Introduction to Industrial Automation

by using another logic coil.

---

310

 

◾

  Introduction to Industrial Automation

contacts I1.4, I3.0, I3.2, I3.3, and M6.0 is replaced by the logic coil M7.1, whereby the circuit of 

Figure 7.40c is obtained that is much simpler than the original circuit. The required program in 

Boolean language to implement the above simplification steps is as follows:

A I0.0

A I0.1

O

A I2.0

= M6.0

A M6.0

A I1.4

O

A I3.0

A I3.2

A I3.3

= M7.1

A M7.1

A I4.0

A I4.1

= Q5.0

BE

Permanent activation of variables. As mentioned in the explanation of the basic instructions of 

Table 2.1, the instructions set and reset for permanent activation/deactivation are the digital imple-

mentation of the latch relay shown in Figure 7.41 (left). The latch relay has two coils, one for actuation 

(which is permanent once it happens, regardless of whether or not there is voltage at the ends of the 

coil) and a second coil to deactivate it, with the same characteristic of the residual state. Therefore, the 

latch relay requires two control signals, which are provided by the switching contacts of the sensors S

1

 

and S

2

. In programmable automation, the relay C

1

 no longer needs to be a latch relay (because of the 

higher cost) but is replaced by a common power relay and the “lock” function is implemented in the 

program using the S and R instructions. For the connection of the I/O devices to the PLC, shown 

in Figure 7.41 (middle), the required LAD program for permanently activating/deactivating a digital 

output (Q0.1) is shown in Figure 7.41 (right). The corresponding Boolean program is as follows:

A I1.0

S

Q0.1

A I1.4

R Q0.1

BE

+24 V DC 

PLC

Inputs

Outputs

Q0.1

I1.4

Implementation with PLC

S

R

Conventional circuit

0 V

S

2

C

1

S

1

Latching

relay

0 V

C

1

+24 V DC

0 V

+24 V DC

I1.0

S

1

S

2

Program (LAD)

I1.0

Q0.1

I1.4

Q0.1