Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs

[THED]

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?

[Dennis_Knutson]

See http://www.control.com/thread/1026235722 for one discussion on the subject.

[K C]

Hi,

I don't know if this is mentioned in the link Dennis provided.

With 4mA as a minimum you can detect an open loop. Actually the complete range is 0 - 24mA. The proces value is 4 - 20mA. Below 4mA is a open line and above 20mA is a shorted line.

Sometimes there are also value ranges like 3-4mA and 20-22mA as transmitter faults.

Kees

[Henrik_Volkers]

you can build sensors that can be powered with the remaining 4mA so only two wires needed to get the signal and power the sensor