We are having a noise or interference on DeviceNet such that the AB, 1756-DNB card shuts down (Bus-Off fault) and has to be re-booted (24 VDC power cycled) to get re-started.

It seem to happen randomly when we start or stop our SEW (MOVIMOT) VFDs (4) that are the only items on the DeviceNet. They appear to be creating High Frequency pulses that are being transferred to the DeviceNet network. We have done all the obvious things, such as having an isolated 24 VDC power supply for the DeviceNet on a Sola XFMR with the PLC power supplies (common grounded). Not run the 480 VAC and DeviceNet wiring together. SEW says we need an additional frame ground (#8 AWG wire) from each of the SEW MOVIMOT VFDs back to our MCP which we have added. We are using AB Flat DN cable which does not have a shield / drain wire.

Please let me know if you have any ideas on how to solve / diagnose this problem.

Thanks.

 

We had the same problem, but it was with another brand of VSD. ADVICE: change the cable to the thicknet cable, the large round shielded cable and install as per AB recomendations and should solve your problem, as we have not had any hickups with ours since we got rid of the flat cable.

Regards

Matthew
matthew.richardson @ nutreco.com

 

Media-wise, the next step is to go ahead and put in shielded DeviceNet cable. I have never had trouble with the unshielded Kwiklink, but clearly you have a high-power noise generator. With only 4 drives the wiring should not be too big a deal.

My favorite tool for analyzing this sort of thing is the Woodhead NetMeter. Rockwell's network analysts carry them. They are an effective way to figure out the noise levels in the network and tell when steps you take (grounding the drives, replacing the cable, changing the carrier freq.) are affecting the DeviceNet signal quality.

 

I think your big problem is regarding your devicenet cable, shielding it would likely minimize your shutdown but you lay-out this cable away from your 480VAC or if you can't do it, install this cable inside a rigid steel conduit pipe suitable to the size of the cable you have and install or terminate earth ground on at least 3-5 meters within the conduit. This is, I think the most possible solution to your problem.

Another thing is on your 24VDC power supply:
1. 24VDC power to your devicenet network must be a dedicated power supply only for this network.
2. As much as possible, this power supply must be installed or connected to the middle of your devicenet network physical lay-out.
3. Check your termination resistor at each end of your devicenet cable. It should be 120-121 ohms only.

I hope this would help....
Good Luck.

 

When you say isolated 24VDC Power Supply, do you mean you have one only dedicated to the Devicenet Network? Are there any other devices on that power supply. If not, then seperate your network power supply from field devices.

Maybe consider flat cable with the drain wire.

Do as SEW suggests? Did you talk to Mike Demao?

He usually has the answer.

David Gulick
402-990-5311

 

I've experienced the Bus Off Fault on DN for two reasons only: DN speed setting too high (excessive accumulated drop length), and termination resistors coming loose. Sounds like you are on the right track with working on noise reduction.

Hugo