Why so many ways to communicate?

[mr_electrician 9]

Good day everyone. I have a question that I would like to have answered. Can anyone tell me the differences between all the ways to communicate, and what advantages/disavantages there are between them all. I have heard of device net, AS i, profi bus, 232 com, ethernet, and so on. I just want to have a good understanding of why you would use one over the other? Thanks to those who reply. Edited 5 Apr 2007 by mr_electrician

[gravitar 3]

wow.. that's kinda like asking the meaning of life :) Some comm standards are very old, some are very new. Some are designed for heavy bandwidth, some for simple point-to-point comms. Some are proprietary to a particular manufacturer, some are "open" to anyone who wants to develop for it. Some are particularly favored by certain industries. Some are favored by certain parts of the world. The reason for them being favored may be practical, traditional, political, or a combination of all three. Some make sense for smaller jobs, some for larger ones. Some are cheaper to implement, some are more costly. So in other words, there's no easy answer to such a broad question. If you could narrow your question in scope, (in other words.. "For Allen-Bradley.. In Canada.. In North American Auto Plants.. For Facility Automation.." etc.) then someone may be able to take a stab at it!

[mr_electrician 9]

Keep in mind that I am learning this, I just want to know, is one faster than the other, more resistant to electrical noise, and so on. It is a broad question, but then again there is alot of broad experience in this forum, so thats why I ask.

[gravitar 3]

Maybe the best way to answer would be in the form of a chart.. the first column would list all the networks and each following column would list subjective criteria, like what you've asked. I don't have such a chart, I wonder if anyone does. As far as speed, I think Ethernet/IP is at the top of the pack. anything riding on RS232 or RS485 would be near the bottom. noise immunity, I don't know. There's more than one way to hook up ethernet, and I think the connection method would have a lot to do with that. Redundant Controlnet would probably be near the top as far as noise immunity. Determinism would be another important criteria. Max cable length is important too. But then there's all the other non-AB protocols that I've never dealt with.. Who knows how they stack up.

[beegee 7]

you can make a difference in type of networks in different ways: 1. time critical aspect 2. size of the data to be transfrred 3. accessability to third party 4. ease of use 5. openess of the standard we tended to divide the industrial networks in 4 different layers PC-PLC high volume (typically Ethernet) PLC-PLC high volume (ex. Profibus FMS, Omron Controller link, AB Controlnet now more and more ethernet based) PLC-Intelligent slaves medium volume (ex. Devicenet, Profibus DP) PLC-Sensor, fast low volume data (ex ASI, Omron Compobus/S and Componet) when coming down from the top you tend to have faster cyclic refreshes but with less data. You could compare it with transport of goods: The top side is more like a heavy truck, you can tranport a lot of data at once you come down to fieldbusses it is more like a car, you can get it faster but if you need the same amount of dat a as the amount of data grows you get more travels to get the data over to the other side. The bottom one is more like a motor bike.... you can see where I'm heading. Profibus DP and Devicenet have expanded both upwards and downwards in these layers but are not always the right solution When it comes to RS232/422/485 it is typically a flexible and easy to program protocol available on many intelligent components (even not industrial) In this domain Modbus and simple STX-ETX protocols are mostly used.

[Mickey 71]

Hope you like to read: http://www.machinevisiononline.org/public/...eature_Grid.pdf http://www.camiresearch.com/Data_Com_Basic...2_standard.html http://www.bb-elec.com/tech_articles/rs422...te/overview.asp http://www.lammertbies.nl/comm/info/RS-485.html http://iatips.com/rockwell.html

[DanW 41]

Just to give an idea of the number of protocols used over time. look at the first slide: http://etr05.loria.fr/slides/vendredi/ETR2005_Thomesse.pdf or scroll down through the list: http://ourworld-top.cs.com/rahulsebos/Links.htm

[rpraveenkum 5]

this may help you

[gravitar 3]

That chart is a good start; I knew someone had to have something like that :) But I wonder if that's even half of what's available.

[TimWilborne 108]

There's a lot missing off of it but it is a great start! Praveen, where did you find this chart or is it something you made?

[Crossbow 295]

It's a excellent start. One more thing of note, is networks are broken up into categories. We typically call them from one of 4 levels. Keep in mind the same network can be used for different purposes on multiple levels. Typically represented on a pyramid. I'll post the picture if I can find it... The enterprise level networks (networks like Ethernet) are designed for controller to PC connection for supervisory or data collection. The controller level networks (like Profibus, MELSECNET (for Mitsubishi) ControlNet (for AB), etc.) are designed for controller to controller communications. The device level networks are designed for PLC to intelligent devices like VFDs, valves, servos, etc. The sensor level networks are the lowest, and these are PLCs to non-intelligent devices, like terminal block input and output modules. Hope this helps.

[OIDMBTRTECH 0]

Can we get that chart in a down loadable version? *.pdf maybe?

[PdL 71]

I think rpraveenkum got it here. A Google search on "Fieldbus Comparison" gives some good results.

[DanW 41]

How about Excel or pdf? Fieldbus_Comparison_Table.pdf Fieldbus_Comparison_Table.xls Dan

[mr_electrician 9]

Thanks for the charts, they will be a big help!!!

[rpraveenkum 5]

Yes PDL is right , & also I have posted it very long back http://forums.mrplc.com/index.php?showtopi...amp;#entry45998

[BobB 58]

Try here - there are plenty of explanations of most busses and links to the various fieldbus sites. This may help. http://ourworld.cs.com/rahulsebos/

[PdL 71]

The particular page your link refers to seems to be dead. Some files are still hosted though, like this comparison matrix. The links page is also still ok: http://ourworld.cs.com/rahulsebos/Links.htm It's not recently updated anymore.

[BobB 58]

Still works OK for me Pim. Whoops! A few of the links do not work anymore. The Fieldbus link still works though. Edited 9 Apr 2007 by BobB

[JesperMP 2]

The previous discussion has mostly mentioned serial type fieldbusses. I have a couple of good links for descriptions of Ethernet based fieldbusses: http://www.ethernet.industrial-networking....play.asp?id=854 (Warning: The author is from Siemens) http://www.ethernet.industrial-networking....lay.asp?id=1165 (Warning: The authors are from Rockwell) As to "why there are so many", then I think there are two reasons: 1. The major automation players each try to push "their" standard. They are only interested in standardisation if they have a heavy influence on it. 2. One standard cannot cover all requirements, in particular when one of the "requirements" is to achieve synergy or migration from older standards (Profibus to Profinet, Devicenet to Ethernet/IP, CANopen to EPL, ..) By the way, there are still new standards being made, the above lists are not complete. Edited 9 Apr 2007 by JesperMP

[BobB 58]

At least now there are a couple emerging from the pile over the last couple of years. Device Net is particularly prominent with AB and Omron. Many are now using Ethernet I/P. Profi flavours are very popular in Europe. I have just started using CompoNet. This is another CIP network from ODVA (they also look after Device Net and Ethernet I/P). It is quite fast on I/O refresh partly because it is token ring based - no crashes anywhere in the system. It also has explicit messaging - looks and feels like an improved Device Net with token ring reliability. Runs on a twisted pair at up to 4megs. I/O refresh times for 1000 I/O over 30 metres on a twisted pair is guaranteed to be 1 millisecind. HALF the price of Device Net, greater complexity and tolerance in system design, repeaters to gain distance, increase I/O numbers and change cable types and speeds along the way. Extremely versatile. Highly recommended.

[Crossbow 295]

For those used to Mitsubishi PLCs, CC-Link is a very robust and fast network, and there is a new version called CC-Link/LT (lite), which uses a flat cable similar to AS-I, and has good update times.