Introduction to Industrial Automation

Figure 7.18  There are limitations to the number of instructions per rung and to the number of 

sub-rungs.

---

Basic Programming Principles of PLCs 

◾

 

297

simple activation of the auxiliary bit M0.0, but the output Q1.0 remains deactivated, as opposed to 

the result of the first program. It should be noted that the reset of output Q1.0 does not depend on 

I2.0 and M0.0, but on some other third variable, and hence it has been omitted from the programs 

o
of the program; however, since this is not the case in many problems, it does not mean that every 

time that the LAD language is utilized, special focus should be provided in the order of rungs. 

Instead, as a general concept, it is important to keep in mind that only in the cases of debugging 

the produced software may the order of rungs affect the automation program and be the source 

of unexpected results. For an automation program written in Boolean language, there are similar 

conclusions while, additionally, it should be considered that among similar activation instructions, 

the last one overrides the previous ones. For example, the following program is considered:

A

I0.0

=

Q3.0

..

.

A 

I2.0

=

Q3.0

where the output Q3.0 may be activated by input I0.0 and deactivated due to input I2.0. Thus, the 

final result will be Q3.0 = “0”, because this is what is finally executed and therefore is saved last.

Additional instructions. Apart from the Boolean instructions OR and AND that have been 

included i

variable, and therefore without the first sub-action of the instruction. This means that with the 

instruction “OR without a parameter”, the CPU does not search in an area of the memory to find 

a desired value (state). For example, the instruction O without a variable is equivalent to creat-

ing a return path, as shown i

S

M0.0

M0.0

Q1.0

I2.0

S

M0.0

M0.0

Q1.0

I2.0

R1

R2

R1

R2

(b)

(a)