Introduction to Industrial Automation

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6.10.9   Parallelizing Digital Outputs of PLCs—

Transitional Protection of I/Os

Another problem is created with the possible parallelism of two digital outputs of a PLC in order 

to increase the power supply to the output device. Let us assume that the digital outputs of a PLC 

have a nominal operating current in the ON state of 0.5 A, while an output device that we want 

to connect to a digital output of the PLC requires a current of 0.8 A. In this case, it is obvious that 

the output of the PLC cannot directly supply the output device. Thus, it is possible to connect two 

digital outputs of the PLC to the output device (0.5 A + 0.5 A > 0.8 A) and program at the PLC the 

simultaneous activation of the two outputs. However, such a solution should be avoided, because 

the switching elements of the PLC digital outputs that have been presented are triac, power tran-

sistors, relays, etc., and cannot ensure a simultaneous operation due to the various electrical differ-

ences they present. Moreover, different initial conditions or differences in the logical update of the 

outputs may cause additional delays in activating the two outputs at the same time. Such a delay 

is able to cause a single digital output to accept the total load of the output device (0.8 A, for the 

examined example) that will resulting in a voltage drop of that digital output.

A last issue that the automation engineer should not overlook, is that of the “transitional pro-

tection” of the digital inputs and outputs when their operation is combined with external control 

or detection devices. In particular, Figure 6.52 shows an output device (inductive load) that is 

connected to the digital output of the PLC via a push button, in order to control the operation 

of the output device when the PLC has activated this output. Such a connection is possible upon 

the closing or opening of the push button contact, generating an electrical noise and, there-

fore, requiring transitional protection. Thus, when it is needed to insert a switch or any kind of 

a switching contact of a sensor between an output device and the PLC, a surge suppressor should 

be connected in parallel to the load (e.g., to the output device) in order to achieve transitional 

protection.

In Figure 6.53, the output device (inductive load) is actuated by both the PLC’s digital output 

1, and the external push button that bypasses the PLC and allows the output device to be activated 

Output module

Surge

suppressor

L1

L2

11

22

33

44

1

2

3

4

L1

L2

Inductive

load 

Output module

L1

L2

1

2

3

4

L1

L2