Current loops are a standard which are used widely in process automation. 20 mA are wirely used for transmitting serial communication data to programmable process controlling devices. Other widely used standard is 4-20mA current loop, which is used for transmitting analogue measurement signals between the sensor and measurement device.
Serial communication using current loop
In digital communications 20 mA current loop is a standard. The transmiter will only source 20 mA and the receiver will only sink 20 mA. Current loops often use opto-couplers. Current is current which matter, the voltages don't matter.
For measurements purposes connect a 1k resistor in series with the RX/TX and a current meter. See what current is flowing into the receiver and that this value is close to the scale you would expect to see.
The data trasmitted though this kind of onterface is usually a standard RS-232 signa just converted to current pulses (current on and of depending the logic state of RS-232 circuit).
4-20 mA current loop
Many process automation sensors use stadardized 4-20 mA current loop interface. This interface works typically so that the process conrolling equipment which reads to the sensor value supplies a voltage to the loop where the the sensor is connected and reads how much current it takes. A typicall arrangement for this is to supply around 12-24V voltage through a resistor and measure the voltage drop on that resistor (the curren can be then detemined from the formula U=I*R).
The current loop is designed so that a sensor take 4 mA current when it is at it's minimum value and 20 mA when it is in it's maximum value.
Because the sensor will always pass at least 4 mA current and there is usually a voltage drop of many volts over the sensor, many sensor types can be made to be powered from only that loop current.
Tomi Engdahl <Tomi.Engdahl@iki.fi>