The maximum CAN Cable Length

An interesting discussion from the CANLIST:

mb Moshe Barazani: According to the DS303-1 and the Bosch protocol and ISO 11898, the 1Mbit should be supported for bus length of 30meters. Practically we could reach not more than 10meters. This also stands with some notes that I could locate on the web.

Does anyone have any experience to share in such architecture? Did you use "Repeaters" to extend the bus length? Any suggestion in how can a bus be lengthen to 30meters? Any idea what were the different standrad calculations when allowing 30-40 meters?

hjo: Did you have done any calculations of your overall signal propagation, e.g. adding all relevant delay times transceiver - opto coupler - cable - opto coupler - transceiver ??

As you already said, having all bad components, you will not reach 30m, in the worst case your calculation will end up with a length of minus some meters.

Repeaters will not extend cable length when you stay with 1Mbit/s. They will add additional delay.

sc Steve Corrigan: A 30 meter bus should not be a problem with a 1 Mbps signaling rate as long as the cable is at the very least 120ohm CAT 5 twisted-pair cable. Shielded is even better. 5ns per meter propagation delay is typical for this cable. The down and back max prop delay is then 400ns. Then, if you add a 150ns total loop delay for a sending transceiver and another 150ns for the most distant receiving transceiver (in this case HVD251s), then the total prop delay from controller to controller is now around 700ns. Easily enough time for a correct sampling point. I have operated at 1Mbps on 40m without any problems at all. This is, by the way, the maximum bus length specified by ISO 11898 (1993).

Problems you may be having 40m at 1Mbps on a standard bus is no trick at all, so something is not electrically correct with your bus arrangement. This is how your bus should be set up for high-speed operation.

sc: Stubs too close together increase the capacitive effects of each -- they become a lump sum and damp the effective signaling rate. What isolators are you using? If they're optos you may want to switch to another part like ADI's ADuM1100.

ob Oliver Betz: is it a must to use isolation, or to use a (maybe specific) optocoupler? TI's parts are much faster, consume less power and cost less than fast optocouplers.

mb: taking the worst case scenario according to the manufacturer you might end up with cable with minus length. The problem is that you cannot get the typicall values of the delay and not even some statistics of "how many components might get to the max delay out of 1000"

ob: with fast couplers and transceivers I see no problem to make a 100% design. ADuM1100 has typ. 10.5 and max. 18ns prop. delay. HCPLx710 typ. 20ns, max. 40ns. Two times transceiver delay (each 120ns..175ns). Still enough room for several meters of cable. Regarding the probability: comparing the "worst case" and the "typical" values gives an idea about the distribution. A rectangular distribution would be a rather pessimistic but still acceptable assumption. After all, I wonder in which case there could be a real need for galvanic isolation of such a small bus - different, distant power sources? AFAIK galvanic isolation suppresses only low frequency noise, RF can be filtered better and cheaper.


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