Application Note

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Application Note

From the Fluke Digital Library @ www.fluke.com/library

Potential faults on a CAN

system

The Controller Area Network

(CAN) was originally developed

by Bosch, Germany specifically

for the automotive market.

Although that’s still its primary

application area, CAN is also

ideal as a general industrial

bus and has found its way into

many applications.

Being a simple two-wire dif-

ferential serial bus, CAN sys-

tems are ideal for reducing

wiring. They offer flexible con-

trol on actuators and readout of

sensors and, in automotive

applications, they prov

ide easy

diag

nosis w

ith a dig

ital tester

However, if communication

isn

’

t possible due to a fault in

the bus system itself, debug-

ging becomes a problem that

can only b

e solved with more

powerful diagnostic tools like

the Fluke ScopeMeter 120.

Verifying CAN bus

signals with a Fluke

ScopeMeter® 120 Series

Figure 1: Two-wire CAN architecture

The CAN (Controller Area Network) 2-wire differential serial

bus system can’t easily be debugged with protocol analyzers

and digital testers. That’s because most of these tools only

look at the protocol layers of the bus — and they can’t even

be used until the physical layers are up and running. How-

ever, the Fluke ScopeMeter 120 Series allows you to take a

detailed look at the actual bus signals to find the real cause

of communication pr

oblems.

Many faults found in CAN

bus systems have physical

causes, such as badly termi-

nated busses, poor signal qual-

ity, inadequate transmission

levels, incorrectly installed

cables, faulty connectors, cable

routing in high EMC environ-

ments and many others.

With the Fluke 120 Series

you can reveal the cause of

these problems by looking at

the bus signals, the so-called

physical layer signals.

Viewing CAN signals

The CAN standard supports

half-duplex communication

ith only two w

ires to send

and receive data forming the

bus. The nodes have a CAN

transceiver and a CAN con-

troller for bus access. At both

ends, the bus must be termi-

nated w

ith a resistor, typically

120

Ω.

The CAN transmits signals

on the CAN bus, which consists

of a C

N-High and CAN-Low.

These 2 wires carry anti-phase

signals in opposite directions to

minimize noise interruption that

simultaneously interferes on the

bus. The CAN bus line can have

one of two log

ical states:

“recessive” and “dominant”.

Typically, the voltage level

orresponding to recessive

(logical “1”) is 2.5 V and the

levels c

orresponding to domi

nant (logical “0”) are 3.5 V for

CAN-High and 1.5 V for CAN-

Low. The voltage level on the

N bus is rec

essive when the

bus is idle.

Summary of content (2 pages)

PAGE 1
Verifying CAN bus signals with a Fluke ScopeMeter® 120 Series Application Note The CAN (Controller Area Network) 2-wire differential serial bus system can’t easily be debugged with protocol analyzers and digital testers. That’s because most of these tools only look at the protocol layers of the bus — and they can’t even be used until the physical layers are up and running.
PAGE 2
To display the CAN bus signals, connect the A and B inputs of the ScopeMeter to CAN_High and CAN_Low, and the ScopeMeter COM to signal ground. To analyze the signals for disturbance, change the time base setting on the 120 and zoom in on the details. measurement, the instrument uses AC coupling to remove the DC offset. Setting the time base to 10 ns per division ensures full signal details. Figure 2: Fluke ScopeMeter 120 showing a CAN bus data packet.