Devicenet dropouts

Good Morning,

I am troubleshooting a Devicenet issue for a customer. The Devicenet network is having dropout issues, mostly in the morning. It has 24 slave.

One issue is that there was a slave that was removed from the network, but had not been removed from the scan list. I removed the slave, but dont think it would cause dropout issues.

I verified that a 120 ohm resistor is on each end of the network, and I even replaced them with new resistors.

Another issue that I thought may have played a part was the L55 processor was running version 15.5 firmware and the 1756-DNB was running version 12.003. I found a 1756-DNB with version 6.002 and replaced it.

Some of the other issues I found was that it appears that the Devicenet cable is not using the shield and is not grounded.

Another issue is that the power for the devicenet is the same 24vdc power that is used for all of the IO. I read in the troubleshooting guide that you should not use the same power for devicenet as IO. How do you normally handle powering the devicenet if you have a battery bank with a charger and need battery backup?

I checked all of the connections and they seem secure. I checked voltages on the Can-L and Can-H and they seem correct. Impedance on the bus is right t 59 ohms so that is good.

I dont have a scope, so I cant tell if there is any noise, but I would think it is a good possibility due to no shield in place.

I think some of these issues may play a part, but this place has been in service for a long time and this issue just recently popped up.

Anyone have any ideas or feedback?

Hi
I had a profibus issue a few years ago.

Basically the 24V power to the profibus remote io unit was seperate from the IO wiring but a Bubba had replaced the unit at some time and crossed the wires over.

The machine had moving parts and a cable had frayed, so the 24V was getting grounded for long enough that the voltage dipped and the profibus unit lost comms....and then came back up.

it maybe something similar

It's been years since I last commissioned a DN network, but the cardinal sin back then was not using an isolated 24VDC power supply. We used to buy those huge AB brick supplies specifically for DN and put nothing else on them. You might be having a loading issue on the shared supply. My first order of business would be to shield the cable and add a separate supply.

diat150,
You are correct that the Devicenet best practices have not been followed and I would look at correcting the cable shielding as well as isolated power supply to correct your problem.
Alot of times these repairs face the "But its been in use for years" argument but Devicenet has a history of overcoming problems like those for a period of time and then causing trouble later on.

Based on the state of the cable installation I would look at things like trunk and drop length also.
I have worked on a system that was good for 5 years before it started dropping nodes off the network and it was found that the trunk line was way longer than the recommended length, we added a repeater with a second power supply and never had another issue.

I have not dealt with a Dnet power supply with a battery back-up before but the solution will be out there somewhere.

I dont think the trunk and drop lengths are a problem. i would say the trunk length is around 100 -150 feet. The drops are all about 1.5 feet. Dc voltage is fine at the farthest end.

I also found that the dc system has 2.5 volts of ac ripple, probably because the batteries are 11 years old and toast... Not providing any filtering. I cant imagine that 2.5 volts of ac ripple in an ungrounded devicenet system will work well.

diat150 said:

How do you normally handle powering the devicenet if you have a battery bank with a charger and need battery backup?

Click to expand...

PULS Battery Control Unit

The major problem I have ever had

The shield of the DN cable needs to be continuious, and then
connected to ground ( AT one point ONLY )

Had a contractor that looked at the cable specs and it said the shield
was bare so at terminations he left a long bare wire so it touched
any earth that was close enough

diat150 said:

The Devicenet network is having dropout issues, mostly in the morning.

Click to expand...

Just recently and mostly in the morning? Environmental, temperature, humidity, Bubba and his coffee leaning on the same drop.

Plus everything everyone else said.

The operator said he drinks coffee from 4am to 4pm so thats a possibility.

Absolutely put your devicenet on it's own power supply, and make absolutely sure it's a devicenet rated one. You'll know it's devicenet rated because it'll look identical to all the other AB power supplies, except it has a devicenet logo on it and is double the price.

But in all seriousness, that's one of the big ones with DNet - as much as it looks identical, there are some other smarts under the hood that the standard power supplies don't have, and a regular power supply might run you OK for a while, but as things deteriorate with age, you could certainly start seeing issues.

My approach would be:

1. Add in correct DNet power supply. Monitor for improvement. Whether there is improvement or not, do step 2 as well (always good to change one thing at a time so that you know what actually helped and how much)
2. Ensure the screen is continuous on every single section of trunk cable and every single drop. Then ground it at one end. Monitor for improvement. If still none (or just to cover off on all your bases), do step 3
3. Check the total length of your trunk and your drops, and compare against the recommended lengths

Another thing is the wire that they used for the drops doesnt have the drain wire. For now I just put the drain on the main trunk and grounded it.

ASF said:

Absolutely put your devicenet on it's own power supply, and make absolutely sure it's a devicenet rated one. You'll know it's devicenet rated because it'll look identical to all the other AB power supplies, except it has a devicenet logo on it and is double the price.

But in all seriousness, that's one of the big ones with DNet - as much as it looks identical, there are some other smarts under the hood that the standard power supplies don't have, and a regular power supply might run you OK for a while, but as things deteriorate with age, you could certainly start seeing issues.

My approach would be:

1. Add in correct DNet power supply. Monitor for improvement. Whether there is improvement or not, do step 2 as well (always good to change one thing at a time so that you know what actually helped and how much)
2. Ensure the screen is continuous on every single section of trunk cable and every single drop. Then ground it at one end. Monitor for improvement. If still none (or just to cover off on all your bases), do step 3
3. Check the total length of your trunk and your drops, and compare against the recommended lengths

Click to expand...

I have had the same problems with a DN system. Ran fine for years. System installed to AB spec. Power supply is stand alone for DN but not a DN rated one to my knowledge. Thanks will try this next, and also let the AB integrator that designed the system know.

I've been through so many of these "but it works some of the time !" arguments that it's disheartening.

Ground the shield. Ground the DC Common. Separate out the network power from the I/O power.

*THEN* you can start troubleshooting.

Ken Roach said:

I've been through so many of these "but it works some of the time !" arguments that it's disheartening.

Ground the shield. Ground the DC Common. Separate out the network power from the I/O power.

*THEN* you can start troubleshooting.

Click to expand...

That is what I checked first. Rockwell have a good guide for installation and testing / checking device net. If I find it I will post a link.

If you have a good relationship with your distributor I would ask them if they have a dnet meter you can borrow. There are several things that the meter will show up and second it'll take you days to figure out.

Some historical issues I have found with devicenet. In no particular order.
Power supplies wrong (not Dnet)
Power supplies weak (slow to start the DC bus)
Bad T Taps
Terrible screw connections after some years
Failing nodes.
Best 1 yet. I designed a small 5 node system for a plant we used a I/O block that pulled power from the network for sensors. 4 in 2 relay out. We powered 1 PE and 2 contactors. The robot where going into a fault (Don't remember the exact code). The conveyor belt lacing had rubbed into the PE cable and caused the 24 Vdc to drop only when the lacing rolled around.

What is the error code on the scanner? Make a little trap logic that logs what the errors are. We are all guessing until we know exactly what the error is.