Porsche P-80 2005, Руководство пользователя

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Page 6.4
Climate Control Systems Diagnosis & Repair
BUS Communications
On 911 Carrera (997) models, there is a separate
gateway controller.
The buses for both High Speed CAN and Low Speed CAN
are two copper wires twisted together, a "twisted pair".
One of the wires in the twisted pair carries the CAN High
signal, and the other wire in the pair carries the CAN Low
signal. The wires are twisted together to reduce
electromagnetic interference. When the signal in one CAN
bus wire goes high, the signal in the other wire goes low.
In other words, the signals mirror one another.
Oscilloscope traces for CAN High and CAN Low signals.
An actual High Speed CAN bus signal is shown in the
example on the oscilloscope below. Whenever the CAN
High and CAN Low signal voltage values are the same (and
they overlap), this represents a "one" or a recessive signal,
and the signal voltage values in both CAN High and CAN
Low will be approx. 2.5 Volts. When the signal voltage in
one wire goes high, the other will go low, and the
difference in CAN High and CAN Low signal voltages will
always be about 2 Volts. This condition represents a "zero"
or a dominant bit. (Note that voltages on the Low Speed
CAN are a little different, and you will see this shortly.)
High Speed CAN Signals
If electromagnetic interference occurs, the voltage on
both wires (both lines on the oscilloscope) moves up or
down together, so the voltage difference between CAN
High and CAN Low always remains approx. 2 Volts. This is
sometimes called a "differential signal". The signals always
mirror one another, and this guarantees signal integrity.
The trace at the bottom of the screen indicates the logic
input that caused the CAN signals to go high and low.
Network Architecture
In a "Master-Slave" network, a master "node" or electronic
component must coordinate communications. All commu-
nications depends on the availability of this specific
component (i.e., the DME in earlier Porsches).
CAN bus systems in newer Porsche vehicles use a "Multi-
Master" architecture. In this design, ALL controllers have
equal ability, and all controllers (nodes) can access the
bus freely. Therefore, event-oriented communication can
originate from anywhere on the bus. Also, a component
failure does not lead to complete system failure. Since any
control unit can attempt to access the bus at any time,
priority status must be assigned to all messages to
prevent message "collisions". Every message on the bus
has a priority assignment. With these priority assignments,
a dominant signal (critical signal) will be transmitted before
a recessive signal.
As previously stated, the High Speed CAN bus and Low
Speed CAN bus use twisted copper wires. A fiber optic
bus can handle more information at higher transmission
speeds and is used in the entertainment system.
Notes:
CAN High Signal
CAN Low Signal