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Comparasion between Modbus Plus and Profibus?
Can any define and Give a detailed comparasion between Modbus plus and Profibus. Which one is suitable in which conditions? which one is better?

Can any define and Give a detailed comparasion between Modbus plus and Profibus.

Which one is suitable in which conditions?which one is better?

Is there any other field bus which is better than these two?

I hope you will guide trully.

Asim

The question is as long as a piece of string

Profibus DP (my preference over DeviceNet and Interbus-S)
- is for I/O and a little Data but complexed
- practicaly 1.5M, 2,500M ish, don't use 12M only 400M and very fussy on wiring & earthing
- many I/O devices but costly
- large number of gateways to other networks
- complications now with version 2 devices
Modbus Plus
- is for I/O and Data both simple
- 1M, 2,500M ish
- limited I/O devices and costly
- large number of gateways to other networks
- same version from the year dot (1972 I think)

Give me some more info, what do you want to do?

PS I am a Certified Profibus Engineer so nearly impartial!

By James Ingraham on 5 August, 2002 - 12:58 pm

> Can any define and Give a detailed comparasion
> between Modbus plus and Profibus.

I won't give detailed, but here are some highlights (thanks to Synegetics Fieldbus Comparison Page at "http://www.synergetic.com/compare.htm":http://www.synergetic.com/compare.htm )

Profibus::

Topology: Line, star & ring
Media: Twisted-pair or fiber
Max Devices: 127 nodes (124 slaves - 4 seg, 3 rptrs) + 3 masters
Max Distance: 100m between segments @ 12Mbaud; 24 Km (fiber) (baudrate and media dependent)
Comm. Type: Master/slave
Data Rate: 12Mbit/sec
Data Transfer Size: 0-244 bytes
Arbitration Metnod: Token Passing

Modbus Plus::

Topology: Linear
Media: Twisted Pair
Max Devices: 32 nodes per segment, 64 max
Max Distance: 500m per segment
Comm. Type: Peer to Peer
Data Rate: 1 Mbit/sec
Data Transfer Size: variable
Arbitration Metnod: ??


> Which one is suitable in which conditions? which
one is better? <

If you ever ask "Which fieldbus is better?" you've already lost. The question is, "Which fieldbus best fits this given scenario?"

As for Modbus Plus vs. Profibus specifically, it goes like this. Profibus is designed for one master to control many slaves in real-time, such as remote blocks of I/O, drives, encoders, etc. You can't connect a PLC to Profibus and expect to be able to program it over the network. And while you can connect Profibs Masters together, it is only a limited fashion. If you need to master to communicate arbitrary data and/or unsolicited messages, Modbus Plus is the answer. If you want to view/change ladder logic over the network, Modbus Plus is the answer.

It would hardly be unusual to have a PLC with Modbus Plus communication for engineers and maintenance to view the ladder, and Profibus for controlling the I/O.

Modbus Plus can in theory do the job of Profibus, but at 1/12th the speed. Plus, bandwidth gets soaked up if you're trying to do the I/O control and master-to-master comm and programming all over the Modbus Plus link.

> Is there any other field bus which is better than these two? <

Here we go with "better" again.

ControlNet is nice because it has the best of both worlds from Profibus and Modbus Plus; you can program over it, you can schedule real-time data to control I/O from a master, you can do Master-to-master scheduled communication, you can do unsolicited messaging. It's also nice because, unlike Modbus Plus and Profibus, you can send meaningful data structures instead of just a bag of bits. But ControlNet is double or triple the cost of Modbus Plus or Profibus, has limited support in the market place, has a difficult "scheduling" procedure, and the coaxial cable is mildly more difficult to work with.

DeviceNet is incredibly popular in the US, so even though Profibus is technically superior in every way, it may well be a better marketing decision to use DeviceNet.

We can go on like this for hours.

-James Ingraham
Sage Automation, Inc.

James-
You might want to rethink your programming answer- I program S7 PLC's over profibus every day. I also download my Protool HMI programs and drive configurations. I get 12 meg speeds too.

But you are right- The wiring cant be tossed together.

By Michael T Mellish on 14 August, 2002 - 4:19 pm

MODBUS PLUS: A peer to peer network designed to provide mapped communications (like I/O or Drives) replacing hardwired links, as well as Async communications such as event based occurances, HMI to PLC, and PLC to PLC. In otherwords the network handles both Fieldbus & Intelligent Device Communications requirements. Modbus Plus is available for communications via Copper wire OR Fiber Optics with a variety of specialized options in between (Radio, T1 etc). So information like distance is really difficult to quantify. For example an Airport Runway lighting system in Denver deployed an 80,000 ft fiber optic Modbus Plus network. Tank farm management system for NATO & certain Middle Eastern countries cover more than 25 miles.
MB+ can operate in both dual cable & redundant interface modes with transparency to the user application.MB+ is very simple to install and provides a lifetime of service with minimal maintence. It does not require "Certified" engineers to design or implement networks.

Likewise the number of nodes is very misleading. A segment of a MB+ network can have 64 nodes maximum. However each segment can connect to four additional segements. If you build a ring of 60 network briges (i.e. four "global hosts") they can reach more than 5000 MB+ networked devices. (One of the largest such network systems is located in Windsor Canada with some 4500 nodes of PCs, HMI, PLCs, Servos all interconnected). On the downside, Modbus Plus is a proprietary, licensed technology so if you or a supplier of your requires a MB+ inteface to be custom created, this may be difficult.

PROFIBUS is a field bus network. It is primarily designed for a SINGLE master device to communicate with "Slave" devices per establish mapped data for that device. Each device on the network has a device description file which predefines the data & format which can be read and written to the device. It is not designed for unsolicted programmed events to be transferred from a device to the master. The profibus standards organization provides a public domain to access designs & standards and construct interfaces to allow any vendor to build a profibus "slave" device (note that head end devices are not so easy to create). As such IF you application is the connection of I/O and I/O like devices, Profibus may offer the more open & available option to complete your project.

IF you require programming, multiple masters, unsolicated writes from devices etc., than MB+ offers the easiest option.

You also ask what other options might be available. THE MOST OPEN & AVAILABLE network is and for the forseeable future will remain ETHERNET. It is the oldest of the three (1980) and has by far more nodes produced new each year than the entire history of shipments of Profibus & Modbus Plus combined. As such the price is cheaper, and due to volume continues to get cheaper. It is now virtually standard on all Personal Computers, and an option for nearly all current PLC models. It can provide speeds of 100 megabits (better than either Modbus Plus or Profibus), virtually any media (including wireless), any distance, unlimited number of interconnected nodes, networks & devices. Studies by EPRI (Electric Power Research Institute) have shown that the EMI/RFI characteristics are as good or better than MB+ & Profibus. Data Determinism can exceed the performance on MB+ & Profibus. And now there is even a TUV approved "Safe" Ethernet for critical safety functions. In short, it is the network of today and the future. (see the IEEE announcement on 10 gigabit Ethernet and you know the performance just keeps getting better). Neither Profibus or MB+ have a 10 gigabit option...

By Jonas Berge on 23 August, 2002 - 1:55 pm

Keep in mind though that Ethernet is not a complete protocol. Ethernet is a bit like RS232 and RS485 in the sense that there are thousands of
incompatible protocols that run on top of it. E.g. you can run Modbus on top of Ethernet and TCP/IP, in which case it is called Modbus/TCP and may run at 10 or 100 Mbit/s etc. PNO has developed an Ethernet solution called PROFInet
but it does not really have anything to do with PROFIBUS. PROFInet uses Windows DCOM and is very much like OPC.

I agree though that for the host-level networking Ethernet is the platform to go for, although I have a preference for FOUNDATION(tm) Fieldbus HSE.

Jonas

By Jonas Berge on 19 August, 2002 - 5:25 pm

Dear Asim,

What is the application? Are you counting bottles flying past on a conveyor belt or are you controlling flow, temperature and level on a distillation column?

If you application is process control then FOUNDATION(tm) Fieldbus is the bus of choice. It has powerful communication features, is easy to install, has an integrated function block diagram based control strategy programming language, independently tested interoperability, and a wide selection of process control instrumentation.

There is a book about it:
"http://www.isa.org/Template.cfm?Section=Shop_ISA&Template=/Ecommerce/Product
Display.cfm&ProductID=3036":http://www.isa.org/Template.cfm?Section=Shop_ISA&Template=/Ecommerce/Product
Display.cfm&ProductID=3036
Best regards,
Jonas