Chapter 1. INTRODUCTION
1 - 2
1.2 Glossary
1.2.1 A - Analog Value
Continuous changeable quantity such as voltage, current, temperature, velocity, pressures and flux is
called an analog quantity. For example, temperature changes continuously with time as shown in Fig.
1.1. The PLC can process that continuous changeable temperature by use of the thermocouple input
module.
1.2.2 D - Digital Value
In the Fig.1.2, the number of man can be counted as 0, 1, 2 and 3. A discontinuous changeable
quantity as such is called a digital quantity. On and Off signals can be denoted as a digital value 0 and
1, respectively.
An analog quantity cannot be directly input to the
CPU module for digital processing. Therefore, an
analog value should be converted into a digital
value to be input to the CPU module. In addition,
for external output of an analog quantity, a digital
quantity of the CPU module should be converted
into an analog quantity.
A/D
Conversion
CPU
(Digital
processing
)
D/A
Conversion
Analog
-200~-1200
℃
400~1800
℃
0~1750
℃
Analog
0~
±
10 V
or
4~20 mA
[Fig. 1.3] Processing in the PLC
time
Number of man
[Fig.1.2] Digital Value
Temperature
time
[Fig.1.1] Analog Value
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