parallel I/O

[sparky970]

We are in the engineering phase of a new HCL synthesis unit. It has been asked that we send the 4-20mA signal from several instruments to 2 separate PLC's. Will the parallel resistances of the inputs cause a problem? How would you do this? We are using AB Control Logix.

 

[danw2]

A couple of considerations:

1) Don't Control Logix processors talk to each other over any number of digital protocols?

2) To have parallel resistances come into play, one would typically convert the 4-20mA current signal to a voltage, and then transmit a voltage signal to the second PLC. The 4-20ma is converted to a voltage at an input anyhow, the CLX uses 250 ohms as a dropping resistance (pg 20, Installation instructions, 1756-IF8H).

So, the 'first' PLC uses 250 ohms on its input to convert the current locally to a 1-5 Vdc signal, the signal across PLC 1's input then is wired in parallel to the 2nd PLC, which has > 1 M? input resistance for voltage signals.

So, doing it that way, no, the parallel resistances are not a problem: 1M in parallel with 250 ohms.

2) Or you could send 4-20mA to both devices, since the 4-20mA is a current signal, the 4-20 wired in series to each device: source + to input 1 +, input 1 (-) to input 2 +, input 2 (-) to source (-).

Either method could be problematic if the 4-20mA signal has HART superimposed on it, and the application intends to use HART for either configuration or for its ability to carry diagnostics or multiple variables, then you need to make sure to confirm whether or not the analog input would 'strip off' the HART before it gets to the 2nd input.

If the 4-20ma signal is an active output, powered internally from whatever it is, try to ascertain whether the output is isolated or not. Signals that are ground to the source device can be problematic when connecting to multiple inputs.

3) The I/O manufacturers make DIN rail mount boxes that will duplicate a current signal, one current in, two current signals out, some people call them splitters.

Dan

 

[sparky970]

Thanks for the information, I think I would prefer to go with a current splitter.

 

[danw2]

Phoenix Contact part number is MCR-FL-C-UI-2UI-DCI-NC order # 2814867

Dan

 

[itsmoked]

You should also review the often ill logic of hooking one sensor to two different PLCs. If it is because the second PLC actually needs the info for control of something else that's fine, if this is some mistaken attempt at redundancy it is a bad idea.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Aren't multiple inputs supposed to be in series? If you put the input resistances in parallel, you cut the output voltage in half.

TTFN

FAQ731-376

 

[itsmoked]

Come on IRstuff, fest up, did you actually read the thread or were you skimming fast?


Multiple 4-20mA inputs can be in series if you don't cause ground reference problems. If each PLC references to ground it's not going to work. Hence danw2's whole point of run one 4-20mA and run the other PLC 1-5V, essentially no resistor.

Or am I missing something now?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

>Aren't multiple inputs supposed to be in series?

For a single current signal through two inputs, yes, they can be in series.

>If you put the input resistances in parallel, you cut the output voltage in half.

There aren't two current inputs, there's one current input and one voltage input. Hence there's only one 250 ohm resistoron the one input. The other voltage input resistance is > 1Mohm, paralleled across the 250 ohms of the current dropping resistor.

The inputs can be either a voltage input or a current input.

 

[IRstuff]

My bad, I got mislead by your usage of "transmit a voltage signal to the second PLC."

TTFN

FAQ731-376

 

[roydm]

One further thing to consider if you wire in series is the possibility of breaking the loop. If PLC 2 is remote I would follow Danw2's suggestion and use a current splitter or signal isolator. That way if anything happens to PLC 2 wiring the loop to PLC 1 remains working.
Roy

 

[GTstartup]

The OP didn't say anything about "transmit a voltage signal to the second PLC."

How I read it, he wants so send transmitter outputs to two separate PLCS. (at least that's what he typed more or less)

Sending two 4-20mA to 2 PLCs would be input resistances in series, not parallel. So I agree with IRstuff.

That being said, I don't see a problem doing this, but I would suggest a zener in parallel with the inputs in case one input card fails open or is pulled out or whatever

 

[RL0010]

Another product that would repeat the signal to two different sources is one from Stahl that also provides IS protection to the hazardous location. That can be found for the model 9160/19-10-11s at

http://www.r-stahl.com/products-and...cts_pi[product_id]=911&cHash=d0dca2056b#panes

this would provide one signal with hart if supplied and the other with only a 4-20 signal.

RL

 

[ScottyUK]

Any reason why both posts of yours that I have read today are pushing Stahl products? There are plenty of suppliers of this type of equipment - MTL and P&F to name just a couple - so why the big push on Stahl? Salesmen are most unwelcome on this site.



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If we learn from our mistakes I'm getting a great education!

 

[JLSeagull]

... although Stahl makes good products too.

 

[ScottyUK]

Absolutely - I like Stahl products: the ones I have encountered are well designed and well made. I am just very suspicious when someone makes two posts, ever, and both recommend the same specific manufacturer.



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If we learn from our mistakes I'm getting a great education!