I'm writing my dissertation over Industrial Networkd and I'd like to talk about the regular devices used in networks, so if anybody could just make a review and list them for my checkings I would be glad. And also I'm in doubt about the usage of terminators: why should we use it? Do we need in each terminal of the cable? Do we need to use terminators in spurs?
Thanks a lot!

 

The electric cable has a impedance. If there is no resistance with that value at the end of the line the electrical wave will be reflected. Thus the information will be disturbed.

Compare it with waves of the sea than run onto a beach or onto a wall in a harbour. If they run onto a wall the water will also be "refelected".

I found something in english
http://atw.huebsch.at/Tipps_Tricks/Terminator.htm

but the german text here is better
http://de.wikipedia.org/wiki/Wellenimpedanz

 

If you are writing a dissertation on the subject, shouldn't you be telling us why you use terminators on controlled impedance networks?

Regards
cww

 

Cww, I'm just discrbing the components of an industrial network, and I found some trouble on justificating and describing the usage of the terminators, wich is regarded to the signal reflexion avoidance at the bus endings. But I ain't got those information from the field, and I'd like to hear some professional opinion about the topic and about networks as well, in general.

But I can't see the point of your question at all, hope to hear it from you.

All the Best
Felipe

 

Right, pvb, I appreciate your help. I got the point over the reflections on a barrier, but what would be the barrier on a bus ending with no terminator? I mean, this is the problem about not having seen this before, I cannot conceive a bus ending differently than a cable cutted, what gives me the impression that nothing will reflect back the signal (unless the is a metallic cover the, or sometinhg like that).

So, can you also tell me wheter we need a terminator at all the spurs or only at the trunk? And is it neccessary to be place only 1 per segment (considering the other extremity is linked to something else)?

Thank you very much!
Felipe

 

A good starting point is the wikpedia entry:
http://en.wikipedia.org/wiki/Transmission_line

While this article refers to maximum power transfer, the matched impedance condition also means that the line length is not significant (other than for line losses). Note the special cases of 1/2 wavelentgh and 1/4 wavelength (and 1/2 wavelength multiples of those) where it is possible to ignore the terminations or characteristic impedance (this is used to construct antennas, 1/4 wavelength is desirable for the most efficent radiation).

The "short" and "open" load ends up having the input impedance be purely imaginary, thus causing the "reflection" of the wave back up the line. When you think of a wave at the seashore hitting a wall, it is the same result whether it is an open or short as far as transmission lines are concerned.

This topic takes me back to my college days (many years ago). The only class that I had to take a second time was transmission line theory. The professor who taught the class also wrote the texbook. He has forgotten more than I have even known on this topic but he made it sound so simple. After putting in the requsite effort to learn, I "got it" but only after learning a tough lesson. I haven't done that again!

 

If you can afford buying an old copy of my instructor's book, do so. You can find some offered at Amazon at:

http://www.amazon.com/Transmission-Schaums-Outline-Robert-Chipman/dp/0070107475

When I checked there were 7 used copies being offered starting at $28 US. Reading the comments they re-enforce my views; Dr. Chipman simply described a complex subject in very understandable language. He was selected as Professor of the Year at the University of Toledo while was there - that is how the students viewed his abilities to instruct us. Winning that award is tough anyway, but an instructor in the Engineering school? It was almost impossible (due to the small number of students vs. the rest of the campus).

Regards,
Russ

 

For most "fieldbuses" the terminator is NOT there to avoid reflections. Reflections start becoming a problem at a few hundred kbit/s. Most Fieldbuses run slower than that.

In the case of Foundation fieldbus H1 running at 31.25 kbit/s, the purpose of the terminator is as a shunt. FF devices communicate by transmitting current but receive voltage. The terminator main purpose is to convert 20 mAp-p current from devices transmitting to a 1 Vp-p voltage to devices receiving the communication.

To learn more take a look at chapter 11 of the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance." buy online: http://www.isa.org/fieldbuses

Cheers,
Jonas

 

You also have to qualify that with:
At the line lengths used with these fieldbuses.

Regards
cww