Introduction to Industrial Automation

Figure 7.19  (a) A two-rung program, whose result depends on the writing alignment of the 

rungs, while in (b) the rungs have an opposite order than in (a).

I1.0

I1.1

I2.0

I2.1

A I1.0

A I1.1

O

A I2.0

A I2.1

O I4.0

O I4.1

I4.0

I4.1

(b)

(a)

A I1.0

A I1.1

O

A I2.0

A I2.1

O I4.0

O I4.1

Figure 7.20  The instruction OR without (a) or with (b) variable assignment.

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  Introduction to Industrial Automation

command is with reference to a variable. A similar logic also applies to the A instruction without 

a variable, for which we will see examples i

In some PLCs and in LAD language, the instructions of “transient contacts” from OFF to ON, 

and vice versa, can also be found. They are mainly utilized in order to generate a pulse with the 

duration of one scan cycle when a condition is satisfied. These instructions concern the classical A 

and AN ones or 

 and 

, with the difference that when their condition is satisfied, 

they give RLO = “1” for only a single scan cycle, regardless of whether the condition continues to 

be true
language. The instruction 

I0.0

, if the input I0.0 is activated (OFF to ON), produces an 

RLO = “1” for one scan cycle. The instruction 

I0.0

, if the input is disabled (ON to OFF), 

gives an RLO = “1” for one scan cycle. The above instructions are usually used in programming 

applications with sequential operations, mathematical operations, presetting of counters, etc.

Familiarity with the RLO. I
process by which it is created by the CPU. This subsection will examine the specific RLO created by 

executing some instructions in the LAD and Boolean languages. Let’s consider the PLC configura-

tion i
open). Then the execution of the instructions 

 and 

 will generate the RLOs that 

appear next to the instructions respectively. If the input I0.1 is activated and remains continuously 

activated (switching contact S is permanently closed) as shown i
instructions will give different RLOs and, in particular, those that are displayed next to the instruc-

tions. The conclusion that can be drawn is that in programmable automation, if an input is activated 

or not (either closed contact or open contact), whatever RLO can be generated serves the logic of 

operation, provided that the appropriate AND or AND NOT instruction has been utilized.

The same conclusions and the same observations are valid if instead of the input we have as 

variable an auxiliary bit, as shown i

I0.0

OFF-ON

I0.0

ON-OFF