Porsche P-80 2005, Manual

Page: 93 / 108

BUS Communications
Climate Control Systems Diagnosis & Repair
Page 6.3
CAN Bus Networking
The CAN (Controller Area Network) is a digital bi-directional
communications system used on Porsche vehicles. CAN
allows many control units to send and receive different
information over a single pair of wires (called a bus). Old-
style hard-wired circuits transmit one piece of information
over each wire. When information is needed in more
places, more wires must be added. In a network, informa-
tion can be shared by many units at the same time, and
over the same pair of wires.
Compare the examples in the figures below. In the first,
temperature sensor information is needed by three
different control units, so three different hard-wired
sensors are needed. In the second, the temperature
sensor data is hard-wired to ECM 1 and is made available
to all units on the CAN Bus network. Only one sensor input
is needed.
Advantages of CAN over hard-wiring include:
• Less wiring and fewer connectors and terminations
(less weight and space, lower cost)
• Information from sensors can be shared by all other
control units
• High data transmission rate (up to 100 M baud)
• Diagnostics and communication errors can be managed
better
• High resistance to electromagnetic interference (the
bus wires are twisted)
Without CAN, each control unit needs its own temperature sensor
and related wiring
With CAN, only one control unit needs to receive temperature
input. It processes this information and shares it with all units on
the bus.
CAN communication messages over the bus are digital
signals, a combination or pattern of high and low voltages,
"ones" and "zeros".
Since more than one control unit might try to transmit data
over the network at the same time, the system uses a
message-specific addressing system to rank the data in
order of priority or importance. Each message is coded to
identify its content and priority. More important messages
override less important ones, and control units pay
attention to specific messages that are addressed to
them.
Porsche uses two CAN systems. High Speed CAN (CAN -
C) is used for power train control (engine, transmission,
ABS, etc.). High Speed CAN transmits and receives data
at a baud rate of 500 K bits/second (500 K baud). Low
Speed CAN (CAN-B) is used for body features (comfort
and convenience). Low Speed CAN transmits and receives
data at a baud rate of 83 K bits/second (83 K baud).
Control units called "gateway" units are connected to both
networks and can pass information from one network to
the other. On Cayenne models, the instrument cluster is an
example of a gateway. It can share High Speed CAN infor-
mation such as road speed from the DME with the
alarm/locking system on the Low Speed CAN.
Coolant Sensor 1
Actuator 1 Actuator 2 Actuator 3
ECU 1
(Fuel System)
ECU 2
(Climate Control)
ECU 3
(Transmission)
Coolant Sensor 3 Coolant Sensor 2