Measure CAN bus bit pulse length

[qwaspen]

Is there a way to measure the CAN bus bit pulse length with for example an oscilloscope, without affecting the can bus?

 

[IRstuff]

Why would the oscilloscope affect the bus?

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[qwaspen]

Because of the 120 Ohm termination of the CAN bus. If I connect an oscilloscope with for example a 50 Ohm input impedance, won’t the bus be affected?

 

[IRstuff]

Depends on what you are trying to see. Most oscilloscopes can disconnect their 50-ohm terminators, and you can add an external compensator of your own.

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[kontiki99]

I don't know much about the aviation CAN bus, but if it's anything like the 429 bus, make sure all the intended LRU's are connected when you make your measurements. Long wire runs, if left unterminated at one of the equipment trays, can add a capacitive load that distorts the signal wave forms.

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[qwaspen]

@IRstuff what do you mean by “external compensator”?

 

[IRstuff]

You can put the oscilloscope into high impedance mode and use something like this:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=7312.

Or, you can wire your oscilloscope directly to something like this:

https://www.amazon.com/Grid-Connect-GC-CAN-TERM-GC-CAN-Terminator/dp/B078WHM3CP?th=1.

However, as a general rule you can simply put your oscilloscope into high impedance mode and live with the ringing, since CAN bus isn't a particularly high frequency architecture.

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[WKTaylor]

qwaspen ...

Your question is odd... all CAN's are supposed to have a diagnostic system [connector, test equipment, software, etc] which should be [made] capable of answering Your basic question.

NOTE1. The ISO and SAE... and to a lesser degree ARINC... are authoritative sources of info for all things CAN. Over the years, I've found listed MANY documents on this subject... guaranteed this issue has been addressed/defined by SAE.

NOTE2 to A/L... An advisory 'warning' for You. ANYTIME a technically unique abbreviation is used, there should always be a clear definition for that abbreviation so it can never be mistaken for any other 'similar-to' abbreviation. Also IF You are referring to a particular item or system, then that should also be stated explicitly.

PLEASE ADVISE>>> In this case, A/L, are you discussing =/similar-to ARINC 825* GENERAL STANDARDIZATION OF CAN (CONTROLLER AREA NETWORK) BUS PROTOCOL FOR AIRBORNE USE … ??? or SAE xxxxxxxxxxx???

Regards, Wil Taylor
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