Re: Line impedance measuring & matching -- techniques

Hi, 
I once was asked to evaluate if it was possible to use a Industrial Ethernet cable for CAN highspeed. 

So first we have the 120ohm impedance. So this is at a rated Mhz, CAN have them listed for 1Mhz and Ethernet usually for 10 or 100Mhz 

So one need to find out what the Ethernet cable impedance is at 1Mhz. I just did as a article on the Internet said. I put a trim potentiometer in one end of the cable and a Frequency generator in the other end and put in on 1Mhz and square wave. I have a scope on both ends of the cable and trim the potentiometer so I get the best square signal. Then I measure the resistance of the trim potentiometer. This is a very crude way of getting the impedance of the cable. 

Then if one think of impedance problems in networks. We once was issued with finding out problems with a CAN network. That time we saw sporadic error frames on the network. This boiled down to some nodes was only 10cm away from each other. 1Mhz is a high speed signal and one have different impedance in parts of the network. When nodes are to close to each other they can be touch of as being in parallel. Messing up the impedance in that part of the network. 

Ref. 
Minimum distance between CAN nodes 
http://v2.can-newsletter.org/uploads/media/raw/9fa400cd22515f17f2c9732bc52c68fd.pdf . 

Quick & dirty cable length & impedance measurement 
http://www.edn.com/electronics-blogs/benchtalk/4431974/Quick---dirty-cable-length---impedance-measurement 

Br, 

Bertil Bäck 
R&D Manager, Hardware 
+358505886895 

TK Engineering solves your CAN, CANopen, J1939 and NMEA2000 needs by providing the hardware, software and know-how you need to design, manufacture and troubleshoot your industrial- marine- and heavy machine CAN networks. Call +358 6 357 6300 or email [email protected] to learn how we can help you solve your CAN related tasks. 

----- Original Message -----

From: "Dinesh Guleria" <[email protected]> 
Cc: "CANLIST" <[email protected]> 
Sent: Wednesday, February 3, 2016 11:07:44 AM 
Subject: Re: [CANLIST] Line impedance measuring & matching -- techniques 


>> How do you know that the communication problems are due to mismatched impedance? 

I think you did not got from my mail, I am asking case in which one have to investigate what is causing mismatching of impedance on line. 

Then how one use to proceed. 


Regards, 
Dinesh 


On Wed, Feb 3, 2016 at 1:24 PM, John Dammeyer < [email protected] > wrote: 


How do you know that the communication problems are due to mismatched impedance? 
That’s like saying. “I know the bananas aren’t ripe yet because they are green so perhaps that’s why the bread has green mold on it” 

Is this a school class assignment? 
John 

From: [email protected] [mailto: [email protected] ] On Behalf Of Dinesh Guleria 
Sent: February-02-16 11:29 PM 
Cc: CANLIST 
Subject: Re: [CANLIST] Line impedance measuring & matching -- techniques 

Thanks John for your reply. 

>> I’m not sure I understand your question. 
I mean to say that if CAN network is having problem in communication, and i need to investigate what is causing mismatching of impedance. Then for that :-- 
1> I will have to measure the line impedance in loaded & unloaded condition 
2> Then check what is causing impedance mismatch. 
3> correct these factors. 

So how to measure line impedance in loaded & unloaded condition ? 
Also How to correct this ? 

Regards, 
Dinesh 



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