Velpandian P

How should I matching the impedance?

7 posts in this topic

 

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 been 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 how should match the impedance successfully?

Edited by Velpandian P

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Have you measured the signal at the AI module? You can disconnect the AI module and just use your meter in its place to rule out the AI module. If the device is too far away and the resistance of the wire is too high, the device may not be able to drive the current. The input impedance of that module is 250 Ohms, so add that to the cable resistance to see if your field device can drive through that much resistance.

The 1769-IF8 technical specifications are in this manual, starting at page 24:
https://literature.rockwellautomation.com/idc/groups/literature/documents/td/1769-td006_-en-p.pdf
Verify the wiring is correct and that the module is configured correctly in the Logix project.

There may also be some useful information in the installation instructions:
https://literature.rockwellautomation.com/idc/groups/literature/documents/in/1769-in067_-en-p.pdf

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Sure you wired it right?

Is it a loop powered transmitter?  Have you wired a DC power supply into the loop? 

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he asked the same thing in several times here and also on other forums. got same responses but does not seem to respond to questions in responses

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17 hours ago, Joe E. said:

Have you measured the signal at the AI module? You can disconnect the AI module and just use your meter in its place to rule out the AI module. If the device is too far away and the resistance of the wire is too high, the device may not be able to drive the current. The input impedance of that module is 250 Ohms, so add that to the cable resistance to see if your field device can drive through that much resistance.

The 1769-IF8 technical specifications are in this manual, starting at page 24:
https://literature.rockwellautomation.com/idc/groups/literature/documents/td/1769-td006_-en-p.pdf
Verify the wiring is correct and that the module is configured correctly in the Logix project.

There may also be some useful information in the installation instructions:
https://literature.rockwellautomation.com/idc/groups/literature/documents/in/1769-in067_-en-p.pdf

Joe E, I had try the wire as short, but there is no change. And your attached manual is helpful thank you.

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12 hours ago, DanW said:

Sure you wired it right?

Is it a loop powered transmitter?  Have you wired a DC power supply into the loop? 

No, it's Non-Loop Powered transmitter

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There’s all those status bits, channel enable or input type selection that affect card operation.  

With respect to impedance matching, if the device AO is a standard commercial 4-20mA output, then its output impedance is what it is and is presimably rated to be capable of driving 21mA of DC current through a reasonable load resistance, typically a minimum of at least 500 ohms, sometimes as much as 750-1,000 ohms for AC powered devices.  

Unless the copper wiring is excessively long and small diameter (high resistance), any 4-20 mA loop driving a 250 ohm input resistance should  work OK.   There is no impedance to ’match’; the total loop resistance just has to be lower than the maximum load resistance the AO can drive (spec’d somewhere in the spec sheet).

The installation guide shows the ”I in 0-” terminal jumpered to the ”ANLG Com” terminal for loop powered transmitters.   I’d probably try a temporary jumper to see if it makes any difference.  

The warning about voltages on V/Iin+ and Iin+ having to be plus/minus 10Vdc is valid; that’s the common mode limitation for this module.   If the device ground is more than 10v from the AI card ground, the 4-20mA current signal disappears in resulting ground loop, which may be happening.   Ground loops between a single ended, non-isolated analog output and an AI are quite common.

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