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 that’s 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 ofsensors and, in automotiveapplications, they provide easydiagnosis with a digital tester.

However, if communicationisn’t possible due to a fault inthe bus system itself, debug-ging becomes a problem thatcan only be solved with morepowerful diagnostic tools likethe Fluke ScopeMeter 120.

Verifying CAN bus signals with a FlukeScopeMeter® 120 Series

Figure 1: Two-wire CAN architecture

The CAN (Controller Area Network) 2-wire differential serialbus system can’t easily be debugged with protocol analyzersand digital testers. That’s because most of these tools onlylook at the protocol layers of the bus — and they can’t evenbe used until the physical layers are up and running. How-ever, the Fluke ScopeMeter 120 Series allows you to take adetailed look at the actual bus signals to find the real causeof communication problems.

Many faults found in CANbus systems have physicalcauses, such as badly termi-nated busses, poor signal qual-ity, inadequate transmissionlevels, incorrectly installedcables, faulty connectors, cablerouting in high EMC environ-ments and many others.

With the Fluke 120 Seriesyou can reveal the cause ofthese 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 bothends, the bus must be termi-nated with a resistor, typically120 Ω.

The CAN transmits signalson the CAN bus, which consistsof a CAN-High and CAN-Low.These 2 wires carry anti-phasesignals in opposite directions tominimize noise interruption thatsimultaneously interferes on thebus. The CAN bus line can haveone of two logical states:“recessive” and “dominant”.

Typically, the voltage levelcorresponding to recessive (logical “1”) is 2.5 V and thelevels corresponding to domi-nant (logical “0”) are 3.5 V forCAN-High and 1.5 V for CAN-Low. The voltage level on theCAN bus is recessive when thebus is idle.

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

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To display the CAN bus signals, connect the A and Binputs of the ScopeMeter toCAN_High and CAN_Low, andthe ScopeMeter COM to signalground.

Peak-to-peak voltagemeasurementsWhen analyzing the CAN bussignals, also take time to meas-ure the peak-to-peak voltagesand to verify that the CAN sig-nals are disturbance free.

The menu structure on theFluke 120 Series makes it easyto select peak measurement.Each input allows for twomeasurements to be madesimultaneously. The meter’sunique Connect-and–View™

triggering automatically gives astable signal display even withcomplex signals.

To analyze the signals fordisturbance, change the timebase setting on the 120 andzoom in on the details.

Rise time measurementDetermining the quality of theedges of the bits involvesdetermining the steepness, bymeasuring the rise and falltimes between recessive anddominant levels and vice versa.

These rise and fall times aredetermined in 1/32-nds of thebit time. Rise or fall times ofgreater than 5/32-nds are con-sidered to be a bus fault.

The Fluke 124, top model ofthe 120 Series, can make cur-sor measurements and has a40 MHz bandwidth, providing a10 ns time base range andautomatic rise and fall timemeasurements. This makes iteasy to verify the rise and falltimes. To enable the full signalspan for accurate rise time

measurement, the instrumentuses AC coupling to remove theDC offset. Setting the time baseto 10 ns per division ensuresfull signal details.

ConclusionMany faults found in CAN bussystems have physical causes.Finding and troubleshooting thereal problem requires examin-ing signal details. The Fluke120 Series provides an easy,battery-operated 20 or 40 MHzdigital scope that allows you to examine signal details whilesimultaneously making fullbandwidth measurements onboth channels.

Figure 2: Fluke ScopeMeter 120 showing aCAN bus data packet. CAN-High on Input Band 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 allowsyou to zoom in and analyze signal details

Figure 5: Fluke 124 measuring rise timeautomatically