question re. active 4/20ma loop..

[Gus A]

Greetings,
Sorry to sound like such a noob...but I am....
I have a conveyor belt scale integrator that I am trying to get to communicate with a PLC. The analog input card is a AB 1756-IF16. The integrator has an active 4/20ma signal and from what I understand, if the analog card has been designated to be active as well, they will not communicate. Sound correct? Is there a device which I could place between them and allow them to communicate? I already tried an Omega DR900, no luck.
Thanks in advance..

 

[George Graziano]

I think what you have is a scale that puts out 4-20mA. Doesn't need any additional power. The AB module should be able to take either.
George

 

[robertmee]

I think you're confusing terminologies and applications.

4-20ma loops come in many different configurations. They can be referred to as Active, Passive, 2-Wire, 4-Wire, single ended and differential.

First off you need to determine the type of 4-20ma loop you have.

An Active loop, also commonly denoted as 4-wire, uses an external power supply connected to the device using two wires, and then sends a 4-20ma signal on the other two wires.

A Passive loop, also commonly denoted as 2-wire, uses a power supply usually generated by the PLC module (but can be separate) and the power and signal are carried through the load across the same 2 wires.

For the IF-16 you can have single ended loops or differential loops. In the case of single ended, multiple or all channels share the same return/common. In the case of differential, each pairs of channels are used to isolate separate returns/commons.

So, what to use? Most times, you'll use differential mode if you are using 4-wire devices that have their own power and are generating their own current loops. In this case, your scale integrator likely has its own internal 24VDC supply that is generating a current loop. You may have other devices doing the same, and for that reason, you should set up the IF16 as differential. Just realize that by doing so you are pairing up the channels so you are only able to use 8 inputs.

If you had a bunch of loop powered 2-wire level probes that relied on the same 24VDC supply, then you could wire them all single-ended to the IF16 and you would have 16 inputs.

You can wire separate 24VDC supplied loops single ended, but then you have to worry about tieing the commons/returns together and other considerations, so rule of thumb for me, is one supply, single ended 16 ch configuration, separate supplies, differential 8 ch configuration.

If you read through the IF16 manual it will detail this more.

 

[Gus A]

Thanks Robert,
The IF16 has been set up for single ended operation. There is a 24VDC common power supply and all returns are common. That seems to be the issue. The integrator has it's own power supply and the card has been designated via software for single ended operation. Looks like the only option will be to add another card and change software, allowing the two wire integrator powered loop to be terminated directly to the new Analog card. I was hoping for another hardware option that would allow communication in the current software/hardware configuration. Any ideas?

 

[dmargineau]

For analog input devices powered up by "third party" supply power(different supply power than the Input Module)there are available Isolated Analog Input Modules -> 1756-IF6I for example.
Refer to Chapter 5 of
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um009_-en-p.pdf
You will have to "jumper" all the module's points RET terminals and connect them to the third party Power Supply's COM terminal.

 

[robertmee]

What he said ^. You'll have to make sure the common/returns of the supply for all the single ended loops is jumpered and connected to the common/return of the scale integrator.