Introduction to Industrial Automation

Table 6.1  Usual Analog Devices and Standard Ranges of Analog Signals

Analog Input Devices

Analog Signals

Temperature sensors

4–20 mA

Pressure sensors

0–5 V DC

Flow meters

0–10 V DC

Transducers of any kind

± 10 V  DC

Electronic devices with analog output

± 50 mV DC

± 500 mV DC

Input N

Input 1

PLC memory

Analog input

module memory

Thermocouple

1

N

Analog input module

Bus for data and 

control-signals transfer

Input

selector

PLC

±50 mV

Shielded

cable

Heated

object 

Module

power supply

–V

+V

A/D

BUS

CPU

Figure 6.22  Analog input module which contains an A/D converter common for all input 

channels and memory for temporary storage of digital values.

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  Introduction to Industrial Automation

this analog voltage through a shielded cable in an analog input of the module. The utilization 

of shielded cable is suggested in cases of transmitting weak electrical signals in order to block 

the corrupting electromagnetic noise generated from nearby conductors, and has the potential 

to cause unwanted and important operational errors (wrong corrupted measurements). Every 

analog input module contains only one A/D converter that is common for all the analog inputs. 

This approach is followed by the majority of manufacturers, mainly for purposes of reducing 

the cost of the hardware that is significantly increased if each of the analog inputs has its own 

A/D converter. Since all the input signals are applied at the same time in the input module, the 

input selector is responsible for routing a separate analog signal to the A/D unit in every clock 

cycle. The digital values of the analog signals provided from the A/D converter are stored in the 

memory of the input module, which is of a restricted size and, on many occasions, is referred to as 

a data register. With timing and control signals transmitted from the central processing unit, the 

content of the module’s memory is transferred through the data transfer bus to the memory of the 

PLC where the values are processed according to the executed specific program instructions. Some 

manufacturers use the term “input channel” rather than the analog input one. In 
11 22 33 44 55 66

Ο

Ν

1 2 3 4 5 6

Ο

Ν

Signal Conversion Circuit

Channel 0

Channel 7

+V

DIP Switches

MULTIPLEXER

POWER SUPPLY

BUS DRIVER

CLOCK

ISOLATION

MEMORY

ADDRESSES

DECODER

ISOLATION

ISOLATION

ISOLATION

Α

D

0V +5V -5V

ADDRESS

BUS

DATA

BUS

PLC  – CENTRAL PROCESSING UNIT 

CONTROL 

PROCESSOR 

OF MODULE 

OPERATION

MULTIPLEXER

ADDRESS

ANALOG INPUT MODULE

1 2 3 4 5 6

Ο

Ν

Signal conversion circuit

–V

DIP switches

Multiplexer

Power supply

Bus driver

Clock

Isolation

Memory

Address

decoder

Isolation

Isolation

Isolation

Α

D

0 V +5 V –5 V

Control

signals

Address

bus

Data

bus

PLC–CPU

Control

processor

of module

operation

Multiplexer

address

Analog input module