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Topic Title: Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs
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Created On: 12 March 2013 01:54 AM
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 12 March 2013 01:54 AM
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Heartnet89

Posts: 11
Joined: 23 July 2012

Hi,

What is the reason behind the 4-20mA current as a standard signal for transferring data? Well for starters, we choose current over voltage because there is less signal / data loss when the signal is being transferred over.

How about 4-20mA?

Well the purpose of having 4mA as a minimum is to be able to make sure that process actuators are "on" so that SCADA is able to detect any possible fault. Is that right?

how about 20mA?
 12 March 2013 09:44 AM
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rogerg

Posts: 27
Joined: 21 October 2008

Minimum of 4mA allows a self powered loop instrument as well as easily detecting line faults.
Using a maximum of only 20mA reduces the energy going out into the field for IS applications.
With a 250 ohm resistor you then get 1-5V DC which was easy for computer interface purposes.
More info here http://en.wikipedia.org/wiki/Current_loop
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