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Topic Title: Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs Topic Summary: Created On: 12 March 2013 01:54 AM Status: Post and Reply |
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 12 March 2013 01:54 AM
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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? |
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12 March 2013 09:44 AM
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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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