Introduction to Industrial Automation

instruction’s execution.

Table 7.2  The Logic Result Differs Only in the First Scan Cycle if 

the LAD Program of Figure 7.14 Is Executed in Serial or Parallel

Serial 

Execution

Scan

1st

Cycle

2nd

Parallel 

Execution

Scan 

1st

Cycle

2nd

Q2.0

Q4.0

1

1

1

1

Q2.0

Q4.0

1

0

1

1

Q0.0

I

I6.0 

I1.0

I2.0

I3.0

I4.0

I5.0

Figure 7.15  A LAD program rung, whose instructions are not required to all be executed if the 

CPU has the related capability.

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Basic Programming Principles of PLCs 

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the way that it is designed to operate within the PLC in combination with the other digital units, 

which is generally a matter for the manufacturer.

The next limitation in the intelligence of a PLC is the fact that the CPU can never activate a 

digital output that requires a “flow of the power in the reverse direction”, which is the realization 

of logic from the right to the left that is in contrast to the philosophy of the ladder logic expressed 

in Figure 7.1. Let’s consider the conventional automation circuit of Figure 7.16 that works perfectly 

without any problem. The relay F is activated if a simultaneous activation of one of the following 

combinations of switching contacts occurs:

 

ABC ADK EK and EDBC

,

,

,

From these, the latter combination (EDBC) requires a “backward current flow”, as it is called, 

which means the current flow is from the right to the left, which is something that does not cre-

ate any problem in conventional automation and wired logic, and the circuit works correctly. 

However, if it is needed to implement the circuit of Figure 7.16 as a LAD program—although it is 

not feasible—then this implementation will most likely result in the hypothetical LAD program 

of Figure 7.17a. In this presented program and, in particular, in the switching element D, the CPU 

needs to apply a logic from the right to the left, which is not possible, based on the PLC’s current 

capabilities. The term “not feasible” means that it is not possible to insert such a program, because 

once there is an attempt to insert a branch to the left, the software environment will indicate a 

A

B

C

F

D

K

E

R

N

Figure 7.16  A conventional automation circuit which can’t be realized in a PLC with an identi-

cal logic.

Α

Β

F

Α

Α

ΒΒ

C

C

Α

Β

C

F

C

E

D

K

Α

Β

C

Α

D

K

E

K

E

(b)

(a)

Β

C

D

Figure 7.17  Non-feasible logic in a LAD program (a) and the modification of the logic in order