Introduction to Industrial Automation

Secondary RLO

Primary RLO

Is the RLO derived from the second 

action (perform AND) and hence from 

the complete execution of the 

instruction?

The logic operation AND is performed 

between the secondary RLO and the primary 

RLO derived by the previous instruction 

execution 

The result of the AND operation is the 

primary RLO of the executed instruction 

If the executed instruction is the first one, 

then the primary RLO that has to preexist 

is created by the machine (CPU)

What the 1st action “look for 1” means?

PLC

INPUTS

OUTP UTS

+24 VDC

0 V

I 2.1

S

Input mapping memory

0

Byte 2

Bit 1

There is no voltage at input I2.1

Therefore, “0” is stored

PLC

INPUTS

OUTPUTS

+24 VDC

0 V

I 2.1

S

Input mapping memory

1

Byte 2

Bit 1

If the content is 0, then 

secondary RLO = “0”

Is the RLO derived from the 

first action (look for “1”) of 

the instruction?

There is voltage at input I2.1

Therefore, “1” is stored

The 1st action “look for 1” means, “scan the content 

of the input I2.1 memory location” and then  

If the content is 1, then 

secondary RLO = “1”

Look for “1”

perform 

with RLO

1st action

2nd action

Α Ι2.1

Ι2.1

BOOLE

or

LAD

PLC

INPUTS

OUTPUTS

I 2.1

0

PLC

INPUTS

OUTPUTS

I 2.1

1

AND

and