ControlLogix Inputs Blinking

Bit_Bucket_07

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Bit_Bucket_07
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A friend of mine inquired about an intermittent problem that he is seeing in his plant in which discrete inputs in 2 remote IO chassis are blinking off and causing nuisance equipment shutdowns. The system uses a single source of 120 VAC control power at each remote location. He has installed a power monitor and determined that the control power is not dropping out when these shutdowns occur.

The PLC equipment is ControlLogix. A 1756-L62 CPU Revision 15.4 with 1756-EBNT/A modules for Ethernet IP distributed IO. They are using N-TRON switches for their IO network. Is it possible that a faulty Ethernet switch or some other intermittent network fault could be the cause of the blinking inputs? He said that he has seen no faults reported when he checks properties for the processor or the EBNT modules.
 
What means "inputs are blinking". Is it the LED on the Input module itself or in RSLogix?

If the first, then, first search what is the electrical/hardware reason for blinking, and if it is normal, apply filtering

If in place the LEDs are stable, but in the program are blinking, then I agree it might be network issue., filtering can help again.
 
It's critical to know what he considers "blinking".

This could be as simple as a loose conductor on a wire that serves the power distribution for the sensors. A power monitor on the incoming main lugs wouldn't show that, but it sure would cause inputs to "flicker on and off".

Check the I/O connection uptime values for the connections made to the 1756-ENBT. That will tell you if the actual network connections are dropping out and re-establishing.
 
Rube said:
What type of device is wired to these inputs? Pushbutton? Limit Switch? Photoeye?
Click to expand...

Yes. All of the above. It is a conveying system.

What I mean by "blinking" is that all of the inputs briefly report an off state to the processor and then return to normal. They suspected an intermittent loss of control power, but that would have been detected by the power monitor, since it was connected to the terminal board common bus in the IO panel that provides power to all of the input devices.

I don't know how ControlLogix systems respond to an intermittent loss of communications to a remote rack. I know that in many other PLC systems, the inputs in the IO image table would retain their last valid state in such an event.
 
How often is the power monitor polling?

Also is the power monitor sharing the same neutral?

When this happens does the IO light on the processor flash green? Might have to set some traps in the logic to catch if you are loosing rack comm. You can also set to fault processor if there is a loss, it's most likely not set that way right now. Then you will know it's a network issue.
 
Maxkling said:
How often is the power monitor polling?

Also is the power monitor sharing the same neutral?

When this happens does the IO light on the processor flash green? Might have to set some traps in the logic to catch if you are loosing rack comm. You can also set to fault processor if there is a loss, it's most likely not set that way right now. Then you will know it's a network issue.
Click to expand...

Nobody has witnessed the event occurring, but traps were set in the program to latch internal bits when inputs go low. This is happening on 2 of the 4 remote chassis, and all of the inputs in the affected chassis go to zero when it happens. Some days, it doesn't happen at all. Other days, it happens several times. Pretty confident that it is not a power or wiring issue. They are planning to start swapping Ethernet switches to see if that is what's causing it.
 
There appears to be a consensus that a ControlLogix Processor will reset all inputs from a remote chassis when comms are interrupted. That is the question that I really wanted answered.
 
Bit_Bucket_07 said:
...for Ethernet IP distributed IO. They are using N-TRON switches for their IO network. Is it possible that a faulty Ethernet switch or some other intermittent network fault could be the cause of the blinking inputs?
Click to expand...


Are the N-tron switches managed or unmanaged?

I only ask this as I have just had a bad experience with Safety I/O over ethernet with unmanaged switches...
 
ianingram said:
Are the N-tron switches managed or unmanaged?

I only ask this as I have just had a bad experience with Safety I/O over ethernet with unmanaged switches...
Click to expand...

To the best of my knowledge, they are unmanaged switches. The last I heard, they have replaced one of the switches. I don't yet know if that solved the problem. This system has been in operation for over 10 years without a glitch. If using unmanaged switches was a problem, then it's odd for the problem to have taken so long to materialize.
 

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