Introduction to Industrial Automation

7.4.2   Boolean Logic Instructions

The first instruction of the Boolean logic instructions group, depicted in Table 7.1, is the AND 

instruction. When the CPU executes this instruction, independently of the programming language, 

A  I3.3 ,

,

I3.3

I3.3

A

 

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  Introduction to Industrial Automation

it will examine whether a logical “1” is stored in the memory location of the PLC’s input I3.3, 

which means, if it is true, then at the corresponding digital input I3.3 there is electrical volt-

age. Thus, in the case that the CPU finds a logical “1” then the secondary RLO (S.RLO) is “1”. 

Subsequently, the AND function between S.RLO and the primary RLO (P.RLO) where P.RLO = 

“1” (this is generated by the CPU, since there is no other previous instruction), will yield RLO = 

“1”. From the latter, it will also depend on the execution result of the instructions that follow.

The second instruction of the group is the AND NOT statement. When the CPU executes 

one of the instructions,

AN  I3.3

,

I3.3

A

I3.3

,

it examines whether there is a logical “0” in the memory location of the digital input I3.3, which 

means that there is no electrical voltage at the corresponding input I3.3 of the PLC. Therefore, if 

the CPU finds a logical “0”, then the S.RLO is “1”. Subsequently, the AND function between the 

S.RLO = “1” and the P.RLO = “1” (this is created by the CPU because it precedes no other instruc-

tion) will give RLO = “1”. At this point, a basic differentiation of the programmable automation 

from the classical one should be highlighted. With the AND NOT instruction, it is possible to 

activate an output via an open input contact (when the contact closes, the output is deactivated), 

which cannot be implemented in conventional automation without the use of an auxiliary relay.

The third instruction in the same instruction group is the OR statement. When the CPU 

executes one of the following instructions,

O  I3.3

I3.3

O

I3.3

,

,

it examines (as in the case of AND) whether there is logical “1” in the memory location of digital 

input I3.3. If this is the case then the S.RLO is “1” and then performs the OR function between 

the S.RLO = “1” and the P.RLO = “0” (which is created by the CPU, since no other instruction 

is preceded). The final result is RLO = “1” that waits for the execution of the next OR instruction 

(for Boolean language) or the “parallel” instruction (for LAD).

The next OR NOT instruction has a similar function to the AND NOT, with the difference 

that instead of the logical AND action, the OR operation is performed. The following instructions 

for creating complex expressions are only found in the Boolean language, and have a very simple 

function that will be presented through examples in Section 7.5.