1734-IE8C Wiring for Device Connection

MTPLCUSER

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I am trying to understand how I wire a device that has a 4-20ma output and provides the 24vdc power as well. The manual for the 1734-IE8C only has one connection for the input, What do I connect to the negative on my device to complete the loop?
 
1734-IE8C Installation Instructions. Wiring diagram is on page 15, although it's pretty vague.

Basically, you'll need to connect the negative on your device to the same negative that's supplying your section of your point I/O rack that the IE8C is on. That is, the 1734-AENT(R) or 1734-EP24DC. NOT the 1734-FPD.
The 24VDC supply for your transmitter must, obviously, come from the same DC power supply from which you're getting the negative.
 
I am using the IE8C connected directly to the 1769-L18ER-BB1B plc expansion slots. Unfortunately I have multiple devices that each provide their own 24vdc thru their own internal transformer, which would not be the same 24VDC powering the plc.

Is there any work around to this? Isolation module of some sort?
 
I am using the IE8C connected directly to the 1769-L18ER-BB1B plc expansion slots. Unfortunately I have multiple devices that each provide their own 24vdc thru their own internal transformer, which would not be the same 24VDC powering the plc.

Is there any work around to this? Isolation module of some sort?
If they're four wire devices, no problem. Just tie the 4-20mA negative wire to the same negative that's on your PLC FP-.
My statement about the 24VDC applies only to two and three wire devices.
 
All of your devices output negative must be tied to the common terminal. This is NOT the ground wire. Typically, sensors do not supply their own power output for the current loop so I would verify that. Perhaps giving the part number of the sensor you are actually trying to hook up. Be certain to make a very good connection to common (perhaps use a good jumpered terminal block) otherwise you may end up with 2 or more sensors causing false readings or even supplying 48vdc or more which will likely blow the input channel. This unit is NOT an isolated type meaning that all 8 channels use one common or -(negative). Some units have isolated common. Dont forget to setup the input parameters in the setup word Ie. filtering, Hi Low and error handling. On older units, if you did not program the PLC to ignore values under the lower limit, you would error the PLC and have to reset it. This is easy to do using a compare statement so if it goes negative, a "0" is put into the memory area used in the scale command. Hope that helps
 
The device is an ABB pH Analyzer, Model# AX460/1.0.0.0.1.0/STD. I need to send the pH signal to the plc. The unit is 120VAC powered and the analog output on the unit has its own 24vdc power for the loop.

I am also trying to hookup an ABB ACS550 drive. I need the speed as an input to the plc, so I am using the AO on it which also provides its own 24VDC as well.

The only thing I can come up with is utilizing an isolator that has 4-20ma in and provides 4-20ma output but no 24vdc. Anybody know of any inexpensive isolators, or another option?
 
I disagree with Johnny's post - four wire devices are quite common. The transmitter you specified is definitely a four wire device, and a VSD is another very common example of a "four wire" analog device.
You don't need an isolator. All you need to do is connect the analog output "+" wire to the IE8C input terminal, and the analog output "-" wire to the same DC negative as you have connected to the FP- terminal on your PLC. It's standard practice when wiring three- or four-wire devices to a single-ended analog input.

You can put an analog isolator in if you want to. There's an argument to say that you shouldn't unnecessarily common up different DC power supplies (i.e. the one in your panel and the 24VDC power supply inside the ABB transmitter supplying your analog output). But in this particular application, I'd view it as totally fine - all you're commoning up is an analog signal that terminates to your PLC anyway.

And if you do use a signal isolator, the signal isolator will then be in the same situation as your transmitter - it's analog signal will need a zero volt reference just the same as the analog signal from your transmitter.
 
I didnt say four wire devices were uncommon and how you came to that conclusion I dont know. What I said was "typically" devices (at least in my experience) do not "supply" their own 24VDC output! Certainly there are but when they do supply their own 24vdc to "drive an output" they are more expensive. What is very common is an isolated transistor output of 0-5 vdc, and 4-20ma.
 
What is the part number of the "sensor" you are using? That ABB AX460 number posted is a "controller" made to drive an external device. I will almost guarantee you you wont need the AX460 anymore once you give me the actual sensor part number that is wire to the unit!
 
PH probes need temperature compensation and a very high impedance AI for the pH circuit which is what pH analyzers are designed to do wel. The analyzer re-transmits the PH value as a conditioned 4-20mA output signal.

Analyzers also provide the HMI interface for pH probe calibration, which nowadays typically includes some intelligence on the state of the probe health.

Direct connection of a pH probe to PLC AI’s needs to take all that into account.
 
Correct danw, that is the "slope" and many conditioners allow multi point correction both at the "PH level" and also medium temperature compensation. Many GOOD signal conditioners allow for 5 or more points for such calibration to allow for a truly linear output which would not need further processing other than to "scale" the input received into "engineering units" for display or control. PLC's are perfectly capable without signal conditioners to accomplish calculating slope IF the programmer actually knows what they are doing LOL. Great point danw
 
JohnnyDrumstix said:
What is the part number of the "sensor" you are using? That ABB AX460 number posted is a "controller" made to drive an external device...

Have you considered...what if this "controller" is actually controlling an "external device"? Would you still say?...

JohnnyDrumstix said:
...I will almost guarantee you you wont need the AX460 anymore once you give me the actual sensor part number that is wire to the unit!

You are lucky you said "almost"!...

I have previous experience with these ABB pH Analyzers, having installed and commissioned them in Effluent Plants. Although they can be used just to retransmit process analog input data, similar to a simple transmitter/transducer, that is often just an "added bonus", or extra feature, more so than their primary reason for being selected i.e. it is quite possible it could be doing more here than just "conditioning" an analog signal to be passed on to the overriding control system. They are usually specified for their very specific abilities in handling process pH electrode probes, and often more so to control the pH level itself. There are analog and relay outputs which could be used in all manner of ways to control, say, dosing pumps or valves, etc. They are often used for closed loop PID control of such critical processes, and quite good at it too. Another feature is a built-in wash facility where the relay outputs can be configured to control a solenoid valve and/or a wash pump to clean the probes. So, in many applications, they are far from just "conditioners", or simple transmitters. Considering this, we should not "lightly" assume we can just eliminate them out of the equation and replace them with some clever programming.

That's just some food for thought.

But, even if in this application it actually is only "conditioning" the pH value for retransmission, we must still consider the benefits to using such devices before the PLC, as danw was good enough to start pointing out...

As danw mentions, the Analyzer's analog input resistance must be very high to handle the pH electrode's high glass bulb resistance. We are talking in the order of 1e+13. That is 10,000,000,000,000Ω or ten quadrillion ohms. Not something you can just easily whip out of the middle of a pH probe to PLC controller circuit, even if it is only "conditioning" the signal being retransmitted, wouldn't you agree?

They also provide built-in calibration, and later recalibration, to meet very high Standards - DIN, Merck, NIST, US Tech, etc. They have built-in zero span and drift compensation to maintain the original calibration accuracy, further reducing the need for future recalibration. This specific unit provides automatic or manual Nernstian temperature compensation (-10 - 200 °C / 14 - 392 °F) and between 40 and 105% Calibration Slope, user configurable. Features you yourself seem to be familiar with, yes? And again, I would also hope you would agree, not features easily replicated by trying to use these pH probes directly with generic PLC controller analog inputs?

And just to add - another key advantage of using these Analyzers is that they are more often mounted close to the probes so the cabling is kept quite short between the probes and the inputs, rather than cabling some distance back to a PLC enclosure, complicating the resistive path during calibration. The provided analog output is then "ready to go" for retransmission to a supervisory controller using suitable signal cabling.

These Analyzers have a very specific and bespoke reason for existing, and as such, we should really give them their due respect, I feel.

In my opinion, I don't think you are going to find too many Process Engineers who are going to be inclined to attempt, or hire a Programmer to attempt, re-engineering all the above "ready made" features using a PLC, when the device was specifically specified in the first place for having those features.

Back more to the topic at hand...

It is this Analyzer's analog output, provided for retransmission, that we must concentrate on here with regard to connecting the analog signal to the POINT I/O module. The specific connections for this output are on:-

Connection terminal block A - Analog Output 1

A13 +
A14 -

So that makes it a 2-wire 4-20mA analog current signal.

MTPLCUSER said:
I am trying to understand how I wire a device that has a 4-20ma output and provides the 24vdc power as well...

How have you come to this understanding? With nothing connected, are you measuring 24VDC across A13 and A14?

The Analyzer's analog output is listed as being an isolated output, and no more. This means the 2 wires (+/-) are isolated from the rest of the Analyzer's circuitry. This should permit you to wire this as a 2-wire loop-powered analog input.
i.e. the PLC may provide the power source. That is how I remember wiring them?...

1734-IE8C%20Wiring%20Example.jpg


Regards,
George
 
Last edited:
They also make a 1734-CTM module which isolates the commons for each point. I have used these before when a Walchem ph meter supplies its own loop
power.
 
In response to: Geospark. Well perhaps the original poster could tell us which probe they are using to first figure out whether or not they need a signal conditioner? After all, there are probes with built in circuitry and others without. I suspect that since they have the signal conditioner already, the probe is matched to the conditioners its input (whatever that may be) but how would we possibly know? Perhaps a salesperson just sold him a bunch of stuff he didnt need or didnt understand the overall process. Perhaps the application started out one way and they discovered the conditioner didnt do everything they needed so then they added the PLC. Its also possible they dont need the analog card but only need to monitor the digital output, who knows. One thing for certain, this little issue which is probably very easy to resolve has spawned a lot of discussion. BTW, Just what the heck do you mean by "your lucky you said almost" anyway dude? Where I come from thats considered a threat! Perhaps following such a statement you could use "hahaha" or a smiley face so I dont think your some kind of techno bully or a *%^^+!! (profane words omitted) because given your choice of words, you certainly come off as arrogant and full of hubris in my opinion...
 

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