Analog input signal means continuous signal such as process data, which is transmitted from a local/field transmitter to a control room (PLC or DCS).
For instance, process variables expressed in physical amount such as opening degree of a sluice gate (0–100%), sewage flow rate (ex: 0– 250m3 /hr), and water level in tanks (ex: 0–10m) are converted into electrical signals and are transmitted to control room.
Standard electronic transmission systems use 4 - 20 mA DC.
There are several transmission systems. One is to connect directly with the DCS/PLC I/O (Input/Output) device via shielded cables. Another is the so-called link system with PLC, in which analog signal is converted to digital signal and is transmitted via coaxial cables or optical fibres