0
\$\begingroup\$

Fault tolerant CAN (ISO11898-3) is terminated differently than high speed CAN (ISO11898-2). Therefore the signal levels for dominant and especially recessive bits differ.

Newer CAN FD compatible transceivers can have baudrates up to at least 8 MBit/s, but none of their datasheets specify minimal baudrates or compliance with ISO11898-3.

Is it safe to assume they are backwards compatible or should I look for some kind of feature indication in the datasheet?

The only problem I can think of is the dominant timeout function of some transceivers. If the baud rate is too low, it might cause problems with longer strings of dominant bits.

\$\endgroup\$

2 Answers 2

Reset to default
3
\$\begingroup\$

Probably not in any sort of reliable way. As you note, they are terminated differently to give a bipolar differential signal. I have previously tested some Microchip transceivers to see if they can operate like this, but the performance was not good (if I recall correctly, the propagation delays and slew rates were mismatched by a large amount, and extremely dependent on the termination resistance). Additionally, fault tolerant CAN has a threshold voltage of something like 3.2V, while regular CAN is about 1V so your noise performance is going to be severely compromised.

However, the only real way is to test it out, but I doubt the manufacturer is going to guarantee any of the performance when operated in this mode, so it's probably not a good idea.

Also you lose all the fault tolerant (ability to lose one wire) benefits if you're not using proper low speed transceivers, so what's the point then?

\$\endgroup\$
1
\$\begingroup\$

Let us go with TCAN1051H for example:

It is

  • Fault tolerant
  • Flexible data rate supported
  • Meets ISO 11898-2 standard as well

The minimum data rate supported is limited by the Dominant timeout timings. Form the datasheet:
enter image description here
enter image description here

Considering the worst case duration of 3.8 ms, the minimum data rate is limited to about ~3 kbps.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Low Speed Fault Tolerant CAN is a different thing from a fault protected receiver such as the TCAN1051H. It is a different physical interface standard that has the specific ability to work with one of the CAN bus wires out. \$\endgroup\$
    – Jon
    Commented Feb 12, 2020 at 11:20
  • \$\begingroup\$ This answer is wrong. "Fault tolerant CAN"/"lowspeed CAN" is a technical term with a defined meaning, not a feature. The part you linked is for 11898-2 ("high speed" = normal CAN). Fault tolerant CAN is 11898-3. They are different, non-compatible hardware standards. Just as CAN FD is yet another hardware standard. \$\endgroup\$
    – Lundin
    Commented Feb 12, 2020 at 11:41
  • 1
    \$\begingroup\$ This answers the last part of my question, but because of @Jon's answer I doubt the data rate will be the main problem. Having much different signal levels the high speed transceivers probably won't work reliably although they probably will work with low data rates. \$\endgroup\$
    – Karsten
    Commented Feb 12, 2020 at 11:42
  • \$\begingroup\$ @Karsten What does it answer? It's the wrong part. The timing spec linked is about how long a bit train of dominant bits it can handle. It is just a random bunch of digits not related to your problem. They think you are asking if the part can handle both CAN FD and classic CAN 11898-2. \$\endgroup\$
    – Lundin
    Commented Feb 12, 2020 at 11:51

Not the answer you're looking for? Browse other questions tagged or ask your own question.