the reactive power meter has an input signal og -20mA to 20mA... can i use 0 to 20mA instead of -20mA to 20mA... it seems the reactive power meter is too old..
The host with the analog input does not know what the ±20mA (bipolar) signal represents nor does it know what a 4-20mA (unipolar) signal represents or any range analog signal, for that matter, you have have to tell it what the signal represents. You assign a meaningful range and engineering units (in the development software) for the analog signal.
You would use a ±20ma-to-4-20mA signal conditioner/converter in order to use a conventional analog input card. But both signals cover the same range, it's up to you to define the range and the units.
A unipolar 4-20mA signal can easily represent a bipolar signal:
4mA = -20 units
8mA = -10 units
12mA = 0 units
16mA = +10 units
20mA = +20 units