Does all CAN communication come with diagnostics?

小太阳yy

Does all CAN communication come with diagnostics? [Copy link]

 

Do all CAN communications have diagnostics? Are the terminal resistor values different for CAN with and without diagnostics?

freebsder

Both are provided, as specified in the protocol specification. The terminal resistance is 120 ohms.

小太阳yy

freebsder posted on 2022-1-12 13:29 Both are provided, as specified by the protocol specification. The terminal resistance is 120 ohms

I see some information that says the diagnostic terminal node resistance is 30 ohms.

freebsder

Xiaoyangyy posted on 2022-1-12 13:36 Why do I see that the diagnostic terminal node resistance written in some materials is 30 ohms

Post your information, or look at the CAN standard information.

chunyang

The value of the terminal resistance is not determined by the chip, but by the transmission cable. The 120 ohm terminal resistance corresponds to a conventional twisted pair. If a non-conventional twisted pair is used, the terminal resistance must be set differently.

annysky2012

There is no saying that CAN has diagnosis or no diagnosis. CAN bus is a standard communication protocol with a fixed format. Diagnosis and matching resistors have little to do with each other. The role of matching resistors is to improve communication reliability.

小太阳yy

chunyang posted on 2022-1-12 17:25 The value of the terminal resistance is not determined by the chip, but by the transmission cable. The 120 ohm terminal resistance corresponds to a conventional twisted pair. If you are not using a conventional...

If you don't use twisted pair, just use normal copper wire bundles, and choose a different resistor value? What I don't understand is that if you use twisted pair, the length will be different. If the length is different, why do you still need to match 120Ω? Shouldn't the resistor value change?

器铭123

See what is written in the information

chunyang

小太阳yy posted on 2022-1-18 08:43 If you don't use twisted pair, just use normal copper wire harness, and choose another resistor value? What I don't understand is, if you use twisted pair, ...

The impedance matching here is to match the characteristic impedance of the transmission cable under AC, not the DC resistance, so it has nothing to do with the length. The wiring distance on the board cannot be long, so there is no need for matching. The condition for matching is that the transmission distance is greater than 1/10 wavelength, and the wavelength is determined by your data transmission rate, and the highest peak value of the transmission rate is used as the criterion.

小太阳yy

chunyang posted on 2022-1-18 15:28 The impedance matching here refers to the matching of the characteristic impedance of the transmission cable under AC, not the matching of DC resistance, so it has nothing to do with the length. The board trace distance...

Why don't you use regular twisted pair cables? If you use regular wiring harnesses, will the resistance change?

chunyang

小太阳yy posted on 2022-1-18 16:07 What if it is not a regular twisted pair? It is a normal wiring harness. Should the resistance value change?

For an exact match, you need to test or check the cable data. However, in most cases, it doesn't need to be so precise, because you probably won't use the transmission rate and transmission distance to the limit. The closer you are to the limit, the more precise the match is, and the larger the margin, the more tolerant the error is.

小太阳yy

chunyang posted on 2022-1-18 17:02 For an exact match, you need to test or check the cable data. However, in most cases, it does not need to be so precise, because you should not know the transmission rate and transmission distance...

It's a bit profound, I don't understand it

小太阳yy

chunyang posted on 2022-1-18 17:02 For an exact match, you need to test or check the cable data. However, in most cases, it does not need to be so precise, because you should not know the transmission rate and transmission distance...

In simple terms, if I use a normal wiring harness instead of twisted pair, and also use this 120Ω terminal resistor, there is no problem. Is that what you mean?

chunyang

Xiaoyangyy posted on 2022-1-18 18:33 What you said is a bit profound, and I can't quite understand it

It is normal. The knowledge behind these seemingly simple sentences is indeed a bit difficult. It requires sufficient mastery of electronic technology. It is best to learn electromagnetic field theory before understanding it.

chunyang

Xiaoyangyy published on 2022-1-18 18:34 In short, I use ordinary wiring harnesses instead of twisted pair cables, and there is no problem using this 120Ω terminal resistor. Is that what you mean?

Yes. But please note that using untwisted pair cables will increase differential mode interference, especially when the bus length is long, and the transmission rate should not be too fast.

小太阳yy

chunyang posted on 2022-1-18 21:02 Yes. But it should be noted that the use of untwisted pair connection will increase the common mode interference, especially when the bus length is long, and the transmission rate will not be...

What are the general limits of length and speed? As for common mode interference, if it doesn't work, you can consider adding common mode inductors.

chunyang

小太阳yy posted on 2022-1-19 11:01 What are the general limits on length and rate? As for common mode interference, I think if it doesn't work, you can consider adding common mode inductors

Common-mode inductors cannot solve the problem of non-twisted pair cables. When using twisted pair cables, the interference coupling strength on the two cables is equivalent, and the common-mode suppression of the differential receiver can eliminate the interference. For non-twisted pair cables, the interference coupling strength is different, and the common-mode interference becomes differential-mode interference, which the differential receiver cannot do anything about. There is a typo in the above statement. The common mode in "connecting with non-twisted pair cables will increase the common-mode interference" should be "differential mode".

chunyang

小太阳yy posted on 2022-1-19 11:01 What are the general limits on length and rate? As for common mode interference, I think if it doesn't work, you can consider adding common mode inductors

The length and speed are a pair of contradictions. If you want to achieve a transmission distance, you have to reduce the transmission speed. If you want to achieve a transmission speed, you have to sacrifice the transmission distance. There is no fixed limit. It depends on the quality of the cable you use, impedance matching, environmental interference intensity, and even the model of the interface chip and circuit design.

小太阳yy

chunyang posted on 2022-1-19 15:11 The length and rate issues are a pair of contradictions. If you pursue the transmission distance, you have to reduce the transmission rate. If you pursue the transmission rate, you have to sacrifice the transmission distance. The specific limit is...

Sir, I want to ask if the maximum current of 1mm copper 35μ thickness is 1.1A and the ambient temperature is 85℃, will there be any problem with long-term use?

chunyang

小太阳yy posted on 2022-1-20 08:55 Boss, I would like to ask if the maximum current of 1mm copper 35μ thickness is 1.1A and the ambient temperature is 85℃, will there be any problem with long-term use?

A better heat-resistant board base is needed, otherwise it is easy to debond under vibration or stress. Widening it will improve the situation. PCB design in high temperature environment should not only consider the current carrying capacity and the temperature rise of the wiring, but also the material aging problem under long-term high temperature, and the vibration and stress problems cannot be ignored. Go to the manual of epoxy PCB substrate and have a look.