Modbus tcpip v Profi

[Steve_D]

Guys, A company I deal with is evaluating the pro's and cons of profi over modbus tcp, reliability, the limiting number of nodes, affect of size of network on comms speed, any installation issues to be aware of, that sort of thing. The suppliers only tell me the good stuff, so I would apreciate any commment on good and bad experiences with both system. Which is prefered and why?
Regards
Steve D

 

[CharlesM]

I would also like to here what everyone has to say. Im using Profibus but I often wonder if another network would be beter. Modbus seems popular but I have never used it. Maybe this would be a good poll, What is your favorite network?

 

[GeoffC]

In my opinion ModbusTCP wins by a long shot, for the following reasons
1. Compatibility - vary rare to find a manufacturer that doesn't support it
2. Speed - It will run as fast as the network, need more speed use faster switches etc, or use routers to split the network. Only really limited by the end device, some devices still only run at 10M, but if that device is attached to a smart switch the rest of the network could be running at 1G
3. Diagnostic tools - Lots of programs are available (most free to download). As an example recently bought a ModbusTCP device and when it came to setting up needed the MacId but only half the number had been printed on the device. Just used Ethereal to discover the rest. For simple tests can't really beat the 'ping' test.
4. Reliability - Yes it must be done properly. Have seperate logical networks for control, scada etc. Use industrial grade network components, and you could even use smart switches with built in redundancy.
5. Can be Cheap - If you dont need all the above then you can use cheap office grade components. Great for testing.
6. PC interface - not many PCs without a ethernet port

The ModbusTCP networks I have put in have all worked perfectly from the start, and were painless to setup. Anyway just my thoughts....

 

[Doug_Adam]

We use Profibus generally since it has a large range of products available to connect to it, and also it is the most convenient connection to use for our most commonly used items, which are Siemens PLCs and SEW servo drives.
We are currently evaluating Profinet, but I don't think we will make the jump for another 5 years.
We use Modbus TCP in areas of the plant that use Modicon PLCs. This is generally for PLC to PLC communications, or PLC to Citect communications. For that it works well. I used it once at a previous employer, and it worked OK. There are many implementations of it since it was the first industry standard Ethernet available.

If I was required to make this descision, I would go with what is best supported by your preferred hardware suppliers.
I don't think either system would be a bad choice.

 

[jolio ST]

Using Modbus TCP is very convinent to setup. peer-to-peer or broadcast is possible. Boss is looking into changing totally from Modbus RTU to TCP/IP for future projects although I think it will still take some time to reach that stage. Overnight changes can never happen.

regards
Sherine T.

 

[JesperMP]

GeoffC, I cannot let this stand unopposed:

1. Compatibility - vary rare to find a manufacturer that doesn't support it

Wrong. Modbus RTU has almost universal support. Modbus TCP is supported by only a few.

5. Can be Cheap - If you dont need all the above then you can use cheap office grade components. Great for testing.

If you talk about a PC that talks with a PLC that has native Modbus TCP, then yes. In all other cases a system based on Ethernet will be significantly more expensive. And I cannot imagine using office grade equipment in an industrial application. Here in the EU, you are not even allowed to do it (EMC directive).

The entire subject is something else.
There is not one universally accepted bus standard that is based on ethernet.
The only widely accepted i/o bus standards are Profibus and DeviceNet.
The only widely accepted non-i/o bus standard is Modbus RTU.

To sum it up for the original question.
The advantage of Profibus is that it is the most widely accepted standard, and you can create up to medium sized systems at an affordable price.
The advantage of any of the Ethernet based networking standards is that you can create very large and performant systems.
And you can combine Profibus and Profinet to gain the advantage of both.

 

[GeoffC]

Supported by only a few???
Have a look at the following article http://www.manufacturing.net/ctl/ar...&industryid=22071&spacedesc=communityFeatures
Personally I have found many devices from many different suppliers all with ModbusTCP compatibility. Strange Profibus doesn't rank in the survey results?

" The advantage of Profibus is that it is the most widely accepted standard "
To me a standard needs to be both open and freely available. ModbusTCP is both.

 

[GeoffC]

Modbus RTU has almost universal support

With a simple bridge all these devices are accessible to ModbusTCP.

 

[Peter Nachtwey]

You are all wrong because you didn't ask what Steve_D's application needs are.

Actually some of you will be right for the wrong reason.

All of you are no better than the salesmen giving answers without knowing the facts. Everyone is telling Steve_D what bus they use when it makes no difference UNLESS they are doing the same thing Steve_D. We support both Modbus TCP and Profibus DP and which one we recommend depends on the needs of the application.

So lets do this like engineers and not salesmen. Lets ask Steve_D
1. How many nodes are there?
With lots of nodes collisions become a problem. Our biggest applications with the most axes are Profibus DP.
2. How frequently must the nodes be updated?
Here again Profibus DP will usually have an advantage depending because of frequent collisions. This depends on the number of nodes too
3. How much data is being transfered?
If there are large packets I favor Ethernet.
4. Is the data message data or I/O data?
If the data is message data I favor Ethernet.
If the data is I/O data then Modbus TCP may work but it is not a good choice. This is Modbus TCP's weak spot and Profibus DP's strength. Ethernet/IP would be better than Modbus TCP.
5. Is there a need for peer to peer?
Can't do that with Profibus
6. Is there a need for multi-master?
Easier to do with Ethernet. This is done all the time in fact.
That should be a start down the right path. Well Steve_D? Given this information, what bus would you choose?

 

[Ken M]

I think Peter's questions are good valid ones. Unfortunately I think GeoffC's post could have been written about Profibus as well as Modbus TCPIP -

"In my opinion ModbusTCP wins by a long shot, for the following reasons
1. Compatibility - all certified Profibus masters and slaves will definitely work with each other, regardless of manufacturer. There's no one I know of will give you that assurance for Modbus.
2. Speed - But we all know that a 1GBaud network doesn't deliver data 1000 times faster than a 1MBaud. But is is nice to know when your data will arrive. Profibus is deterministic, Ethernet is still, regardless of speed, send-and-pray.
3. Diagnostic tools - The only diagnostic tool most of us need is the DP master itself. Oh, and maybe a multimeter. Oh, and a pair of eyes.
4. Reliability - Yes it must be done properly. (At last we agree!) Use the same logical networks for control, scada etc. Use industrial grade network components, (is there any other kind with Profibus?)
5. Can be Cheap - If you dont need all the above then you can use cheap office grade components. So it's got all these great advantages, but if I don't want the advantages I can have none of them. Sounds like a great reason for picking a network and protocol. Great for testing. Yeah, I think I would say Profibus is great for testing too.
6. PC interface - not many PCs without a ethernet port Damn it, you got me there! But there's no lack of choices for DP interfaces either."

Regards


Ken

 

[danw]

I've never done Profiwhatever, so you can educate me, please.

1. Compatibility - all certified Profibus masters and slaves will definitely work with each other, regardless of manufacturer. There's no one I know of will give you that assurance for Modbus.

About a year or two ago, a 'lab' in China became the Modbus certification location. I have yet to see a Modbus product advertised as certified. Does a profiwhatever device have to be certified to claim Profi'x', or can a product claim Profi compatibility it without being certified?

3. Diagnostic tools - The only diagnostic tool most of us need is the DP master itself. Oh, and maybe a multimeter. Oh, and a pair of eyes.

If I get a Modbus device and want to test it on the bench before I install it, I can wire the device to the appropriate port, fire up my PC with Modscan23, a Modbus master utility (one of several commercial software tools), and test poll it.

How does bench testing or in-situ testing work with Profiwhatever?

Great for testing. Yeah, I think I would say Profibus is great for testing too.

Because ? ? ?

6. PC interface - not many PCs without a ethernet port Damn it, you got me there! But there's no lack of choices for DP interfaces either."

And those choices for DP interfaces would be ? ? ?

Thanks in advance, Dan

 

[JesperMP]

Supported by only a few???
Have a look at the following article http://www.manufacturing.net/ctl/ar...mmunityFeatures
Personally I have found many devices from many different suppliers all with ModbusTCP compatibility. Strange Profibus doesn't rank in the survey results?

Your link doesnt lead directly to that article ypu are refering to, so I cannot comment it.
But I am very certain that Profibus outshines Modbus TCP by a wide margin when it comes to the number of supporting manufacturers.
For example, how many of the "big" drives suppliers support Modbus TCP ?

" The advantage of Profibus is that it is the most widely accepted standard "
To me a standard needs to be both open and freely available. ModbusTCP is both.

Profibus is definitely a standard, I do not say that Modbus TCP is not. I say that Profibus is more widely accepted than Modbus TCP.

5. Is there a need for peer to peer?
Can't do that with Profibus

What do you mean by "peer to peer".

6. Is there a need for multi-master?
Easier to do with Ethernet.

I cannot imagnine how it can be easier than the multimaster on Profibus.


I hope that I do not give the impression that I am biased towards Profibus.

 

[Peter Nachtwey]

While we are waiting for Steve_D

Profibus DP certifed products should work with each other. The problem is that there are a lot of uncertified products out there and the PTO and PNO are letting those with uncertified products claim to be Profibus DP. Most of the Profibud DP problems I bet are cause by uncertified products.

There is a difference between Modus ascii, Modbus RTU, Modbus Plus and Modbus TCP. I only know of certification for Modbus Plus and who care any more.

SST and other make master Profibus DP cards that can do diagnostics like a packet sniffer.

I haven't seen any PC with a Ethernet port that also has a dedicated CPU for just running the Ethernet port. I know they exsit but no one wants to pay for them just like they don't want to pay for the Profibus DP master. When ever you move, open or close a window the Ethernet communications stop on a PC.

Modbus TCP should not be used for I/O. Why?

 

[danw]

SST and other make master Profibus DP cards that can do diagnostics like a packet sniffer.

A card like a PCI card for a Windows box? Or a slot card for a specific brand PLC?

 

[GeoffC]

For those that need to know about certification http://www.modbus.org/devices.php