CAN FD wire length

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CAN FD wire length

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javiervallori
Contributor III

Hi,

in our company are considering to improve our product using CAN FD instead of standart CAN with the LPC546xx microcontroller. But we have a critical doubt about the CAN FD physical layer and its maximal length. The standart CAN specifies:

Bit-RateBus lengthMax. stub lengthAccumulated stub length
1 Mbit/s25 m1,5 m7,5 m
800 kbit/s50 m2,5 m12,5 m
500 kbit/s100 m5,5 m27,5 m
250 kbit/s250 m11 m55 m
125 kbit/s500 m22 m110 m
50 kbit/s1000 m55 m275 m
20 kbit/s2500 m137,5 m687,5 m
10 kbit/s5000 m275 m1375 m

But what about the CAN FD? I guess that the arbitration and control bit should respect the same relation rate/lenght from the table, what happens with the data bytes? They increase the the rate in 1:8, inicially I would understand that we have to follow the table with the new actual rate, but I am not sure on that, because the arbitration timing are diferent than the sampling timings on the data, so the wire length delay could affect less on the data byte.

Does anybody knows any kind of information about it?

Thank you.

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MartinHo
Contributor IV

Hi Javier,

A good source is can-cia ( CAN in Automation (CiA): CAN FD - The basic idea ).

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MartinHo
Contributor IV

Hi,

In theory the wire length limit of CAN-FD should be the same as for "standard" CAN, eg. the Arbitration is done on the "base" data-rate. But i think that in practice in many applications the length needs to be reduced because of ringing (specially when stubs are used).

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javiervallori
Contributor III

I agree with your comment, the arbitration is the same as in standard CAN and on the data (with faster rates) the arbitration is already done. But I would like to know if there are documentation about the ringing throubles, because we use CAN bus in building facilities, with long lenghts, and before to adopt the CAN FD, we need to ensure that the CAN FD lenght specitications match with it.

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xiangjun_rong
NXP TechSupport
NXP TechSupport

Hi, Javier,

Regarding the relationship between the CAN-FD bit rate and CAN-FD wire length, I suppose that CAN protocol provides the specs, we only implements the CAN-FD hardware protocol, I do not think we have the data for the relationship between CAN-FD bit rate and CAN wire length. I suppose that you can check the CAN protocol for the specs.

BR

Xiangjun Rong

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javiervallori
Contributor III

On CAN FD specs I haven't found any kind of reference about. At least on the specs version I have (ver1.0 - 2012).

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xiangjun_rong
NXP TechSupport
NXP TechSupport

Hi, Javier,

It appears that the ISO 11898-2 2015 standard talks about the CAN wire length, pls check.

BR

XiangJun Rong

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