This article will be geared towards the curious, I am not going to try to train you how to work with the subject materiel.
But you will be able to converse intelligently after reading this.
4-20ma can be either an input or output from the perspective of a PLC, (Programmable Logic Controller)which was covered in an earlier article.
The PLC specialty cards necessary are referred to as Analog cards(both input and output types).
MA stands for milliamp, a measurement of current flow. 4-20 is the lower and upper limits of this defined scale.
One example is the use of this scale to order a motor to run at a specific speed, with 4ma representing stopped and 20ma representing full speed.