DeviceNet topology questions (teach me)

[industectron]

Ahh Matt, another good point that I've been tossing around. Since I'm new to DN I was wondering about hardware. I don't have a grasp on what the pro's and con's of the different ways to connect these guys. The one thing I saw that might make life better is the connectors. We have, what I quess is the standard, 5 chamber screw type. Getting both wires in one hole is cumbersome. I was thinking that the 10 chamber screw type would make it alot easier for the guys daisy chaining these devices. have you seen them used?
I am going to check out those "T"'s you mentioned. I can see those could help reduce drop length and could be a solution where the thick cable is in a small area and bend radius is an issue.

 

[industectron]

Anybody jump in here,

to clarify,, if we had those remote i/o's daisy chained on the trunk and say we had to unplug one (the cable) for some reason, this would not disrupt the rest of the network? Is this a true statemnent?

 

[LJBMatt]

Yes for "pigtail" connects where you will be daisy chaining down the line the 10 pin phoenix plug-in connectors make life a lot easier. I highly recommend these for hardwire connections. The other thing i dislike about Tap boxes if one goes bad you need to replace the entire box and reconnect everything, depending on if it is all hardwired can take an hour or so to swap out. If an inline "T" tap goes bad your change takes less than a minute.

 

[industectron]

Matt you are appreciated,
****,,, that's a good point... when we started having problems with this DN, When I saw the inside of that port box I wondered if that could be a problem area and how hard it would be to pinpoint and the disruption it would cause. GOOD INFO
Thanks

 

[mellis]

Trunk cable. Another good topic. If you do decide to go with the thick trunk, the 10-slot open connectors are quite good. You will have trouble getting two of the 12 awg power wires into a single terminal. If you go with the T's and the plugs, be very careful to follow instructions of how to prep the cable for termination. I've seen some really bad ones that continually snapped the signal wires. The cable is really heavy and stiff, the strain reliefs have to be installed correctly.

My personal favorite for DeviceNet trunk is the flat QuikLink cable. It has to fit the application of course, and it doesn't sound like a good fit for your application. Adding drops to a running system is quite easy and quick. If you use the O-ring style connectors, it's generally more watertight than the T's.

 

[mellis]

Anybody jump in here,

to clarify,, if we had those remote i/o's daisy chained on the trunk and say we had to unplug one (the cable) for some reason, this would not disrupt the rest of the network? Is this a true statemnent?

It depends on the wiring. If your trunk cable remains intact when you pull the plug, then you should only lose that drop. If the I/O's have a removable 10-slot open connector, then when you unplug it, the trunk is still intact.

 

[industectron]

Hello everybody,
I am trying to sell the idea of abandoning the thin cable and using thick cable going out 350 ft. The contractor who installed this mess is resisting, although they have now aknowledged that the installation is incorrect (after I referrenced the Devicenet spec's).They instead want to install a repeater in the field at about 250 ft., which is where the next to last remote i/o is located.

This should work, but is it the best solution? Has anybody had any experience with these repeaters?
I know installing new cable is expensive and time consuming. I do not kinow what is involved in putting a repeater.

 

[LJBMatt]

The repeater is just a 24VDC power supply to "boost" the voltage of the network. I have used them but only in instances where the network devices "Vampire" power from the network. To me this is a band-aid and not a solution as it might work but the spec on using thinnet cable as your trunk is maximum 100 meters *regardless of repeaters*, but i'm not going to hang my hat on that last part of that statement.
If they can get the thin net trunk to under 100m (328ft) then i'd be ok with that. But point to any thinnet cable manf. (AB, Turck, Woodhead, Lumberg, etc.) and you will see that maximum trunk cable run for thinnet is 100m. That being said I'd suspect the repeater will help boost the power on the trunk cable but if you aren't having massive voltage loss or power consumption by devices the repeater will just be another device to fail or give you fits in the future. Like i said I see this as a bandaid that may work. With new equipment now is the time to do it right. My advice, for what it's worth, use thicknet as your trunk, thinnet for drops as I can tell you that this will work for certain. Help yourself now by reducing the chances for recurring problems in the future. Teach your supplier that ignorance and incorrect design is no excuse to apply bandages for new equipment. My 2cents on the proposed solution.

 

[industectron]

Matt thanks,
I am siding with you on this. Your opinion bolsters my stance on this. This was engineered wrong in the first place and adding something that could go wrong is asking for trouble. Simple, straight forward is what I prefer. Granted a bandaid would probably work, but...
I'm not familiar with these DN repeaters. Thats why input from people like you and Ken and all the others that share their experiances here is much appreciated.

 

[LJBMatt]

Well i can say that most everyone here in this forum tries to help others from going through the same pain they have experienced at some point. Please keep us updated from time to time as the project moves along. I'm a big fan of devicenet myself and I have used it in many different fields of automation with good success. AB seems to have the best Devicenet interface software and hardware that I have used up to this point which hopefully will make your introduction a little easier.

 

[mellis]

I'm afraid I'm going to have to take issue with what Matt said.

A repeater or bus extender is not just a power supply. In fact it doesn't have anything to do with powering the network. Installing a bus extender allows you to extend your trunk to twice the normal length (assuming you stay within the maximum length on both sides). You can even use more than one bus extender in series. The downside is that there is usually a propogation delay across the extender, one model I looked up quoted it as 2ms. Also, it does add complexity. And, it's not always easy to find an accessable spot to mount one.

Powering the network is a whole other topic. With long runs like you have, I expect more than one power supply will be needed. The manual Ken referenced will cover that as well.

Personally, I've never used an extender on any of my projects. My DeviceNets are usually well within the specs. I'm interested in what Ken has to say about them.