Profibus Vs DeviceNet

MorphuisOGrady

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Jul 2007
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Guys,

My DeviceNet Network is going to consist of 17 Nodes with an IO size of 425 Words. This is huge and and it was suggested to try using profibus. Prosoft make a Profibus DP module but what i need to know is, will Profibus support this amount of data? Is there anything i need to consider?
 
MorphuisOGrady said:
Guys,

My DeviceNet Network is going to consist of 17 Nodes with an IO size of 425 Words. This is huge and and it was suggested to try using profibus. Prosoft make a Profibus DP module but what i need to know is, will Profibus support this amount of data? Is there anything i need to consider?
Click to expand...

Sometimes reading articles such as this help. Which network to use may depend types of devices and distance of the network.
 
I just made a test with a single master and 17 slaves with each 32 words, and a Profibus bus speed of 1.5 Mbps.
The Ttr (Target Token Rotation time) for this setup is 40 ms.
The Ttr_typ (Typical Token Rotation time) is 11 ms.
 
I set it up in STEP7.
The socalled Ttr value is absolutely critical with Profibus, and is recalculated every time you make a change to the configuration.
When I have done a hardware configuration in STEP7, I can check the socalled "bus parameters" that have been automatically calculated.
There must be a configuration utility with the Prosoft module that will do exactly the same thing.

And yes, it will be fine. You can make much bigger networks than that.
Btw. When running Profibus cables in the field (which I am guessing that you do) and at 1.5M I recommend that you place socalled "active bus terminators" at the physical ends of the Profibus cable.

edit: I cannot tell if there is a limitation with the Prosoft module. You have to check that yourself.
 
Last edited:
Thanks Jesper,

My only concern now is that its not Profibus cable than is submerged to the remote IO Remote. Its a dark blue 4 core shielded cable that I have never seen before. Unfortunately I cannot change this cable because the prototype is already built. My cable lenght is only 15M. I know this is not ideal but its all I have to work with.What are your thoughts on this?
 
You almost certainly will have to exchange the cable to a Profibus cable.
At 187.5 k profibus will work with almost "any" cable, but at 1.5 M it is much more critical with the cable.
I once had a connection that ran fine at 187.5 ok over just 10 meters, when I cranked it up to 1.5 M it failed. It turned out that the cable had been extended in the middle with plain screened cable.

If you run it at 187.5 Ttr will increase to 297 ms.
Is an update of 3 times per second in worst case OK for you ?
 
I am just getting an idea.

4 wires are enough for ethernet. Maybe the wires are made up of 2 twisted pairs, which i what is used with ethernet.

Is your PLC and AB CompactLogix or ControlLogix ?
You could put the nodes on Ethernet/IP.
You can even test if it is possible without purchasing anything. Just connect with an ethernet RJ45 in both ends and see if you can connect from a laptop to another PC for example.
 
I have a few installations where I have about 100 nodes, each with either a micromaster drive or four 32 bit IO cards and a high speed counter card. So, your "word" count is no problem at all.

I use 12MB, and have update times of around 13ms in the above configuration. As long as you take extra care with the connections and follow the rules on cable lengths and use repeaters when required (you won't need one in your case), you should only be limited by your particular hardware. In my experience, Profibus is fantastic when the installers follow the guidelines exactly, but you are in for a world of hurt when people take shortcuts with it.
 
Thanks Guys

1536

Guys,

Thanks for your interest in this one. I will be using a ControlLogix 1756-L61 for this one so the Prosoft MV156 will be perfect (1536 Bytes I/O). Ideally I'd have stuck to my guns and used DeviceNet but in this case its just not suitable for the job. There is nothing more in this other than monitoring a ton of Analogs i.e PT100s, Vibration and magnetic sensors.

Jesper, I had considered the possibility of using Ethernet/IP but after my IT guy inspected the cable it was a no-no. If this thing falls over after surmerging and something goes wrong it might be 3 months before someone can get out to sea to fault find.

So just to recap. THe longest distance is from CLX rack to Remote I/O enclosure. THis is exactly 15m in length. From there its just 1r runs to each Profibus node of <1M
 

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