Control Panelview Node Limitations


Thread Starter


Does anyone have any experience with limitations the number of Panelview nodes on a ControlNet network?

Allen-Bradely has stated in their literature that Panelview nodes should be limited to 3 per CNB module or 6 per processor. I have spoke with
several people who have seen applications where these numbers were exceeded without any problems. Any actual experience or input with
regards to this issue would be appreciated.



I've seen problems with ControlNet PanelViews in two installations. One was a PLC-5/40C15 with a lot of ControlNet I/O, including a couple of big
CN2DN adapters and a whole lot (27 !) of FORCE drives. The two PanelView 1000's had scheduled connections that just barely pushed the utilization for that PLC-5/40C15's ControlNet channel above 90% and put the whole thing teetering on the edge of it's bandwidth capability.

I was able to tune that one up by slowing down the big CN2DN connections so that they wouldn't outrun their own DeviceNet sampling rates and distributing the bandwidth enough to get down to about 85% utilization.

The other was a ControlLogix system with five PanelViews, all doing unscheduled messaging. Intermittently, some of the PanelViews would give
a "data read failed" error.

All of those PanelViews were set with a polling rate of 100 milliseconds (the default). Every 100 milliseconds, the ControlLogix processor had to swallow five simultaneous connection requests from the 1756-CNB, and when it already had one or two connections taking up that buffer between itself and the backplane (I understand its just five connections deep), one or two of the PanelViews would fail to get their connection request approved and would error out. The easy fix was to stagger the polling rates; 100,
110, 120, 130, 140 milliseconds. That let the ControlLogix get it's connection requests out of turn and service them all.

The same problem didn't happen on DH+ or Ethernet, I think because they don't deliver all the connection requests at once. Ethernet/IP is a godsend for the PanelView because it takes away the bandwidth worries and puts the HMI on an information network where it belongs. My PanelViews sure can tolerate a few dozen (or hundred) milliseconds of jitter, but my web drives can't.

There are some rules of thumb for 1756-CNB utilization you should know about if you're building a system of more than a half-dozen nodes. Lots of folks don't realize how much more powerful the PLC-5C15 ControlNet scanner is than the 1756-CNB and try to hang dozens of fast rack connections off just one CNB. The best systems (Kimberly Clark really has the hang of it at their paper mills) I've seen use individual CNB's for "downlink" I/O scanning on a per-machine basis and then "cross-link" ControlNet or Ethernet/IP networks for system synchronization.

I'm not going to post my recommendations for CNB utilization on the Automation List because I am weary of hearing the Ethernet and PC-control enthusiasts chew on my Company's butt every time one of our guys raises his hand to help somebody with a real application or problem out. I
appreciate the A-List's intent, and have enjoyed it over the past couple of years, but the harangues are more than I can handle these days. I do most of my free-time discussion at Chris Elston's excellently-formatted but underused these days. If you want the technote or opinions on your particular application then e-mail me directly and I'll give you my ideas or set you up with someone local to you who can.

Good luck,

Ken Roach
A-B Seattle
[email protected]