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EEVblog 1622 - Don't Make This Multimeter MISTAKE

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EEVblog:
This is the biggest mistake beginners often make when using a multimeter ohms function.
A customer returned an EEVblog BM235 multimeter because it was "Faulty"
Was it? Let's find out what happens when you try and measuse a CAN bus with a multimeter.

derree:
Hi Dave. In one comment you wrote that the customer still insists that the bm 235 is bad and you don't know what you are talking about. I learned that measuring component values in situ is always prone to errors, powering the circuit up for sure doesn't help. Would be nice from the customer to explain here what you are "getting wrong" ...

DimitriP:
For some, the CAN Bus is a can't bus.

Sometimes I say there are people that should never be in the same room with an oscilloscope - at least not alone!
I now realize some should not be touching multimeters either.
And certainly NOT MY CAR.


To summarize:

--- Quote ---CAN Bus Termination

There should be a 120-ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN HI to CAN LOW on a wiring harness you should measure 60 ohms. This measurement should be conducted with the device power off.
--- End quote ---

https://support.enovationcontrols.com/hc/en-us/articles/360038856494-CAN-BUS-Troubleshooting-Guide-with-Video#IntroductiontotheCANBus


In the next episode we might have to go into why when you have a 120ohm termination near EACH end of the bus, how you end up with 60 Ohms .....    :horse:

vk6zgo:

--- Quote from: DimitriP on June 17, 2024, 04:32:49 am ---For some, the CAN Bus is a can't bus.

Sometimes I say there are people that should never be in the same room with an oscilloscope - at least not alone!
I now realize some should not be touching multimeters either.
And certainly NOT MY CAR.


To summarize:

--- Quote ---CAN Bus Termination

There should be a 120-ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN HI to CAN LOW on a wiring harness you should measure 60 ohms. This measurement should be conducted with the device power off.
--- End quote ---
https://support.enovationcontrols.com/hc/en-us/articles/360038856494-CAN-BUS-Troubleshooting-Guide-with-Video#IntroductiontotheCANBus


In the next episode we might have to go into why when you have a 120ohm termination near EACH end of the bus, how you end up with 60 Ohms .....    :horse:

--- End quote ---

That's easy, the multimeter doesn't care----as far as it is concerned, they are in parallel, whereas to the signal things are somewhat different.

quince:
Thanks Dave. Every time I say "don't measure the bus when it's powered" someone replies "but my other meter worked just fine!". Yeah, but not every meter is identical!

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