Introduction to Industrial Automation

Figure 7.23  The RLO in relation to the status “0” (a) or “1” (b) of an auxiliary bit (logic coil).

S.RLO = “0”

AND

AND

Program

AND

AND

AND

AND

AND

AND

PLC

INPUTS

OUTPUTS

+24 V DC 

I1.0

I2.0

0 V

11

0

PLC

INPUTS

OUTPUTS

1

0

Α    Ι1.0

S.RLO = “1”

Α    Ι2.0

S.RLO = “1”

ΑN    Ι1.0

S.RLO = “0”

ΑN    Ι2.0

AND

AND

AND

AND

PLC

Inputs

Outputs

1

0

S.RLO = secondary RLO

P.RLO = primary RLO

P.RLO

i

P.RLO

i+1

P.RLO

i+2

P.RLO

i+3

P.RLO

i+4

Figure 7.24  The procedure of RLO creation in a Boolean program.

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  Introduction to Industrial Automation

This result is easily checked by looking at the generated RLO per scan cycle. With the utilization 

of this simple program, it is easy to explain why, in the case of PLCs, each program is executed 

only once in every scanning loop without feedback toward the instructions that have already been 

executed, and of the new states created in the activated or disabled variables. If such a situation did 

take place, then the program would lead the PLC to an endless loop of repetitions, from which it 

would never escape and therefore no control action would be applied.

The program of Figure 7.25 can also be used to measure the duration of a scan cycle when the 

PLC has no such diagnostic direct measurement. The auxiliary variable M1.0 has the behavior of 

a pulse train (1, 0, 1...) whose period has a two-cycle sweep time. The pulses of the variable M1.0 

can be counted using a counter. By programming the counting from the counter of a large number 

of pulses of M1.0, and by combining the counter function with the measurement of corresponding 

time based on a timer, it is possible, with simple mathematical operations, to calculate the time 

duration of the PLC scan cycle for the specifically executed program.