Introduction to Industrial Automation

Figure 7.2  Programming elements of FBD language.

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Basic Programming Principles of PLCs 

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instructions are written one after the other as a list, hence the language received the IL name 

from IEC 61131-3. An example of a small Boolean program is as follows:

LD

I0.0

A

I0.1

=

Q0.0

LD

I0.4

O

I0.5

=

Q0.1

  4. Structured Text (ST)—Language Based on Text.

    ST is a high-level language whose syntax resembles that of Pascal language. For program-

mers that are already familiar with programming in a high-level language, such as Basic or 

C, it will be very easy to program in the ST language. In general ST supports complex and 

embedded instructions such as:

–  Repeat-until, while-do

–  Execution under conditions (IF-THEN-ELSE, case)

–  Functions (SQRT (•), SIN (•))

    Any of these programs starts with the declaration of the variables and then follows the main 

text of instructions, separated by a semicolon. Variables can be clearly defined as numeric 

values (e.g., integer, real, etc.), internally stored variables, or inputs and outputs. Empty spaces 

can divide the instructions from the variables, and it is allowed to parse and insert comments 

to increase the readability of the program. An example of the ST language is:

FR D(bcd_input, delay_time);

t.PRE :=delay_time;

IF (test_input)  THEN

t.EnableTimer:=1;

EL SE

t.EnableTimer:=0;

EN D_IF;

TO NR(t);

Set:=t.DN;

    This program reads the “bcd_input” in a form of a BCD code format and is used to change 

the delay time of the TON timer. When the “test_input” input is activated, the timer starts 

counting and after a specific amount of time the “set” is activated. An important difference 

AND          INPUT 1

A      I1

Operand or

variable name

Operation

or function

Operation

Operand

AND          INPUT 1

Writing abbreviation

A          I1