Introduction to Industrial Automation

Figure 7.4  The form of a program in SFC language.

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Basic Programming Principles of PLCs 

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–  When a step is active, it means that the automation program is in this state and that the 

corresponding action to the step is performed.

–  Each transition and step represents or contains a whole (small or large, simple or com-

plex) automation program written in one of the other languages (e.g., LAD or Boolean).

Since the SFC language, in addition to the previous basic elements, contains elements such 

as the “OR branch” and “simultaneous steps branch”, it is suitable for applications with repeti-

tive industrial processing cycles. With these elements of SFC, someone can organize and syn-

thesize large automation programs that include sequential and/or parallel (i.e., simultaneous) 

control sub-processes. Figure 7.5 presents an SFC program for a two-door security system where 

one door requires a three-digit security code and the other one requires a two-digit security code 

to open.

The IEC 61131-3 standard, with the five programming languages described above, helped to 

clear the existing differences in the PLC manufacturers’ software and thus helped to achieve an 

overall standardization in programming. However, there are still necessary steps to be realized 

for reaching full compatibility, especially for transferring programs from one PLC to another 

from a different manufacturer. From the five previously described languages, the main focus of 

Chapter 7 will be LAD, FBD, and IL, since mastering these three will make it very easy to cope 

with the scheduling needs in SFC language, where its graphical programming relies on the three 

START

Door

lock

Step 1

Step 2

1st number

right

2nd number

right

3rd number

right

1st number

wrong

1st number

right

2nd number

right

1st number

wrong

2nd number

wrong

2nd number

wrong

3rd number

wrong

Simultaneous branches

 

 

Step 3

Step 4

5 sec delay

Step 5

Unlock 1

Lock 1

 

Step 6

Step 7

 

 

Step 8

Step 9

 

Unlock 2

Lock 2

5 sec delay

OR branch

OR branch