URGENT - High Speed Counters not working all of a sudden ML1400/Logix 500

[TimD]

f

then the next most likely cause for the "8 volt power supply drop" effect that you reported would be that the power supply is "floating" – and the meter connection is merely providing a path to "ground" ... in other words, it doesn't sound like the power supply is tied to a firm "ground" potential ... in most cases that's a recipe for "noise" and "fluctuation" from the input signals ...

Hi Ron,

I did switch PSUs (same ratings) and the same behavior is present. The grounds on the supplies are tied together. I also noticed that the panel shop did not ground the PLCs or supplies to the panel itself, as mentioned in Allen Bradley's Grounding and Control Wiring standards manual. Maybe doing this will help as well. I will probably tie an additional ground today to earth somehow. (8' rod...) May not have the time for that though.

First thing is for me to use the on-board instrument supply from the controller and hook up one sensor. I am going to hook it up in it's native NPN configuration without pull-up resistor. Apparantly, I have this configuration already, but never got the meters to 'talk' without using a resistor. I have 24v- to common, and 24v+ through sensing element and onto plc input, this is exactly as described in both PLC hardware manual, and Omega's Flow Meter manual. I never could get the plc to read anything until I introduced a resistor, which kept the input high. I am still perplexed as to why I need the resistor, when both manuals do not call for one, and the inputs can be configured NPN or PNP on the controller...

 

[TimD]

It works again!

OK, Got it!

So, I did the following panel modifications:

1. Changed wiring from Current Sinking to Current Sourcing.
2. Removed "earth Ground" from flowmeter wiring terminals, and replaced that connection with 24V-
3. Isolated HSC input commons on each controller (I have two Micro1400's) and wired appropriately.
4. Removed all 2.2k pull up resistors

My site maintenance guy is going to add an 8' copper rod and ground the panel, and sub panel. (door is already bonded to panel)

I still am left with some questions however,

1. HOW did this happen? I may have one bad flow sensor, maybe it caused some voltage spike that messed with ground potential?

2. Why does the Omega manual state (see earlier in thread, page 2) that a pull up resistor is needed for current SOURCING, when in fact.. at least for now... the opposite is true?

Conclusion,

I want to take a moment to personally thank all of you who responded, or even just followed along. I learned a great deal in the past couple of days, and now have some pretty good ideas for any future panels I have put in with respect to grounding. I also know better to get all the cut sheets to the panel shop prior to assembly... and doing some of my own homework is equally important.

I have a long path ahead of I&C learning. I thought I knew a fair amount, but no way.

Thanks again forum for a job well done. Happy Friday!

Tim

 

[mordred]

Glad to help though my contribution was small this time.. One thing make sure your grounding only one side of sensers to avoid ground loops

 

[kamenges]

Point #2 in you list is the magic bullet. Somewhere along the line you lost ground integrity between your machine and your control panel. In effect your sensors were not consistently operating because they didn't have a consistent DC common. By running the 24VDC common to the sensors (which is the way it should be done) you guaranteed the supply path to the sensors regardless of that the machine ground is doing.

Sourcing and sinking rend to be a bit relative. But generally speaking, sinking input modules will accept signals directly from sourcing sensors. A sinking input module is generally consider to be a module that will activate when source voltage is applied to it. Conversely, sourcing input modules will accept signals directly from sinking sensors. A sourcing input module is generally consider to be a module that will activate when the input point is attached to DC common.

As I said, the sink/source thing is relative. The Omega manual considered the SOURCING configuration to be the configuration of a module designed to interface to a sourcing sensor. This use of "sourcing" is opposite of the description I gave above. So the SOURCING configuration from the manual is needed ot interface to a sinking style input module (sinking as defined by AB).

Keith

 

[bernie_carlton]

I always get the schematics, or equivalents of the output circuitry of the sensor and the input circuitry of the card and work it from there. I don't trust the Sinking/Sourcing nomenclature for exactly the reasons noted - inconsistency of usage.

 

[Gil47]

Wiring diagram attached:

On the first line of your posted diagram at post 13
The words IF the PLC only accepts

Well in your case the PLC does accept sinking and sourcing ,
I prefer to call it PNP and NPN ,as then you know whether you will get, two of Pos or Neg and your common will be the one, that has only appeared once