drivers that results in setting a DTC,
and the attendant latching of the instrument malfunction indicator lamp (MIL) ON,
is leaving the fuel tank cap off while driving. The resulting loss of pressure in the
fuel tank vapour recovery system (purge canister) shows up as a vapour recovery
system gross leak. To correct the problem a scan tool with personality card for the
particular make and model of vehicle is needed so that the DTC can be reset.
