Canbus fuss

Caution - mild rant ahead:

A note to the industrial networking codifiers of the world - why, oh why, do all of your terminating resistors need to be some hairball value that is impossible to find at Radio Shack?

CanBus - 121 ohms. Not 120 ohms (or better yet, 100 ohms) but 121. Specifcally. With tight tolerances.

It seems like when "they" were designing the network, somebody at the lab could have pointed out how much easier it would be to find 100 ohm resistors, and they could have adjusted the circuit design at that point.

Yes, I know it's several years too late to complain now, but if all I can do about a problem is gripe, then I want to contribute where I can.

Grrrrrr.

TM

Hi,

Don't know where you got the 121 ohm or those tolerance requirements from, but we've probably installed 40+ CANBUS networks over the last 10 years and only ever used 120 ohm terminating resistors (without any problems)

Kevin H

Hi,

Yeh, I know... but then I wouldn't have anything to gripe about

TM

The value of the terminating resistor is a result of the impedance of the cable. As with most things there isn't a black and white cutoff between works and doesn't work.

121ohm is to get the maximum safe output from the network.depending on load and traffic you might not need it.

I am using a can network for learning purposes without any resistors and is working perfectly. however I am not using even 2% bandwidth. I do not know the implications if I were to use 80% what will happen but I will intend to find out.

However, can and all others industrial networks they were developed with a purpose.And people who built them they gave you an idea how to use them. If you like to use 120 or 100 or nothing is your problem, they do not guarantee anymore the network to work within promised parameters.

And if you are designing a industrial network who would care if your resistor would cost 10c(Radio Shack), or 5$(Allen bradley)??

So test your network with a low tolerance 120ohm you will see it will do just fine.

Uh, I think that you may be mistaken. The value of the terminating resistor(s) has nothing to do with the amount of traffic on the network. It is there to correctly terminate the line with the charateristic impendance of the cable, without which, reflections caused by a high impedance would cause data errors. That network that you are using probably is very short in length with respect to the wavelengths of the signals on the wire. Drop in about a couple hundred feet of cable, depending on the data rate, but keep the same network layout and traffic patterns and see if it continues to work without the termination resistors. The effect of termination resistors can be easily seen on transmission lines with a scope.