Analog Input Card with Analog Output with HART Protocol

H

Thread Starter

Hamid Ankouti

Hi Dears

I have a sensor that send 4-20 mA with HART protocol. if i connect this to AI without HART, can i read correct value?
Please guide me.

Best regards
Hamid Ankouti
 
Your analog input card will read the 4-20mA value as well as the transmitter 'makes' it and in conjunction with the analog input card's calibrated state.

The HART signal would only be useable with a HART communicator.
 
I
Hi Hamid,

If you connect non HART analog input channel to transmitter that have HART, your analog input will be able to read values that transmit by 4-20 mA signal only and HART signal will be discarded. Sure, as long as your 4-20 mA scaling in both side are correct and your loop resistance within specification, you will read correct value.
 
Hamid,

The 4-20 mA signal would still work with no problems. There are manufactures that make devices that tap into the wiring so the digital HART signal can be accessed. The hand held calibrator the sensor manufacture probably sells is a must to purchase. By the way there needs to be at least 250 ohms resistance in the loop beside the sensor.

Dale Feucht
 
J
Most of the time no problem. However, there are control systems that have AI cards which are affected by the HART communication signal. In this case you need to install a HART communication filter on the input terminal of the AI card. We have used it a couple of times.

They are available from P+F (and maybe others too).

http://www.pepperl-fuchs.us/usa/en/classid_1362.htm?view=productdetails&prodid=32778

Don't just connect a capacitor on the terminal block. Will not be good and will surely get damaged. Buy something encapsulated and rugged.

Cheers,
Jonas
 
>However, there are control systems that have AI cards which are affected by the HART communication signal.<

Interesting. What are the symptoms of 'affected by HART'?

Noise on the signal? Jitter? Offset?

HART noise component from aliased sampling?

 
J
Some systems do not have filter on the AI so they may sample on the peaks of valley of the HART signal rather than on the DC components. P+F etc. make filters to deal with this.

Some systems do not have filters on the AO so they move the output too fast such that it interferes with the HART communication. Fisher (Emerson) makes filters for this.

See full explanation on filters and line conditioners etc. in this essay:
https://www.linkedin.com/pulse/new-tips-tricks-get-value-out-hart-jonas-berge

Cheers,
Jonas
 
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