ControlNet anomaly


Thread Starter

Ed Mulligan

In publication 1785-RN522C-EN-P dated 8/01 Allen Bradley states:

"Under extremely noisy conditions on the ControlNet channel, processor may fault with memory loss or lose communication."

I have seen this (complete memory loss) happen from time to time, sometimes on just one PLC on the network and sometimes to more than one. I am looking for information on how to measure the health of the ControlNet system. Are there any hardware/software products that can tell me the system health? Are there any specs available for me to use an o-scope or other test gear to test the system?



Speaking for me, not for Starbucks. . .
Yes, Allen Bradley has an equipment that can help you to troubleshoot ControlNet, it's like a tester and it measures the status of the network and gives you diagnostics. Cat. No is 1788-CNCHKR


I have had very good luck with the handheld 1788-CNETCHKR from Allen-Bradley. RA's European Design Center came up with it. It gives you, with six LED's, both a "Error Type" and a "Signal Strength" indication. There's an oscillioscope-tap plug on the side that will tune in the signal from each node (a pushwheel designates that) to help you nail down the source of noise. Usually you need to go around and unplug the taps from the adapers or PLC's one at a time to investigate the signal from that node's perspective. Having a spare tap really helps if you can't take down any of the network nodes for analysis.

I have investigated many questionable ControlNet networks and almost without fail I find poorly-crimped BNC connectors and loose co-axial center
pins. Yes, noise can be a factor in ControlNet systems, but cruddy (and I mean fall-off-with-a-gentle-tug cruddy) crimps are #1. These would
have shown up immediately in a co-axial video cable installation as bad picture quality, but because ControlNet will green-light even under poor conditions it seldom gets diagnosed.

See if Rockwell GMS will rent you one of these, and spend a half-hour reading the user manual first; it's not intuitive.

Go into RSLinx, then RSWho, then right-click on a PLC or I/O adapter and select "Station Diagnostics". Those numbers that you see are internal counters kept by every ControlNet node, which RSLinx is reading. That data is also available in the PLC-5 data table, though they're formatted in very packed-in way that makes them an effort to decode. The decoding table is in Appendix F of the 1785-6.5.22 ControlNet PLC-5C15 User Manual.

Probably you can read this data out of 1771 and 1794 adapters and such with a CIP message, but I haven't researched how to do that.

Good luck,

Ken Roach
Technical Specialist
Rockwell Automation / Seattle
[email protected]