Modbus, Profibus, OPC

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Modbus, Profibus, OPCPosted by Anonymous on 15 August, 2006 - 12:53 am

I would like to know the difference between MODBUS, PROFIBUS and OPC and area of application.

Posted by Gerhard Burle on 17 August, 2006 - 1:17 am

MODBUS and PROFIBUS are fieldbusses.

In wikipedia

http://en.wikipedia.org/wiki/Fieldbus

you'll find an explanation and additional

links to MODBUS and PROFIBUS.

OPCis an international standard for

excange data between devices of different

suppliers:

http://en.wikipedia.org/wiki/OLE_for_process_control

Best regards

Gerhardhttp://www.deltalogic.biz

Posted by Jonas Berge - F on 28 August, 2006 - 11:23 pm

OPC: communicates software-to-software for operator display Modbus/Profibus: communicates hardware-to-

hardware for control

OPC is principally a standard software interface between a driver (mainly for device hardware and networks, bu

also a databases) and a display application such as HMI/process visualization software. However, there are

hundreds of applications from lots of suppliers performing different functions that are based on OPC. The beauty

is that you can easily plug these application together with all kinds of hardware. However, OPC is typically not

used in closed loop control.

OPC has a few sub-types:

*OPC-DA for live data access

*OPC-A&E for live alarm propagation

*OPC-HDA for historical data

*OPC-XML-DA for live data access across the Internet
http://en.wikipedia.org/wiki/OLE_for_process_controlhttp://www.deltalogic.biz/http://en.wikipedia.org/wiki/OLE_for_process_controlhttp://en.wikipedia.org/wiki/Fieldbus


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*and a few more

For closed loop control you would use either Profibus, Modbus, or some other fieldbus such as Foundation

fieldbus.

Profibus and Modbus are quite similar on the surface but there are some differences under the hood. Both come

from the PLC-world with Profibus

originating from Siemens and Modbus originating from Modicon.

Both use RS-485 media although Modbus can also use RS-232. However, due to timing requirements Profibus

require dedicated interface cards while Modbus can use the standard PC serial port and RS-232/RS-485

converters.

Both are master-slave protocols although Profibus makes a distinction between cyclic and acyclic commands. In

Profibus the cyclic commands are repeated over-and-over used for real-time I/O where read and write is done b

a single command on an ad-hoc basis. Non-real-time communication like configuration and diagnostics is done

using acyclic commands.

Both of them have a few sub-types:

*Modbus/ASCII, Modbus/RTU, ModbusPlus, Modbus/TCP*Profibus-FMS, Profibus-DP, Profibus-DPv1, Profibus-PA, Profibus-DPv2.

Profibus-DPv2 is isochronous - all the other Profibus versions and Modbus are not. I have a paper explaining thi

term if you are unsure. Contact me directly on address below.

Profibus-PA does not use RS-485, it uses IEC 61158-2 type 1.

Profibus-FMS and ModbusPlus have peer-to-peer commmunication, the others do not.

Profibus and Modbus have a similar scheme to access data based on (input, output, holding, and ) registers forModbus and Slot+Index for Profibus.

Essentially both are single master protocols although Profibus has a scheme for token rotation between masters.

Not sure if that feature is used much.

For both Profibus and Modbus you set the address manually.

Profibus also has an optional safety layer that permits it to be used within safety instrumented systems.

To learn about all flavours of OPC and how to use it, take a look at the book "Software for Automation:

Architecture, Integration, and Security". Preview, see contents, and buy online:www.isa.org/autosoftware

To learn about Profibus-PA take a look at the yellow book "Fieldbuses for Process Control: Engineering,

Operation, and Maintenance" buy online:

http://www.isa.org/fieldbuses

Jonas

[email protected]
http://www.isa.org/fieldbuses


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Posted by Tracy Lenz on 18 August, 2006 - 12:13 am

Modbus is a legacy protocol and has been around for many years. It is all software driven and does not require an

scanner cards, Just RS-232 or RS-485. Many companies make software drivers for Modbus or if you are good you

can write your own driver.

Profibus has been around for many years but not as long as Modbus. This is a good protocol but does require ascanner card, GSD file, and configuratin software. This can have a cost to build a system.

OPC is interface software that converts tag names to talk a protocol,Address or to a scanner card. Example: HMI

software Tag-name, OPC-Tag-Name-to-Address, Address to Protocol or Scanner. OPC can make life easy to

interface.

Posted by Rondrob on 18 August, 2006 - 1:21 pm

You can more information at http://www.opcfoundation.org for OPC and http://www.profibus.com

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