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Using music to understand CAN & vise/versa
Posted to Technical Theory Forum on 8/27/2011 17 Replies

Over the past few months, there have been several very interesting threads on CAN that's really pegged my interest. CAN is an extremely reliable communication network. One reason is it's resistance to interference due to using the voltage differential between the High & Low CAN lines. Here is a MS CAN capture I took on an 08 F450 with no known issues:

[2008 Ford F-450 Super Duty XL, ECM/Inputs/Outputs Waveform]

In that capture, Channel 1 was on Pin 3 of the DLC and channel 2 was on Pin 11 of the DLC. My ground was a chassis ground, Pin 5. The purple trace is a math channel within PICO; Channel 1 voltage (-) Channel 2 voltage or the voltage differential. I thought it was very interesting how clean the signal is now.

Here is another capture I took with the Pos lead on Pin 3 & the Neg lead on Pin 11:

[2008 Ford F-450 Super Duty XL, ECM/Inputs/Outputs Waveform]

BTW, the tall peak at the end is an acknowledgement bit. The sending module with do a check sum for the data field. The receiving module(s) will also do a check sum and if the check sums agree, there will be a dominant "ACK" bit.

This is my theory about the very brief peaks at every rising edge. I believe this is the modules doing bit-by-bit monitoring. Each module will receive the data at virtually the same time. Each module is expecting a certain voltage level. If the modules receive the expected voltage level, they will bring the voltage down to the where it flattens off. The peak is also about 160 ns.

So where does the music come in. I love music. I deal with live sound with our church, I have an amateur recording setup, and I love to play. Today, I had some time to play around. Most everyone has seen an XLR cable (aka: microphone cable). This is a 3 conductor cable capable of carrying a balance signal. I couldn't wrap my head around what a balance signal was, so I asked someone about it a couple of years ago. He said a balanced signal is carrying 2 identical signals of the sound source, except one is 180 degrees out of phase. That way, any interference will be blocked out. That sounded pretty cool and smart, but then I learned things about CAN (mainly voltage differentials). The 180 degree thing started not to make sense.

My feelings is that the other signal was not 180 out of phase, but simply inverted. I have a PICO, a dynamic microphone, and a dirt cheap XLR cable that I didn't care to cut up:

[Test leads hooked to a cut XLR cable]

[Test leads hooked to a cut XLR cable]

This is what I got.

[Test leads hooked to a cut XLR cable]

I got the pitch of A below middle C (which is 440 hz at standard pitch; aka A440) in my head and hummed it into the microphone. My frequency matched (I was on key!) and sure enough, the signal is inverted.

I thought this was very interesting, my wife wasn't as thrilled :)

BTW, some of the info on CAN came from this book:[...]

It's a very inexpensive book with tons of info on how CAN operates. It's not that bad of a read either. CAN is used in a ton of applications: aerospace, space craft, appliances, etc. One thing to keep in mind, SAE J2284 defines the CAN standard for automotive. Other applications have their own standards. All are designed/built off the CAN standard. There are differences in how each application does things. So if you read something about CIA (CAN in Automation), it my disagree with what you know about automotive CAN.

Feel free to correct anything here, I'm still learning. I hope we all can learn something here.

Robby from Alabama

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