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Answered

CAN Communication Distance

Joe 2 years ago in IQANdesign • updated by Gustav Widén (System support) 11 months ago 4

What is the maximum distance that the IQAN bus can transmit (in terms of cable length)? I can't find it in the documentation and I have a project that requires a display to be away from a hazardous area.

I have not done much with CAN on IQAN. I have some experience with DeviceNet (which is based on a CAN model).

Simply based on the standard, not specific hardware:


Depends on the operating communication rate. Using an industrial cable with shielded wire (in a static noise free area) you should not attempt more than 100 meters at 500kbps and not more than 200 meters at 250Kbps.

Effective length will be reduced where electric noise is present or wiring is susceptible to outside effects. I think many people use coaxial shielded cable when trying to go far.


Be sure to use correctly placed terminating resistors.

Answered

As Neil wrote, 100 m at 500 kbit/s and 200-250 m at 250kbit/s are in some sources given as a theoretical upper limit for CAN.


In brief, a longer bus has a longer the propagation delay (the time for any two nodes to see the same bit). On a bus with a higher bitrate, there is less time for the bit to propagate before the nodes must sample it.


In the IQAN mounting and maintenance instruction book, we give more conservative estimates on CAN bus length, under the assumption it is a 250 kbit/s bus (expansion module or regular J1939), we are stating 100 m as the maximum, but with 40 m as the recommended length.


In both SAE J1939-15 (Physical Layer, 250 Kbps, Un-Shielded Twisted Pair) and SAE J1939-11 (Physical Layer, 250 Kbps, Twisted Shielded Pair), the maximum backbone length is specified as 40 m. This is based on requirements on cabling as well as on the controllers.


By taking controller specific parameters into consideration, and assuming low cable propagation delay per unit length, we have made calculations showing 150 m as upper length for an MD3-XA2 system, and 190 m for an MD3-MC2 system. But given that the cabling is probably going to be less than ideal, I would not advise designing a system with these lengths.

I am trying to investigate using the new can routing option as a solution to this - can you tell me which modules support CAN routing?

The CAN routing feature was added in IQANdesign 5, it exists on MC4x modules. If splitting the single bus into two, joined by one module routing the messages actually works will depend on how sensitive the higher layer protocol is to latency of the messages.