Application Note

2 Fluke Corporation Verifying CAN bus signals with a Fluke ScopeMeter
®
120 Series
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
F
or more information call:
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Fax [phone]
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-
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1 40) 2 6
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ax (9
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From other countries [phone] or
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©2004 Fluke Corporation. All rights reserved.
Printed in U.S.A. 8/2004 2392165 A-US-N Rev A
Fluke. K
e
eping your world
up and running.
To display the CAN bus
signals, connect the A and B
inputs of the ScopeMeter to
CAN_High and CAN_Low, and
the Sc
opeMeter COM to signal
g
round.
Peak-to-peak voltage
measurements
When analyzing the CAN bus
signals, also take time to meas-
ure the peak-to-peak voltages
and to verify that the CAN sig-
nals are disturbance free.
The menu structure on the
Fluke 120 Series makes it easy
to select peak measurement.
Each input allows for two
measurements to be made
simultaneously. The meter’s
unique Connect-and–View™
triggering automatically gives a
stable signal display even with
complex signals.
To analyze the signals for
disturbance, change the time
base setting on the 120 and
zoom in on the details.
Rise time measurement
Determining the quality of the
edges of the bits involves
determining the steepness, by
measuring the rise and fall
times between recessive and
dominant levels and v
ice versa.
These rise and fall times are
determined in 1
/32-nds of the
bit time. Rise or fall times of
greater than 5/32-nds are con-
sidered to be a bus fault.
The Fluke 1
24, top model of
the 120 Series, can make cur-
sor measurements and has a
40 MHz bandwidth, providing a
10 ns time base range and
automatic rise and fall time
measurements
. This makes it
easy to verify the rise and fall
times. To enable the full signal
span for ac
curate rise time
measurement, the instrument
uses AC coupling to remove the
DC offset. Setting the time base
to 10 ns per division ensures
full signal details.
Conclusion
Many faults found in CAN bus
systems have physical causes.
Finding and troubleshooting the
real problem requires examin-
ing signal details. The Fluke
120 Series provides an easy,
battery-operated 20 or 40 MHz
digital scope that allows you to
examine signal details while
simultaneously making full
bandwidth measurements on
both channels.
Figure 2: Fluke ScopeMeter 120 showing a
CAN bus data packet. CAN-High on Input B
and CAN-Low on Input A
Figure 3: Easy menu selections for max-peak,
min-peak or peak-peak measurements
Figure 4: Changing the time base allows
you to zoom in and analyze signal details
Figure 5: Fluke 124 measuring rise time
automatically