Introduction to Industrial Automation

Figure 7.37  Mixed AND and OR logic in three basic languages.

+24 V DC

Q3.0

FBD

OR

OR

OR

AND

OR

OR

AND

BOOLE

A(

O I1.4

O I1.5

)

A(

O I2.0

O I2.1

)

= Q3.0

BE

Ι1.4

Ι2.0

Ι2.1

Q3.0

0 V

Ι1.5

I2.0

I2.1

OR

I1.4

I1.5

OR

AND

LAD

I1.4

Q3.0

I1.5

I2.1

I2.0

Figure 7.38  The OR logic in combination with AND logic in three basic languages.

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Basic Programming Principles of PLCs 

◾

 

309

Logic coils are very useful in programming and simplifying complex automation circuits. 

Parts of the composite automation circuit can be replaced at no cost by logic coils (as opposed to 

the cost of auxiliary relays in conventional automation) leading to a drastic simplification of the 

form of the circuit and hence of the overall PLC program. Let’s consider the automation circuit 

of Figure 7.40a, which includes nine switching contacts as inputs to the PLC in a combination 

of branches of parallel and serial logic. Instead of programming the classical logic of the automa-

tion circuit faithfully, logic coils can be introduced as intermediate steps of this logic. In particu-

lar, the combination of the switching contacts I0.0, I0.1, and I2.0 is replaced by the logic coil 

M6.0, as shown in Figure 7.40b. In the next simplification step, the combination of the switching 

PLC

INPUTS

OUTPUTS

0 V

+24 V DC

I 1.0

I 1.4

C

1

Q 0.2

I 1.2

PLC

INPUTS

OUTPUTS

I 1.0

I 1.4

Q 0.2

I 1.2

+24 V DC

PLC

Inputs

Outputs

+24 V DC

0 V

I1.0

I1.4

S

1

S

3

Q0.2

S

2

I1.

2

0 V

S

1

d

0

S

2

S

3

d

0

C

1

(a)

(b)

Figure 7.39  The auxiliary relays of conventional automation are the internal digital elements 

(bits) in a PLC. (a) Automation circuit and (b) corresponding wiring for a PLC-based automation.

0 V

Q5.0

I4.0

I4.1

+24 V DC 

0 V

+24 V DC 

Q5.0

I4.0

I4.1

M7.1

M6.0

I1.4

I3.0

I3.2

I3.3

0 V

+24 V DC

I0.0

I0.1

Q5.0

(a)

(b)

(c)

I1.4

I4.0

I4.1

I3.0

I3.2

I3.3

I2.0

Figure 7.40  Use of auxiliary bits (logic coils) for the simplification of complex circuits/programs. 

(a) Example automation circuit, (b) first simplification by using a logic coil, (c) second simplification