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Remote I/O choice


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#1 Buckaroo

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Posted 12 May 2009 - 12:44 PM

I have an upcoming project where I will be using some Flex I/O. I have DH+ as well as Ethernet available to use. There is the 1794-ASB communication adapter for DH+ and the 1794-AENT for Ethernet IP. Does anyone have any suggestions or ideas as to one or the other? I have not used either product so ease of setup is somewhat important. Any suggestions or comments are welcome.

Thanks

Tom

#2 Eddie Willers

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Posted 12 May 2009 - 02:24 PM

DH+ and Allen-Bradley Universal Remote I/O run on the same kind of wire, but are different protocols for different purposes.

1794-ASB is a Remote I/O adapter. You cannot connect it to a DH+ network.

I presume your controller is a ControlLogix-family controller. PLC-5E and SLC-5/05 do not support I/O scanning on Ethernet.

I'd choose the Ethernet adapter for the speed, the flexibility, the diagnostic features, and the ease of support for specialty and analog I/O.
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#3 BobLfoot

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Posted 12 May 2009 - 02:42 PM

DH+ and Allen-Bradley Universal Remote I/O run on the same kind of wire, but are different protocols for different purposes.

1794-ASB is a Remote I/O adapter. You cannot connect it to a DH+ network.

I presume your controller is a ControlLogix-family controller. PLC-5E and SLC-5/05 do not support I/O scanning on Ethernet.

I'd choose the Ethernet adapter for the speed, the flexibility, the diagnostic features, and the ease of support for specialty and analog I/O.

Given you're doing this new I'd agree with Eddie on using Ethernet. That said I have a lot of concerns when it comes to Ethernet I/O. be sure and read all the warning about network layout and connection limitations. Personally I'd use Controlnet over Ethernet just because of the history of ethernet in industrial settings. But I know that is an old "joe relay" attitude and I need to "get with the flow" and use Ehternet one of these days.
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#4 bikerdude

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Posted 12 May 2009 - 04:43 PM

Bob,

old "joe relay" attitude and I need to "get with the flow"


i hear ya, I just can't get there myself either. Ethernet is for comms in my "Joe Relay" or "Tunnel Vision" way of thinking. I would rather have my I/O wires screwed down tight instead of just plugged in thru a RJ-45 jack. Any old "Bubba or Cletus" can come along & pull the cable out just to see if it is loose or needs the pins shines up.

Use the 1794-ASB is my vote.

BD

#5 paulengr

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Posted 12 May 2009 - 05:40 PM

I have an upcoming project where I will be using some Flex I/O. I have DH+ as well as Ethernet available to use. There is the 1794-ASB communication adapter for DH+ and the 1794-AENT for Ethernet IP. Does anyone have any suggestions or ideas as to one or the other? I have not used either product so ease of setup is somewhat important. Any suggestions or comments are welcome.

Thanks

Tom


Long term, Remote I/O (1794-ASB) is going the way of the dinosaur. The better comparison would be between DeviceNet and Ethernet/IP.

If you use Remote I/O (1794-ASB), you must go to program mode to change your configuration around in the future. If you use Ethernet/IP, then you can add additional modules or other extensions to the PLC without going to program mode/offline. That's a major limitation with Remote I/O if your intent is to maintain maximum uptime and still be able to make relatively small changes and additions to the system while it is running.

With DeviceNet (and Controlnet) in most cases you must also shut down first to make any changes to your I/O, and you need extra software (RSNetworx) to do all the configuration on your network.

Ethernet is MUCH faster than any other network. If you are running 100 Mbps (no reason not to), you will be 20 times faster than Controlnet, 200 times faster than DeviceNet, and 2000 times faster than Remote I/O.

In terms of reliability, Ethernet and Controlnet are almost equals. You can create ring networks (always a redundant path) with either one, and you have an almost arbitrary topology available with Ethernet.

When/if you run Ethernet, you are running with a VERY general network with lots and lots of options that allow you lots of configuration flexibility. This is a good thing but also can leave you vulnerable to unforeseen issues. For instance, you definitely want to keep your general plant traffic and your I/O traffic separate. If you don't, you could potentially have network problems (and loads) from the rest of the plant affecting your I/O...not a good thing. One option is to use 2 separate 1756-ENBT's, one for the general network, and one for the I/O.

If you want a single ENBT, then you should consider how to control the general/I/O network interactions. I strongly suggest you have at least one managed switch in the PLC network if this is the case. You can isolate traffic using overlapped VLAN's or by careful choices in terms of traffic prioritization and broadcast traffic control. I actually suggest you do this regardless.

You should also have a switch with IGMP querying capability capable of serving your I/O, and I suggest you don't rely on the general network router exclusively for this function. And if you use any dumb switches, they should have IGMP snooping capability as well. Otherwise without this function, IGMP packets get treated as broadcast traffic which can cause unintended consequences (overloading the Ethernet adapters in your I/O network).

I also strongly suggest you use industrial grade switches. Brand names that I have used and can recommend are Hirschmann, N-Tron, and Cisco. In terms of capabilities, features, and ease of use, I'd also rank them in that order. There are some others out there (Sixnet, Moxa, Garrett, and Advantech come to mind) but I have no experience with them.

As to DeviceNet, get a copy of the "Ethernet Technical Bible". Read it religiously. Get a "netmeter" from Brad. Use it and get a good survey ahead of time. Realize that it's a bus network, and if any device on the network has a communication problem, it will JAM the network giving you the dreaded "Bus Off" error which is notoriously hard to fix. DeviceNet is good when it works but very tough to deal with when you've got a problem with it.

Edited by paulengr, 12 May 2009 - 05:43 PM.


#6 Buckaroo

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Posted 12 May 2009 - 07:28 PM

DH+ and Allen-Bradley Universal Remote I/O run on the same kind of wire, but are different protocols for different purposes.

1794-ASB is a Remote I/O adapter. You cannot connect it to a DH+ network.

I presume your controller is a ControlLogix-family controller. PLC-5E and SLC-5/05 do not support I/O scanning on Ethernet.

I'd choose the Ethernet adapter for the speed, the flexibility, the diagnostic features, and the ease of support for specialty and analog I/O.


My mistake, long day and I did not compute all the particulars. Some of what we have is a 1756-ENBT(I think that is the number) with the DHRIO card. So I haved DH+ going to a handfull of controllers and panelviews. I guess then the question is what is the best option comparing Ethernet remote I/O and DH+ remote I/O options. I must give this some more thought and figure on the best course.

Paul,

I will have to re-read your post tomorrow after I recharge. You offer very good information

Thanks to all for the replies so far.

Tom




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