Introduction to Industrial Automation

Figure 7.8  Content of accumulator 1 in bytes after some instruction execution.

C

0 V

+24 VDC 

S

1

S

3

S

2

V

    

“Existing or absent voltage” in wired Logic

Relay “C” is energized if there is voltage at point A 

“RLO” in Programmable Logic

Output “C” is activated if last RLO = “1”

Logic instruction      “S1”

Logic instruction      “S2”

Logic instruction      “S3”

Activate “C”

IF  RLO = "1" 

Circuit

Program

RLO = "1"

RLO = "0"

RLO = 

….

A

V

    

Is there voltage at this point? YES  

  

RLO = “1”

Is there voltage at this point? NO  

  

RLO = “0”

Figure 7.9  The RLO plays the role of existing or absent voltage metaphorically in program-

mable logic.

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  Introduction to Industrial Automation

To make the concept of RLO more understandable in connection with Boolean logic instruc-

tions, Figure 7.10 will be utilized. As presented in Table 7.1, each of the Boolean logic instructions 

includes two sub-actions: the sub-action to examine the state of the variable (0 or 1), and the sub-

action to execute the logical action specified by the instruction. Each instruction specifies what 

the CPU searches for (logical “0” or “1”) by examining the status of the variable specified in this 

Instruction operation 

Instruction

Α Ι2.1

Ι 2.1

BOOLE

or

LAD