CAN NETWORK FAULT
DETECTION
Mehmet DURMUŞ
Electrified Powertrain Systems
25.04.2024
•
•
•
•
agenda
Example CAN Network Layout
Basic Informations
The Importance of Termination Resistor
Fault Types
2
Example can network layout
3
•
•
•
•
•
•
•
Basıc ınformatıons
Network voltage level is 2,5 V if there is no transmitting message.
CANL-GND = 2,5-1,5 V
CANH-GND=2,5-3,5V (While transmitting)
CANH V > CANL V
Network line should be as short as possible
Termination resistors must be 120 ohm.
Network resistance value is 60 ohm.
CAN Network wires must be twisted.
4
The ımportance of termınatıon resıstor
In the CAN (Controller Area Network) protocol, a 120-ohm termination resistor is used at each end of
the bus to match the characteristic impedance of the network. CAN bus is intended to implemented
by a twisted pair of wires. As speed is increased and the length is enlarged reflection occurs. For
avoiding reflection we use Transmission Line theory. For the High speed CAN bus the maximum
length is 40m, with 1 Mbps speed and maximum of 30 nodes.
So here by using 120 ohms resistor as terminating resistor we can avoid all the reflections easily
without fail.
This helps to minimize signal reflections and ensure signal integrity by preventing the signal from
bouncing back and forth along the bus. The termination resistors also help to reduce electromagnetic
interference (EMI) and radio frequency interference (RFI) in the network. The 120-ohm value is
chosen to match the typical characteristic impedance of the CAN bus, which is usually around 120
ohms.
Twisting helps minimize electromagnetic radiation and resists external interference. It also helps
limit interference with adjacent twisted-pair cables.
5
Fault types
1.
2.
3.
4.
Verifying Termination Resistors
CANH-CANL Wired Backwards
CAN Signal Missing
CAN Signal Shorted
6
Fault types
1.Verifying Termination Resistors
If there is a missing termination resistor, network line total resistance value will be 120 ohm.
Diagnostic:
Cut the line before termination resistors and measure the line.
7
Fault types
2. CANH-CANL Wired Backwards
CANL-GND = 2,5-1,5 V
CANH-GND=2,5-3,5V (While transmitting)
If the connection is reverse, you will measure irrelevant values such as CANH:2,46V
CANL:2,61V
Diagnostic:
Every module provide voltage to the bus. You need to disconnect modules’ connector and try
to finde reverse connection.
8
Fault types
3. CAN Signal Missing
When there is a error regarding to lack of bus comminication/signals, we measure 0 V where
loose connection is.
Diagnostic:
Disconnect connectors to find loose connection.
9
Fault types
3. CAN Line Shorted To Ground
When one of the cable(CANH or CANL) is shorted to ground, we see voltage values around
0.1-0.2 V for both wires. Because they connected each other.
Diagnostic:
Remove terminating resistors to isolate CANH and CANL each other. When we connect
multimeter in voltage measurement mode to CAN lines, we can find which CAN line shorted
problem has.
After that we remove all modules’ connector to find the problem.
Internal resistance of CAN port of the module is 28k ohm. We take resstance measurement
from a modüle CANH to ground and CANL to ground. Values shall be OL if the modüle is okay.
If it’s not okay, resistance value will be lower.
10