BMW G12 2015 Technical Training Manual page 13

G12￿Voltage￿Supply￿and￿Bus￿Systems
1.￿Bus￿Systems
Hardware￿advantages
One￿advantage￿is￿the￿transmission￿of￿vehicle￿camera￿data.￿Vehicle￿cameras￿(IP-based)￿are￿not
connected￿to￿one￿control￿unit￿,￿but￿several￿control￿units￿by￿Ethernet￿which￿allows￿these￿units￿to￿access
the￿camera￿data￿flow￿efficiently.￿The￿network￿is￿therefore￿very￿flexible￿and￿is￿well￿prepared￿for￿the
future.
Another￿important￿advantage￿is￿the￿fast￿programming￿of￿control￿units.￿The￿programming￿times￿are
reduced￿significantly￿via￿Ethernet.￿Whereas￿a￿CAN￿bus￿can￿only￿transmit￿data￿with￿a￿maximum￿length
of￿8￿bytes￿per￿message,￿Ethernet￿messages￿have￿a￿length￿of￿over￿1500￿bytes.￿The￿user￿data￿rate￿thus
increases:￿Considerably￿more￿message￿packages￿can￿be￿transmitted￿in￿a￿shorter￿period.
The￿OABR￿Ethernet￿connection￿equates￿to￿additional￿weight￿and￿cost￿savings,￿due￿to￿only￿using￿2
wires.
Data￿transfer
For￿data￿transfer￿in￿the￿standard￿Ethernet￿100BASE-TX,￿one￿line￿pair￿is￿needed￿for￿transmitting￿data
and￿one￿pair￿for￿receiving.
In￿contrast￿to￿the￿standard￿Ethernet￿described￿above,￿with￿the￿OABR￿Ethernet￿the￿data￿is￿now
transmitted￿with￿one￿line￿pair.￿This￿means￿with￿the￿2-wire￿OABR￿Ethernet,￿a￿simultaneous￿transmit-
receive￿mode￿is￿done￿via￿each￿of￿the￿two￿single￿pair￿cables.￿The￿two￿lines￿transmit￿a￿sum￿signal
differentially.￿If￿a￿line￿is￿interrupted,￿a￿signal￿can￿no￿longer￿flow￿back￿through￿the￿second￿wire￿via
the￿differential￿transmission.￿A￿symmetrical￿transmission￿would￿thus￿no￿longer￿be￿possible,￿the
communication￿via￿the￿data￿bus￿comes￿to￿a￿standstill.
During￿the￿transmission,￿messages￿with￿100 MBit/s￿are￿transmitted￿simultaneously￿in￿both￿directions
via￿the￿lines.￿In￿total,￿this￿gives￿a￿maximum￿transmission￿speed￿of￿200 MBit/s￿of￿data.￿In￿the￿transceiver,
the￿transmitted￿message￿is￿subtracted￿from￿the￿bus￿signal.￿The￿result￿is￿the￿message￿of￿the￿opposite
transceiver.￿A￿schematic￿diagram￿of￿the￿transceiver￿of￿the￿OABR￿Ethernet￿shows￿how￿the￿data
exchange￿takes￿place:
Transceiver￿in￿the￿OABR￿Ethernet￿schematic￿diagram
Index
1
2A,￿2B,￿2C
3
4
5
Explanation
Message￿from￿transceiver￿1
Message￿on￿the￿OABR￿Ethernet
Subtracted￿message￿from￿transceiver￿2
Message￿from￿transceiver￿2
Subtracted￿message￿from￿transceiver￿1
7