Comparison between ASI, Fieldbus and Profibus


Thread Starter


I need some information to compare the mostly used bus-based systems such as ASI, Profibus, Fieldbus.
I would greatly appreciate it if someone could advise me on this matter.

Best regards,
Vahid Zare
Hello Vazid;
It will be difficult to give you a single comparison tool, as you are addressing different types of buses.

For example, As-i and Profibus-DP are what is conmsidered automation buses (for manufacturing industries, digital and analog signals transported as fast as possible between a master (PLC, generally) and remote I/O units. This link will show you a comparison of various industrial buses of this type:

Whereas Fieldbus Foundation and Profibus-PA are FISCO-compliant (explosion-proof buses), slower and more complex networks for the process industry (instrumentation). Both use telegrams that piggy-back on fasster netorks to connect to their controllers: H1 ethernet for FF, Profibus-DP for Profibus-PA.

This link has a good discussion of both instrumentation networks:

For more details on Profibus (DP and PA) look on
For details on FF, look at Emerson's Plantweb University (

Hope this helps,
Daniel Chartier
Mr. Daniel,
Thank you for your concern. Since I can not access to the link(PDF file) which you introduced me,could you mail me it as an attachment. I have some questions.

In fact I do not know that can we say that Fieldbus is transmission technique or a communication protocol?I suppose fieldbus is a technique like 4-20 mA,Hart for signal transmission. My other question is about Modbus or DiviceNet. If we consider fieldbus as general technique what can we say about Modbus or DiviceNet or even ASI.

Once again I thank you for your co-operation. I would greatly appreciate if you or any well experienced expert in this field.

Best regards,
Hello zare;

Difficult to e-mail you a document without having an e-mail address to reach you. If you cannot open suggested files because they are in pdf format, I suggest you go to and download Acrobat reader. It is free and allows you to open pdf files without modifications.

One interesting and important point you make: the distinction between physical layer and protocol. Rs-232 or Ethernet describe physical layers without mentionning the way data is formatted and transmitted. Which is why we must specify Ethernet TCP/IP and Ethernet UDP, for example, when we design a data transmission network.

On the opposite side, a protocol such as Modbus must be specified to the physical layer (Modbus RTU RS-485, Modbus/TCP)for users to understand what they are dealing with, and what physical network must be set up.

To better understand the different components of a protocol, you should spend a little time studying the OSI seven layers model for communication protocols, for example here:

Hope this helps,
Daniel Chartier
Mr. Daniel Chartier,

Please accept my thanks for your reply. My email is vahid_zare @ yahoo. Since I cannot access to the site which you addressed me in your last email, I would be thankful if you could mail me the file via my yahoo email address. Yes, you are right about distinction between protocol and physical layer, I had not concerned it.

Referring to your explanation, could we say that Fieldbus or ASI is a communication protocol? In parallel that I will go through the OSI model, could you advise me how can I find whether something is considered as protocol or a technique which is used in physical layer? Once again I thank you for favour.

Best regards,
Zare, by your definition, what is the difference between "transmission technique" and "communication protocol"? Isn't it the same thing? Or are you distinguishing the physical electrical signal (transmission) from the software procedure (protocol) of the network?

Note that the term "fieldbus" has been confused because many different industrial networks wanted to be "fieldbus". As a result "fieldbus" with lower case 'f' has instead become a generic term for any digital industrial network. Therefore: -HART = fieldbus -PROFIBUS = fieldbus -Modbus = fieldbus -DeviceNet = fieldbus -ASI = fieldbus -Foundation fieldbus = fieldbus or sometimes Fieldbus -Etc.

Terminology has been confused. Note that Foundation fieldbus is sometimes abbreviated as Fieldbus with capital 'F'. Maybe this is what you are confused about.

Of the standard/open bus protocols, HART is the only one superimposed over 4-20 mA analog signal. All the others (Fieldbus, PROFIBUS, Modbus, DeviceNet, and ASI etc.) are purely digital. No analog 4-20 mA.

Some dispute HART is a fieldbus because it uses analog for real-time, not pure digital.

As far as physical signaling technique is concerned, HART uses Bell 202 FSK, PROFIBUS-DP and Modbus/RTU use RS485, Fieldbus uses Manchester Biphase-L encoding with additional requirements for amplitude etc. That is, they are all different resulting in different characteristics: -Speed -Distance -Topology -Bus power -Intrinsic safety -Number of devices -Etc.

As far as software procedure (protocol) is concerned, HART and Modbus are master/slave, PROFIBUS is token passing, Fieldbus is delegated token, etc. That is, they are all different resulting in different characteristics: -Number of masters (including redundancy) -Determinism -Precisely periodic (for PID control and motion control) -Communication relationships (master-slave [client-server], publisher-subscriber [producer-consumer], report distribution [report by exception]) -How data is represented etc.