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help:fieldbud technology

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rectifier

New Member
hello everybody

this is my first post here ,, I dont know if this is the right place to put my question

First of all ,, I'd like to thank everyone in this forum ,, this place is very interesting and I got benifit from it,,

I'm about doing a research about ((fieldbus technology))
I searched in the net ,, and all the informations were the same ,, I couldnt find a good book in library ,,

What I understood from the websites ,, that fieldbus technology ,, is a new tech. that is going to replace ((4-20 mA analog signal))

Am I correct ?

thanks for all of you ,, I appreciate your help
 

mechie

New Member
Fieldbus and Profibus

Hi rectifier...

Fieldbus is advertised by fieldbus people as the future in industrial instrumentation, Profibus people would argue that profibus is better and modbus people will say their system is best ...

As I understand it all these (and more) systems are competing for the same market and none has a big advantage over the others. They are all just re-inventing some form of industrial control network, RS485 could be used just as well if you could get the instruments and control elements to understand it. This is probably the big downfall of these sytems, some instrument manufacturers can afford to develop interfaces for several 'systems' (and pay royalties for using the names!) but others simply choose their favourite, make all thier stuff work with it and so end up 'tied in' to a standard, fieldbus, profibus, modbus, whatever. This makes mixing different manufacturers' instruments difficult or impossible - one insisting on modbus, the other insisting on controlnet. You end up being limited to a subset of the instrument manufacturers you used to enjoy!

These 'standards' have been annoying me since the mid 80's and I am still not convinced that they are a good idea for large sites. The common theme is that a twisted pair of wires can act as a network, connecting to a hundred instruments and valves, each having a unique address. Any setting up can be done from a central control point (re-ranging transmitters or tuning valve controllers), it's all just serial data over this local area network. It saves a fortune on cable as a bit of telephone wire carries everything.
[Ever heard of Z domain? sampled data instead of S domain and its continuous, proper data ?]

If the wire gets broken everything DIES - instantly!
There are ideas that a valve could have its own processor running a control program that can run independantly of the main control room if the cable breaks, even still talking to local transmitters on the same fragment of cable but this requires careful cable routing and intelligent programming to predict likely failures and strategies to cope.

With 4 to 20mA loops every transmitter has a pair of wires back to the control room (SCADA, DCS, whatever) as does every valve, injector, everything. That pair can carry only the one signal (unless you start to look at HART communications- serial data superimposed over the current loop) so more cable is required - lots of big, heavy multicores - and lots of junction boxes. Big, fat multicore cables are more self-protecting than a tiny telephone cable, scaffolder's boots don't snap them and laggers can't cut bits off them to secure lagging mats :wink:

Take a look at ...
http://www.synergetic.com/compare.htm

I don't think fieldbus will replace 4-20mA, it is just an alternative which will suit some applications and not others.
Hope that helps --- just my point of view.
 
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