The temperature in your database was, at some point, a current in a wire. Not as a metaphor. Actual milliamps through copper. Almost everything your code knows about a physical process traveled as an electrical current through a cable before it became a number, and the rules for that journey were written in the 1950s.
The standard is called 4-20mA. It is old, it is analog, and it still carries most of the world’s industrial sensor data. I have pulled the cables for it as an electrician and consumed the numbers from it as a developer, and the two jobs taught me very different things about the same signal. This post is what I wish the developer side knew.
Words in this post, in plain language
Transmitter: the measurement converter mounted on or near the sensor. It turns the sensor’s raw signal (the resistance of a PT100, for example) into a standardized 4-20 mA current.
PLC (programmable logic controller): the industrial computer that runs the control logic of a plant. It reads inputs, executes its program and sets outputs, over and over, in real time. In this post we care about its analog inputs.
