Introduction to Industrial Automation

10.6   Retentive Reciprocating Movement of a Worktable

Retentive operation in a PLC means that in the case of a power interruption, its energized internal 

elements (M, T, C, and O) preserve their state that existed before the interruption. When the 

power supply is retrieved, the program of the PLC runs based on the conserved states of these 

internal elements. In other words, the PLC operation is continuous from the point that it had 

stopped. Suppose that the memory bits M10.0 to M10.7 have retentive property. The mecha-

nism shown in Figure 10.6 has a worktable that performs a continuous reciprocating movement 

which (up-down) starts and stops by the same button alternately. The LAD program of the figure 

only performs the reciprocating logic without the exiting input and does not satisfy a retentive 

LS

2

LS

2

LS

1

LS

1

M

0 V

0 V

Start/

stop

+24 V DC

+24 V DC

Inputs

Outputs

PLC

C

1

 = Up, C

2

 = Down

C

1

C

2

I0.0

Q2.0

Q2.0

Q1.0

Q1.0

Q1.0

I0.1

I0.1

I0.1

I0.2

I0.2

I2.0

I0.2

LAD