Device Net Communications Failure

[nightfox1925]

Hi all. We have an HVAC MCC supplied by one of our vendors. In their systems, they have provided Device Net Devices for communication and provided 24Vdc power supplies within the 600V MCCs. The power supply is supplied by a 600V-120V step down transformer feeding two units of 120VAC-24Vdc power supplies. These power supplies feeds the Device Nets.

There is a Buffer that is connected in parallel to the 24Vdc Power Supply output terminals. The device nets are powered and there is 24Vdc voltage on their power inputs. The problem is that, the device nets are communicating with DCS. Howeverm, if they bypass the buffer (disconnected) and wire the device nets directly to the 24Vdc power supplies, the communication is working.

The buffer spec is in this link:

http://literature.rockwellautomation.com/idc/groups/literature/documents/ap/1606-ap029_-en-e.pdf

Is this because of the buffer emitting EMIs or noise of some sort that it disrupts the device net comms? or there is something we are missing here?

 

[jraef]

Who designed the system to have that buffer? Was it approved by the ODVA? If not (and I doubt it), then you cannot expect it to work with Devicenet. Just because it says Allen Bradley on it and A-B is the primary promoter of Devicenet doesn't automatically mean that the device is compatible. A-B doesn't make that line of power supplies, Puls is the manufacturer, and they show no ODVA approval for their version. There are only 2 versions in that entire product line that are ODVA approved, no mention of the buffer module being approved.


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[nightfox1925]

Hi Jaeref, what is ODVA stands for?

 

[xnuke]

http://www.odva.org/

xnuke
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[avogel]

It's possible the buffer is causing your issue, there isn't enough information from you to really make a decent conclusion, but at the get go, verify they haven't violated any devicenet wiring rules, ie: trunk length and terminating resistors on both ends for the Tx and Rx. You may need RSNetworkx for Devicenet to do any useful troubleshooting. AB's Devicenet overloads are evil, I won't use them again, because I want my stuff to just work.