Introduction to Industrial Automation

Figure 7.42  The conventional START-STOP circuit with self-holding logic can be implemented 

in a PLC via instructions S and R.

PLC

Inputs

Outputs

0 V

+24 V DC 

0 V

+24 V DC

I1.0

I1.4

S

1

S

3

C

1

Q0.3

S

2

I1.2

e

C1

I2.0

I2.4

I2.2

START

STOP

1

STOP

2

I2.6

Melt level 

Water level

Temperatur

e

Blender

Figure 7.43  Operation of an industrial blender using instructions S and R.

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  Introduction to Industrial Automation

The motor C

1

 is switched on by pressing the START button, AND the material level sensor is acti-

vated, AND the water level sensor is also on. The machine stops operation if any of the two STOP 

push-buttons are pressed, OR if the temperature sensor is activated, OR if the thermal sensor of 

the motor is activated. All sensors and the thermal overload switch are shown in the deactivated 

state in the corresponding connection diagram in the PLC. The required Boolean language pro-

gram to achieve the above function is the following one:

A

I1.0

A

I1.2

A

I1.4

S

Q0.3

ON I2.0

ON I2.2

O

I2.4

ON I2.6

R

Q0.3

BE

Impulse generation. This term denotes the activation of a variable (logic coil or output) for a 

single scan cycle if a condition (RLO = “1”) is satisfied regardless of the length of time that the 

condition is satisfied. It essentially refers to the implementation of the transitional contacts, men-

tioned previously in Section 7.5.2. The variable that is triggered for a single scan cycle (impulse 

generator) can then be used to perform functions that have to take place only once. As an exam-

ple, let’s suppose that the switching contact of the sensor S

1

, shown in the classical automation 

circuit of Figure 7.44a, closes and activates the relay C for a sufficient time. It is desired that 

upon activation of S

1

, the logic coil M11.0 and the output Q0.2 (indicator light) of the PLC 

(shown in Figure 7.44b) are momentarily activated for a single scan cycle. The behavior of the 

logic coil M11.0, with respect to the input I1.2, is depicted in Figure 7.44d. The LAD language 

Sensor

N

C

S

1

R

(a)

(b)

(c)

(d)

1 SC

Ι1.2

Μ11.0

Program (LAD)

I1.2

M11.0

I1.2

S

R

M11.1

M11.0

M11.1

M11.1

M11.0

Q0.2

Ι1.2

Q0.0

PLC

Inputs

Outputs

0 V

C

+24 V DC

0 V

+24 V DC

Q0.0

I1.2

Q0.2

Sensor

S

1

SC = scan cycle

Figure 7.44  Creation of one-scan cycle pulse in relation to the long-term activation of a digital 

input. (a) The automation circuit, (b) the electrical connections to the PLC, (c) the LAD program,