plc analog io module
Originally small plc logic controller, which is handling the switch. But now the PLC can handle analog signals, but the PLC computing power or weak, so-called analog is the analog computing simple arithmetic or comparison, and some introduced PID operation. So now the PLC can also analog, such as temperature, etc. to control.
Analog scaling
Because the correspondence between A / D (analog / digital), D / A (digital / analog) conversion between, S7-200 CPU internal representation of the external value of the analog signal, a certain mathematical relationship between the two. This relationship is the conversion between analog / numerical quantity.

For example, using a 0 - 20mA analog signal input, the S7-200 CPU inside, 0 - 20mA corresponds to the numerical range 0--32000; for 4 - 20mA signal, the corresponding internal value for 6400--32000.

If there are two sensors, the range is 0 - 16MPa, but one is 0 - 20mA output, and the other is 4 - 20mA output. They are at the same pressure, the different transmitter analog current magnitude, the value in the internal representation S7-200 different. Obviously scaling relationship that exists between the two. The case of the analog output is also roughly the same.