AB (1756) Devicenet Bus Failures

People, I have been experiencing numerous bus faults on my DeviceNet card installed in a system that has been running for about 6 months. The problem has been reoccurring since the conception of the project, but was never taken care of properly.

All of the busses are powered and configured, and when the system is working, it works flawlessly. However, if there is a power down, the system does not see the bus-system in full, and sometimes blinks a red light on the bus-fault indicator.

I have researched the PS issue, knowing that the Devicenet is very suceptable to noise. Currently, we are using an OMRON PS that feeds the whole 24VDC circuits.

I am investigating replacing the OMRON with a AB 1787-DNPS As described below.

What do you think?

The DeviceNet stand alone power supply (1787-DNPS) provides 24V dc network power for devices connected to DeviceNet taps. It is a UL/CSA class-2 power supply, which is required by the National Electrical Code (NEC) for a power bus such as the DeviceNet cable system. If the DeviceNet cable system is installed as building wiring, it must be meet NEC requirements; in particular, article 725.

I worked on conveyor/packing system that used DNET, the engineer originally used one power supply and we had "issues". At www.odva.org if you click on DeviceNet then Planning and Installation manual it will greatly assist on determining power requirements and noise protection. I got the engineer to replace the single PS with a dual and all was well.

NEC has specific requirements for DNET which are also mentioned in that manual.

I am not sure if I am understanding you correctly, you are using 1 power supply to power the DC Bus and Devicenet Power? This is absolutely not the way to set it up.


What you will need to do is set up 2 power supplies. One to feed the Devicenet Network power requirement and one power supply for your DC Bus(devices). If you do not do this, any noise, spikes, solenoid transients, shorts, or any event on the DC Bus will cause the Devicenet Scanner to fault. It really doesn't matter what power supplies you use as long as they have a clean regulated output. And that you seperate the network power away from your device power.

David

Actually it does matter which PS is used, NEC Article 725 and DNET has specific requirements on class and output. Class 1 is limited to 8A and Class 2 to 3.3A

I actually consider DeviceNet to be very noise-resistant. The only systems I've actually found noise problems in are ones where the system was purposely installed without grounding or proper cable.

As David pointed out, it is incorrect to use the same 24VDC power supply for the DeviceNet network and for other 24VDC loads.

It is also incorrect to power more than one DeviceNet segment with the same power supply. That V- line is part of the signalling system and cannot be shared between signalling systems.

If either of these instances are the case, just changing the power supply won't improve anything.

I don't recommend the 1787-DNPS, really. It is very large and very expensive, and was offered by A-B only for installations where the customer required that A-B provide *all* of the components. In A-B MCCs, for example, MURR Electronik 10A supplies are used. I also like the SOLA SDN-4 and SDN-8, and I absolutely refuse to work on anything with an IDEC supply attached.

After several years of working with DeviceNet I have settled on a system that works well for me. I normally use one power supply for DeviceNet and PLC inputs and a second supply for PLC outputs. I prefer large simple linear supplies (I use Legrand) to small switched mode types and I would not recommend the AB units as I have had to replace quite a number of these when they have been used by panel builders.
I would agree entirely with Ken about DeviceNet being very noise tolerant but would add that it is essential that the 0Vdc line and the shield wire are connected firmly to ground. Failure to do this is the most common cause of problems I have seen and in some cases can actually cause damage to the connected hardware. AB Armourblock IO modules seem to be particularly prone to this type of damage.

Also I found that with some power supplies that on startup of the system the votage either rises or falls below the allowable level which will bring an immediate bus-off which had to be reset. Only way to get system going (before replacing power supply) was to disconnect 24V from power supply, energise power supply then reconnect 24V. Regards Alan

Thanks for the heads up people.

I have ordered two of the AB devicenet Power Supplies, for testing.
I am now looking at the schematics, and a seperate power supply is being used for the DeciceNet bus.

Ok, now I am going to wait for the PS to arrive....2 weeks!!

I will let you know in the next little while what the outcome was...

Cheers