CAN databus cable/connector specifications

[tomleijen]

I am looking at setting up a 1Mb/s CAN databus for an industrial application. Distance between the nodes on the bus will range between 10cm to 10m.
What are the main things to look out for when choosing a cable for a CAN bus?
The distance between the nodes will vary from directly adjacent to eachother to up to 10m apart, I am worried that this could cause issues such as propagation delays and reflections etc, is this much of an issue with CAN?

Ideally I would want to be able to use standard CAT-5 or CAT-6 cabling with RJ45 connectors, is it possible to reach the 1Mb/s with this type of cable? Or is there similar cable out there which can? I want to avoid using the really thick DeviceNet style cabling.

 

[itsmoked]

1Mb/s those distances.. Not a problem. CAT5 does 10Mb/second tied in a knot.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[tomleijen]

From what I have found through research is that Impedence is quite a big issue when it comes to communication speed and minimising reflections, is this correct?
Because the databus will be used for an industrial application I have been looking at some DeviceNet specifications which say that I need to use cable with an impedence of 120 ohms (or 108 to 133 ohms), is this related to communication speeds or is this only specified because DeviceNet uses 120 ohm termination resistances?
Does this mean that I can run CAN over CAT5 as long as I use 100 ohm termination resistances?
I am looking at running the CAN signals through the circuit boards of the nodes (i.e. the T is built into the node) am I right in saying that I need to match the impedence (or thickness) of my tracks to the Impedence (or diameter) of the cable?

 

[itsmoked]

The characteristic impedance of typical twisted pair is 120 ohms so that's why we terminate with 120.

Generally you don't look for 120 Ohm cable unless you are running RF and matching is even more important. Instead what you want is cable with low capacitance. Capacitance is what reduces the speed that signals can change at. Why? Because each time the signal reverses state the line has to charged or discharged and as you know the voltage across a cap can't change instantaneously.

But I digress, don't sweat 1Mb/s for 10 meters. Try it with garbage found-it-in-a-drawer CAT-5. If you unexpectedly have issues then look for lower capacitance CAT-5 cable.

Keith Cress
kcress -

http://www.flaminsystems.com