One 4-20mA analog signal to two PLC inputs possible?

spaceacealpha · Dec 13, 2010

I'm trying to hijack a laser's analog signal from a Modicon to a CLX5000. I ran the wires and did the hook up as normal parallelling in with the original. Seems like my resistance drops forcing my mA to double. Is there a trick way of doing this without knocking out the signal?

My other option is to send the analog signal direct to the CLX5000 and use its analog output to relay the information back to the Modicon. I just don't like the idea of my CLX5000 'needing' to be on all the time.

OkiePC · Dec 13, 2010

Since it's a current signal, you'd want to series the connections so both receivers see the same current flowing through them. This may exceed the capability of the transmitter. In that case, use a signal conditioner with more than one output like this:
http://www.acromag.com/catalog/219
or this:
http://www.actionio.com/products/actioniq/original/q501.html

rpoet · Dec 13, 2010

With 4-20ma signals, or any system that measures current, you need to place the receiver(s) and the transmitter in SERIES, not parallel (as you do with VOLTAGE measurement). Be careful in placing multiple receivers in series however. You may develop ground loop and/or differing ground reference problems leading to inaccurate measurements, or in the worst case equipment damage. Those issues can be solved with a signal isolator, such as the one here:

http://www.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners/FC-11

AD also sells some higher-end conditioners if you need to convert to/from voltage / mV / current.

You could use the CLX5000 to "relay" your signal, but it's not the most elegant solution, as you MAY have resolution loss based on input and output resolution. Are they both 16 bit?

Easiest solution is to wire everything in series (+ to - to + to - etc...) and use the signal conditioner if you find you need it.

-rpoet

rpoet · Dec 13, 2010

Looks like OkiePC beat me to it.

-rpoet

sthompson · Dec 13, 2010

spaceacealpha said:
I'm trying to hijack a laser's analog signal from a Modicon to a CLX5000. I ran the wires and did the hook up as normal parallelling in with the original. Seems like my resistance drops forcing my mA to double. Is there a trick way of doing this without knocking out the signal?

My other option is to send the analog signal direct to the CLX5000 and use its analog output to relay the information back to the Modicon. I just don't like the idea of my CLX5000 'needing' to be on all the time.

As well as the good advice given above, a number of companies make 4-20 mA Splitters. A quick Google search sould turn some up.

Stu...

ROBERT_PLS · Mar 29, 2012

Same scenario with anaolg vDC

Hello all,

I know this thread is like 2 years old but I have a similar question I've been needing to ask someone who would know.
I am new here so forgive me if I am doing this wrong. Not sure if I should just start my own thread. Well anyways my situation is this:

I have an array of various sensors, mostly displacement sensors. Analog output is 0-10 vDC. I am presently feeding the output signals to a single DAQ (GW Instruments iNet 555) and recording test data. This system functions ok.

Now I am setting up a PLC to automate other systems and would like to be able to take the output from my DAQ wired sensors and parallel it over to the PLC and do other stuff with the sensor outputs.

Is this feasible? Is there a way to do it without degrading/corrupting the original signal?

Thanks,
Robert.

Mark Buskell · Mar 29, 2012

What type of processor?

If its on AB 5000 controllers using ethernet. You can add the module from one processor into the tree for the other processor. Over than that you could use a signal splitter.

ROBERT_PLS · Mar 29, 2012

My PLC is a Turck BL20 system. I was thinking I could just take the raw signals from the sensors output leads where they are dropped on my terminal blocks (before the DAQ) and parallel that directly out to the PLCs analog input channels. I kind of tested this scenario with a single sensor output paralleled to my DAQ and an independent, external panel meter and it seemed okay but I am not sure if it will be 100% reliable under actual operating conditions doing it with all channels.

geniusintraining · Mar 29, 2012

ROBERT_PLS said:
My PLC is a Turck BL20 system. .

Do you have an analog output? if yes then move the input word on the first PLC to a output and that would give the input to the second PLC...

I did this not that long ago, but they had spares that they were not using (A LOT) and it was on the fly (middle of the night) keep in mind that you need to document it well as if you loose power on the first PLC you will loose your input on the second

Just a thought...

ROBERT_PLS · Mar 29, 2012

geniusintraining said:
Do you have an analog output? if yes then move the input word on the first PLC to a output and that would give the input to the second PLC...

I did this not that long ago, but they had spares that they were not using (A LOT) and it was on the fly (middle of the night) keep in mind that you need to document it well as if you loose power on the first PLC you will loose your input on the second

Just a thought...

My DAQ doesnt have any output channels so, the only way I can see to get these particular signals into the PLC is to parallel off the raw sensor outputs.

Lancie1 · Mar 29, 2012

Robert,

I have use DAQ recording equipment before. Yes, if you wire your DAQ voltage signal inputs in PARALLEL to the new Turk PLC voltage inputs, it will probably work okay. It will help if you can run the DAQ inputs and the PLC inputs from the same 10 VDC power supply. Otherwise you might need some signal isolators.

OkiePC · Mar 29, 2012

I concur with Lancie. It should probably work okay.

Most analog voltage inputs are very high impedance but I am unfamiliar with both devices so you would need to research that and/or test it. The signals would be wired in parallel as you stated, and as long as both devices receiving them have a high impedance, the sum of that impedance should not overload the source in the sensors nor affect the resultant voltage by a great deal.

I think it would be safe to test this if you can set up a sensor with a known and precise signal and measure it when to connected to each input device and again with both connected. The results may also depend on your required resolution...if the signal drops by .002 vdc when paralleled is that too much?

ROBERT_PLS · Mar 30, 2012

Thanks Lancie. Yes The parallel outputs from the one sensor to both the DAQ and the PLC will be from the same 10 vDC supply. Thanks for your input. I think this might actually work. I will test it again.

ROBERT_PLS · Mar 30, 2012

Thanks OkiePC, for all the input. It is really good to hear feedback from you guys who know about these things. Now I finally know that I am not crazy thinking of this idea and that it might actually work. I will test it as you suggest and see how it works. Fortunately I have a little more time to play with it.

OkiePC · Mar 30, 2012

Last night, I looked up the spec on a Turck BL20 analog voltage input and the "burden" was about 98kohms IIRC, so that should have a minimal impact on voltage. I don't know about the DAQ input.

And, you're welcome.

One 4-20mA analog signal to two PLC inputs possible?

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