Chapter 1 Introduction 1-2
1.2.1 Analog value : A
Analog value is a continuously changing value such as voltage, current, temperature,
speed pressure, flux, etc. Temperature, for example, is continuously changing according
to the time. Because this temperature itself is not available for input of the PLC, the
temperature should be converted an analog electrical signal by transducer. Moreover, the
converted analog signal (vol tage or current) has to be converted into a digital value with
the A/D module because the PLC can handle only digital values.
Figure 1.1 Analog value
1.2.2 Digital value : D
Digital value is a discrete data that are described in numbers such as 0, 1, 2, … In
general, a binary code system that are consist of 0(off) and 1(on) is used for expressing
digital value. Also, BCD or hexadecimal values are used.
Because the CPU module of PLC can handle dig ital value only, the analog signal from
external devices should be converted into analog signals with A/D module.
In the other hand, the digital output of CPU module should be converted into analog
signal to be used for external devices. The D/A module (digital-analog converter) is used
for converting digital values into analog output (voltage or current).
Figure 1.2 Digital value
time
value
time
value