Introduction to Industrial Automation

Figure 6.41  A small-size industrial enclosure with a modular PLC.

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  Introduction to Industrial Automation

PS

C

P

U

C

P

M

INDUCTIVE

COUPLING

Network

Ε

RADIATIVE

COUPLING

PLC

C

R, S, T, N

R, N

Galvanic coupling

PS

C

P

U

C

P

M

Inductive

coupling

Capacitive

coupling

Η

Radiative

coupling

C

R, S, T, N

R, N

Figure 6.42  Various forms of electric or magnetic coupling and interference creation.

Plc

Ps

C

P

U

C

P

Ps

C

P

U

C

P

Power supply 

Grounding and potential equalization system 

Signal wire

s

Inputs

Outputs

PLC

PS

C

P

U

C

P

Electromagnetic environment

Lightning protection system versus

electrostatic and electromagnetic fields

Electromagnetic environment

Power supply 

Grounding and potential equalization system

Signal wire

s

Inputs

Outputs

Figure 6.43  Passageways of electromagnetic interference between a PLC and the existent elec-

tromagnetic environment.

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Basic Operating Principles of PLCs 

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types. For this reason, in cases of debugging, it is not easy to identify when something is the source 

of electromagnetic interference, which therefore causes general malfunction. The frequency of an 

interference signal may range from a few Hz to 100 MHz, while the currents and voltages gener-

ated are not easily measured. It is therefore preferable, in order to avoid the debugging task for 

identifying the problematic hardware, to design the entire automation system from the beginning 

according to the rules for avoiding electromagnetic interference. For example, the connection 

cables of analog inputs and outputs must always be fitted with a protective copper or aluminum 

braided shielding. Digital I/O modules already have a basic protection against interference, due to 

the optical isolators that are inserted in the corresponding circuits, as we have seen in Sections 6.3 

through 6.7. The communication cables should also withstand electromagnetic interference and 

be connected according to the manufacturer’s instructions. In Figure 6.44, the RS 485 connec-

tion interface is shown between two PLCs through a twisted pair cable with a protective braided 

shielding that has to be grounded at both ends.