CAN bus backbone vs daisy chaining
This thread is a spinoff from the below thread since it is departing from the original topic of that thread.
http://www.chiefdelphi.com/forums/sh...87#post1121087
Quote:
Originally Posted by EricVanWyk
(Post 1121087)
I'm curious how the CAN hub is holding up during testing. It is an interesting tradeoff between cable reliability and signal reflection. How many places are you terminating? What are your cable lengths?
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The four-port hub is used in our test bot for a while and it doesn't seem to give us any trouble. Regarding termination, the whole backbone is terminated in two places. One is built-in to the 2CAN, the other is the 100-ohm resister I soldered at the end of the bus. If a port is unused, it is not terminated either. Each port is connected to a Jaguar. There is no termination on the other port of the Jaguar. At first I was thinking of terminating it but then I considered that the two ports on the Jaguar are probably a passive connection. If that's the case, terminating each Jaguar would lower the impedance of the bus. So I decided I don't have to. Let me know if I am wrong. The overall length of the backbone is really short, probably within 2 to 3 feet including the pig-tail that plugs into the 2CAN. The cable length from the hub to each Jaguar depends on how far the Jaguar is from the hub. Typically, it's about 1 to 2 feet.
Quote:
Originally Posted by EricVanWyk
(Post 1121087)
If I remember my CAN math correctly (and honestly it has been >2 years since I've done any form of line reflection analysis), at 1 megabit those taps should be limited to a third of a meter. But I think that was assuming an electrically significant distance between the taps.
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It's been over a few decades for me, so I can't remember any of the wave reflection math I've learned :). To make things worse, I am now in the software industry. So I am really rusty on this. But my impression is that since the cable length is really short and the CAN bus is running only 1Mbps, so if there is any reflection, it would be electrically insignificant. Correct me if I am way off :)
Quote:
Originally Posted by EricVanWyk
(Post 1121087)
Could you scope the bus and see what the reflections look like? If it works, this could be really handy.
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I will when I have some time. This is very interesting. |
