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What is the Maximum Cable Length For a CAN Bus?



Hardware: Industrial Communications>>CAN

Problem:
I am building a CAN bus and I need to know the maximum length of the CAN Bus, or the maximum distance between end Nodes. Additionally, what is the maximum distance that any Node can be from the main CAN bus? Lastly, what values should I use for my termination resistors?

Solution:
The CAN bus is designed to be setup using a bus network topology.  You cannot use a star or a ring topology. The CAN specifications limit how far a device can be attached from the bus. Below is an illustration that depicts how the bus should be setup.

Can Bus Configuration Map

If you are using a fault tolerant or low-speed bus it will require you to provide termination at every node. If you have a high-speed bus it will require termination at the two ends of the bus. Generally you would place the Master at one end of the bus (node 1) but It is also possible to have the master connected in the middle with no termination, with termination at the nodes at the end of the bus line.

Can Bus Maximum Length
Bus Speed Bus Length
(L)
 Cable Stub Length (l)
 Node Distance
(d)
1 Mbit/Sec 40 meters
(131 feet)		 0.3 meters
 (1 foot)
 40 meters
(131.2 feet)
500 kbits/Sec 100 meters
(328 feet)		  0.3 meters
 (1 foot)		 100 meters
(328 feet)
100 kbits/Sec 500 meters
(1640 feet)		 0.3 meters
 (1 foot)
500 meters
(1640 feet)
50 kbits/Sec 1000 meters
(3280 feet)		  0.3 meters
 (1 foot)		 1000 meters
(3280 feet)

If a cable stub (un-terminated cable) or T-connector is used to tap into the bus line, then the stub distance should not exceed 0.3m at 1Mbit/s rate per. DS 102 Version 2.0.

Termination Resistors

Termination Resistors are used to impede reflections on the line.  To determine the value of the termination resistors look at the Impedance of the cable and match the resistors to it.  For a normal CAN bus cable with a 120 ohm line impedance you should use 120 ohm resistors. 

For greater understanding of the entire CAN Physical Layer please read this concise comprehensive 15-page paper authored by Texas Instruments: Controller Area Network Physical Layer Requirements. For complete understanding of how National Instrument Products interface with the CAN BUS please consult the NI-CAN Hardware and Software Manual.




Related Links:
Product Manuals: NI-CAN Hardware and Software Manual
White Paper: CAN Physical Layer and Termination Guide

Attachments:





Report Date: 12/01/2011
Last Updated: 06/03/2016
Document ID: 5S0KAAX0

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