How should I matching the impedance?


I have a problem for getting the 4-20mA input via 1769-IF8 I/O module. My field device is in good condition, but my AI card doesn’t respond. When i have to browsing about it. most of them said, the impedance of the field device and AI module are not matched.
I don’t have any idea about how to match the impedance of both, explain to me about how should match the impedance successfully?



This forum is mostly for the Ignition software itself. I really don’t think you’ll get much of a meaningful answer to your question here.



I think this is the correct forum to post this question.

General Forum Discussion. Talk about whatever you’d like here.



Impedance matching is a requirement for certain kinds of communications wiring, not 4-20mA loops. Whoever offered that advice is clueless. It is unfortunate that bad advice is mixed with good on the internet, even on this forum, but you get what you pay for. /:

However, while it’s not an Ignition question, Ignition is pretty popular with engineers who understand such things. And some of us hang out on the forum. (:

All that aside, there is an existing topic from @velpandianp. It is inappropriate to repost. Further answers should be over there:



how did you say my post is inappropriate. Actually, there are different questions are raising in my mind. sometimes, there are the different answers will come from the different questions. it may be solve my problem.

And my doubt is, some peoples said it may be solve by matching the impedance between I/O module and Field device ( source for 4-20mA). If it’s right advice means, how should i’m do it?



No, just different aspects of the same problem: a non-working 4-20mA loop.

Good luck getting an answer: impedance matching is not applicable to 4-20mA loops.



In my experience, the most likely issue is a wiring problem. You have to find out if your analog device is active or passive and wire it accordingly. The easiest way to do that (other than looking in the manual) is to measure on the analog output of the device with nothing connected to it. If you measure any voltage being produced by the device then it is an active loop device and you can wire it directly to the analog input card on the PLC. If you do not measure any voltage being produced by the device then it is a passive loop device and you will have to provide your own voltage for the current loop, i.e., placing a 24 volt DC power supply in series with the device and the PLC input card to create a closed loop for current to flow.

If everything is correctly wired you should, typically, measure around 1-5 volts (which will correspond to the 4-20 mA signal with 1 volt being regulated to produce 4 mA and 5 volts being regulated to produce 20 mA) across your PLC analog card input.

Some devices are a little tricky to wire. For example, IFM Efector devices output current loop analog on the white wire but the analog negative is just 0 VDC so on the PLC card you would take the white wire and connect that to analog input + but analog input - you would just tie to 0 VDC so physically it doesn’t look like a “loop”, but electrically it is.



Hey folks-
This post started at: ( add the h for https )
and is in process at post #26. It was made in two places because it really is not an Ignition problem per se, yet rather a PLC input issue. Personalities aside, I think it is safe to say that the problem has been resolved as insufficient input current for the I/O module. There was a side-track about impedance because the actual circuit distances nor input device(s) were not described. As pturmel pointed out in post 4 of this topic, there was an existing topic. There is also knowledge to be gained for some in looking at the wiring requirements in schematics and highlighted specifications which pertain to the Analog Common terminals. In this case, an external power supply was in use to power the loop.