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Allen Bradley Analog Input Card Series Wiring with another Controller

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#1 Chris Elston

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Posted 28 August 2008 - 07:48 PM

Has anyone ever used an analog sensor in series with another controller?

I think what I'd like to do is bring an LVTD sensor back to a motion controller which contains it's own close loop system, then series the wiring over to my Allen Bradley INPUT card 4-20ma, then to the power supply.

Should I be concerned with anything or is this a lame brain idea? In my mind it should work. Basically I want the PLC to know the same information the motion controller knows and the motion controller does not support "pass through analog or a repeater analog wiring feature".

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#2 b_carlton

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Posted 28 August 2008 - 08:52 PM

It's fine as long as at least one of the input devices (the microcontroller or the PLC) can take a current input which is floating. On most that I've seen there is a common between multiple current inputs, with that common tied to the analog common.

Secondly the sensing device must be able to supply the current across the now larger (possibly doubled) resistance. Check for maximum drive capability.

As long as bot of these are met then it should be ok.
An output is a PLC's way of getting its inputs to change.

#3 JeffKiper

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Posted 28 August 2008 - 09:16 PM

As long as the total resistance doesn't get to high as Bernie stated. Thank about the basics current is equal in all nodes. Right? I have done this several times with Newports going to Honeywell single loop controlers.
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#4 BobLfoot

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Posted 29 August 2008 - 09:17 AM

Has anyone ever used an analog sensor in series with another controller?

I think what I'd like to do is bring an LVTD sensor back to a motion controller which contains it's own close loop system, then series the wiring over to my Allen Bradley INPUT card 4-20ma, then to the power supply.

Should I be concerned with anything or is this a lame brain idea? In my mind it should work. Basically I want the PLC to know the same information the motion controller knows and the motion controller does not support "pass through analog or a repeater analog wiring feature".


We have done something very similiar here with our flow rate and viscosity meters. Only we conenct an Analog input card and a Hart Module Card to a single flowmeter. This allows us to pull running data thru analog and tune the meter and read and set parameters thru the hart module.
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#5 panic mode

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Posted 29 August 2008 - 08:48 PM

as barnie mentioned this can work and i've been using for example with chart recorder etc but there are some restrictions. you need to check what load can sensor drive and still push 20mA. so far i've not seen this as a problem. most analog inputs seem to be 250Ohm and usually current outputs will be able to drive up to 1000Ohm (most will do at least 500 Ohm).
bigger concern is if they can operate stacked up. if one of them is isolated this is not a problem (you can isolate it using additional device for $150-200).
if there is nothing else wired between the two (each is powered from separate and isolated power supply and at least one of the two is not grounded) you can also connect them. you will of course loose isolation and have one of the power supplies floating.
however this is probably not going to be your case as the motion controller will have other signals tied to PLC etc. and most PLC analog inputs are not isolated. in other words if the motion controller doesn't have isolated analog input, you probably need to get 3-way isolation device.

#6 paulengr

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Posted 30 August 2008 - 07:01 AM

Has anyone ever used an analog sensor in series with another controller?

I think what I'd like to do is bring an LVTD sensor back to a motion controller which contains it's own close loop system, then series the wiring over to my Allen Bradley INPUT card 4-20ma, then to the power supply.

Should I be concerned with anything or is this a lame brain idea? In my mind it should work. Basically I want the PLC to know the same information the motion controller knows and the motion controller does not support "pass through analog or a repeater analog wiring feature".


This is totally valid and it's supposed to be the way things work with 4-20mA current loops. This is text book. The only reason that it might not work is if you don't have ISOLATED inputs. An example of how this is achieved is with an optoisolator. The 4-20mA loop drives a very small low power LED on the input side of the optoisolator which shines onto a phototransistor in the output side. The card circuitry is connected to the phototransistor. There is clearly no electrical connection whatsoever. There are many other designs but this is one of the simplest to describe.

However, AB doesn't really do analog very well. I don't know if the CLX cards have improved any but at least in PLC-5 and SLC are some of the most poorly constructed ones available. They offer no isolation whatsoever and the electrical noise on the card wipes out the LSB by itself. I haven't seen evidence of common mode problems between the card power supply and the 4-20mA system but I've definitely fought problems with very, very poor channel-to-channel isolation. And yes, this is in so-called "differential mode".

All is not lost even in this situation. As long as the misbehaving device is always the LAST device in the system (so that the return side is connected to common), it will still work because the device will see what it is expecting.

#7 Chris Elston

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Posted 09 September 2008 - 12:57 PM

Alright, you guys have cast some concern into me.

I think maybe I'll look into a 2 way splitter.

This should do the trick:
http://rd.phoenixcon...ets/MCR-2UI.pdf

#8 Ken Moore

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Posted 09 September 2008 - 01:50 PM

I like the products from Moore Industries (hmm.... wonder why...)

http://www.miinet.co...ca_signal.shtml








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