Porsche P-80 2005, Руководство пользователя

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BUS Communications
Climate Control Systems Diagnosis & Repair
Page 6.5
Node 1
CAN High
CAN Low
120
Ω
120
5
4
3
2
1
0
rec
CAN High
Bus Level
(V)
CAN Low
domrect
Ω
Node n
(max. 30)
High Speed CAN
Node 1
CAN Low
CAN High
5
4
3
2
1
0
rec
CAN High
Bus Level
(V)
CAN Low
dom
min. 3.6V
min. 4.8V
max. 1.4V
max.
0.2V
rec t
2.2k
2.2k
Node n
(max. 20)
Low Speed CAN
Bus Logic - High Speed CAN
High Speed CAN control units recognize the two bus signal levels: dominant (zero) and recessive (one). As stated previously, a
dominant signal occurs whenever the unit sends out a "0" (zero) onto the bus. The voltages on the twisted wire pair are then 2
Volts apart (approx. 3.5 Volts on CAN High and 1.5 Volts on CAN Low). Even when another unit sends a recessive "1" (one),
the bus signal remains dominant. The figure below shows a bus signal changing from recessive (both CAN High and CAN Low
values approx. 2.5 Volts), to dominant (CAN High signal approx. 2 Volts higher than CAN Low signal), and back to recessive.
Up to 30 control units can be present on the High Speed CAN network. Both ends of the High Speed CAN bus have 120 ohm
termination resistors to prevent reflections. ISO 11898 is the international standard for high speed CAN in motor vehicles.
Bus Logic - Low Speed CAN
Low Speed CAN control units recognize dominant (zero) and recessive (one) bus signal levels differently than units on High
Speed CAN. Low Speed CAN also works at different voltages than High Speed CAN. The voltages on CAN High and CAN Low
bus wires do not coincide as they do on the High Speed CAN. When a recessive (one) signal is sent by a node, CAN High
voltage is 0.2 Volts and CAN Low voltage is 4.8 Volts. When a dominant (zero) signal is sent, CAN High voltage goes to 3.6
Volts and CAN Low voltage is 1.4 Volts. The difference in voltage between the two wires in the pair goes from approximately
4.6 Volts to 2.2 Volts.