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Application Note

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Potential faults on a CANsystemThe Controller Area Network(CAN) was originally developedby Bosch, Germany specificallyfor the automotive market.Although this is still its primaryapplication area, CAN is alsoideal as a general industrialbus and has found its way intomany applications.

Being a simple two-wire dif-ferential serial bus, CAN sys-tems are ideal for reducingwiring. They offer flexible con-trol on actuators and readout of sensors and, in automotiveapplications, they provide easydiagnosis with a digital tester.However, if communication isnot possible due to a fault inthe bus system itself, debug-ging becomes a problem that

can be solved only with morepowerful diagnostic tools likethe Fluke ScopeMeter 120.

Verifying CAN bussignals with a FlukeScopeMeter ® 120 Series

The CAN (Controller Area Network) 2-wire differential serialbus system cannot easily be debugged with protocol analyzersand digital testers since these look only at the protocol layersof the bus and require the physical layers to be up andrunning before they can be of use. Here’s where the FlukeScopeMeter 120 Series can prove invaluable by allowing you

to take a detailed look at the bus signals themselves to findthe cause of communication problems.

Many faults found in CANbus systems have physicalcauses. Like badly terminatedbusses, poor signal quality,inadequate transmission levels,incorrectly installed cables,faulty connectors, cable routingin high EMC environments andmany others.

With the Fluke 120 Seriesyou can reveal the cause of these problems by looking atthe bus signals, the so calledphysical layer signals.

Viewing CAN signalsThe CAN standard supportshalf-duplex communicationwith only two wires to sendand receive data forming thebus. The nodes have a CANtransceiver and a CAN con-troller for bus access. At both

ends, the bus must be termi-nated with a resistor, typically120 Ω .

Figure 1. Two-wire CAN architecture

The CAN bus signals aredesignated CAN-High andCAN-Low. These 2 wires carryantiphase signals in oppositedirections to minimize noiseinterruption that simultaneouslyinterfere on the bus. The CANbus line can have one of two

logical states: “recessive” and“dominant”. Typically, the volt-age level corresponding torecessive (logical “1”) is 2.5 Vand the levels corresponding todominant (logical “0”) are 3.5 Vfor CAN-High and 1.5 V forCAN-Low. The voltage level onthe CAN bus is recessive whenthe bus is idle.

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2 Fluke Corporation Verifying CAN bus signals with a Fluke ScopeMeter 120 Series

Fluke CorporationPO Box 9090, Everett, WA USA 98206Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The NetherlandsFor more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800) 36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116

Web access: http://www.fluke.com

©2003 Fluke Corporation. All rights reserved.Printed in U.S.A. 12/2003 2126072 A-ENG-N Rev A

Fluke. Keeping your world up and running.

To display the CAN bus sig-nals, connect the A and B inputsof the ScopeMeter to CAN_Hand CAN_L, and the ScopeMeterCOM to signal ground.

To analyze the signals fordisturbance, the instrumentallows zooming in on details bychanging the time base setting.

Figure 3. Easy menu selections for max-peak,min-peak or peak-peak measurements

Figure 4. Changing the time-base allows youto zoom in and analyze signal details

making cursor measurementsand has a 40 MHz bandwidth,providing a 10 ns time-baserange and automatic rise- andfall-time measurements. Thismakes it easy to verify the riseand fall times.

To be able to use the fullsignal span for accurate rise-time measurement, AC couplingis used on the instrument toremove the DC offset, and thetime base is set to 10 ns perdivision, so full signal detailsare revealed.

Figure 5: Fluke 124 measuring rise time automatically

Rise-time measurementTo determine the quality of theedges of the bits, the steepnessis determined by measuring therise and fall times betweenrecessive and dominant levelsand vice versa. These rise andfall times are determined in1/32 nds of the bit time. Rise orfall times of greater than 5/32nds are considered to be a busfault. The Fluke 124, top modelof the 120 Series, is capable of

ConclusionMany faults found in CAN bussystems have physical causes.To find and troubleshoot theseyou need to look at signaldetail. The Fluke 120 Seriesprovides an easy batteryoperated 20 or 40 MHz digitalscope, which allows you tolook as signal details andsimultaneously make fullbandwidth measurements onboth channels.

Figure 2: Fluke ScopeMeter 120 showing aCAN bus data packet. CAN-High on Input B and CAN-Low on Input A.

Peak-to-peak voltagemeasurements

When analyzing the CAN bussignals, it is of interest tomeasure the peak-to-peak volt-ages and to verify that the CANsignals are disturbance free.

The Fluke 120 Series has aneasy menu structure thatallows the peak measurementto be selected. Each inputallows two measurements to bemade simultaneously. Themeter’s unique Connect-and–View™ triggering automaticallygives a stable signal displayeven with complex signals.