AB 1756 Analog input IF8 iRTN & RTN

[jc77]

Hello! I am trying to wrap my head around how to wire a 4-20mA sensor
to my AB 1756 ControlLogix IF8 Analog Input card. Could someone educate me on the purpose and use of the iRTN and RTN terminals on the card? (user manual image attached).

For my purpose, I have a 4-20mA ultrasonic flow meter that gives signal to a controller that connects to my analog input card via a single output wire (4-20mA). The controller has a wire for +24 and another for 0V which I plan on hooking up to +24 and 0V terminal boards connected to my power supply the PLC uses. I want to use channel 2 so I am planning on connecting the output wire to Input 4 (terminal 12) and having Input 5 (terminal 14) connect to 0V to complete the circuit. I am wondering if I am missing something by not utilizing the RTN/iRTN but I don't know because I am not sure what the purpose of those terminals is for.

From the user manual, it seems like those terminals are used primarily for differential mode, but I am not sure how to tell if that mode is what I need
to use for this ultrasonic sensor. Any answers you could give me or resources to understand this concept would be appreciated!

 

[id10t_error]

The entire 1756-IF8 is in either Differential or Single-Ended mode. This mode is set inside the Studio/RsLogix 5000 software. Changing this mode in software changes how the hardware is connected internally. In differential, two op-amps are used for one analog input, in Single-Ended one Op-Amp is used per analog input, and twice as many channels are available.

Once you have that figured out you need to figure out the sensors 4-20ma loop type: Feild powered or Loop Powered. active or passive. Once that is known, you can hook it up just like in the diagram. https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um009_-en-p.pdf

 

[Bit_Bucket_07]

Hello! I am trying to wrap my head around how to wire a 4-20mA sensor
to my AB 1756 ControlLogix IF8 Analog Input card. Could someone educate me on the purpose and use of the iRTN and RTN terminals on the card? (user manual image attached).

For my purpose, I have a 4-20mA ultrasonic flow meter that gives signal to a controller that connects to my analog input card via a single output wire (4-20mA). The controller has a wire for +24 and another for 0V which I plan on hooking up to +24 and 0V terminal boards connected to my power supply the PLC uses. I want to use channel 2 so I am planning on connecting the output wire to Input 4 (terminal 12) and having Input 5 (terminal 14) connect to 0V to complete the circuit. I am wondering if I am missing something by not utilizing the RTN/iRTN but I don't know because I am not sure what the purpose of those terminals is for.

From the user manual, it seems like those terminals are used primarily for differential mode, but I am not sure how to tell if that mode is what I need
to use for this ultrasonic sensor. Any answers you could give me or resources to understand this concept would be appreciated!

All PLC analog inputs are really voltage inputs, even when you're using 4-20mA transmitters. In order to convert the transmitter's current signal into a voltage signal, you insert a 250Ω resistor across the analog input positive and negative terminals and measure voltage drop across the resistor. 4mA = 1V and 20mA = 5V.

The iRTN terminals internally connect a 250Ω resistor between the analog input positive terminal (IN-0 for channel 0) and the iRTN terminal (iRTN-0 for channel 0). The iRTN terminal, when used, must also have a jumper installed to a RTN terminal in order to connect the negative side of the built-in resistor to DC common.

The RTN terminals are all common to one another and to PS negative.

So, you only use the iRTN terminals with a transmitter that outputs a current signal. For a voltage signal, you don't need the 250Ω resistor in the circuit, so you don't use the iRTN terminals for voltage signals. Just tie all of your negatives to RTN and connect the positives to the IN-x terminals.

In differential mode, every other IN-x terminal (beginning with IN-1), which is normally a positive terminal becomes the negative terminal. The inputs must be used in pairs in differential mode. IN-0 & IN-1, IN-2 & IN-3, IN-4 & IN-5...

You don't connect to the RTN terminals when using differential mode.

 

[jc77]

All PLC analog inputs are really voltage inputs, even when you're using 4-20mA transmitters.

That's a key concept I was missing, I have a much better understanding now thanks to you two.