Introduction to Industrial Automation

Figure 6.3  Cyclic operation (or scanning) of a PLC means continuous and repeated reading of 

inputs, user program execution, and updating of outputs: conventional circuit (a), I/O and user 

program memory in relation to I/O cards (b), and scanning cycle (c).

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  Introduction to Industrial Automation

from the PLC. However, it should also be noted that the scanning time is directly dependent on 

the speed of the microprocessor, while for a specific PLC, this time is dependent only on the size 

of the program (number of instructions) and the type of these instructions, since different instruc-

tions demand a correspondingly different execution time. As an indication of the scanning speed 

of a PLC, manufacturers usually provide the scanning time for a program that contains a set of 

instructions of 1 KB of memory.

PLCs are not programmed according to an internationally standardized programming lan-

guage that is adopted by all manufacturers. Instead, there are various forms of programming 

languages that vary from company to company under various names, even if they are similar 

in their functionality. Also, there is a significant incompatibility between similar programming 

languages developed by different manufacturers. The International Electrotechnical Commission 

standard 61131 (IEC 61131-3), which was adopted in 1993, deals specifically with PLC program-

ming languages, and defines the most basic forms that fall into two categories: graphic languages 

and text-based languages.

Despite the lack of an absolute standardization in the matter of scheduling, we can distinguish 

three main programming languages, which are: the cascade ladder diagram language (ladder), 

the instruction list language or Boolean (IL), and the language of logic elements or function 

block diagram (FBD). The most popular of these is ladder language, since it is very similar to 

the classic implementation of an automation circuit. This was also the reason for the adoption of 

this language by almost all manufacturers in the early years of PLCs, because in this way it was 

easier to spread the novel PLC technology and have it adapted by older engineers who were not 

familiar with the programming. In Figure 6.4, the general format of the three programming lan-

guages is presented, where the specific program represents a conventional automation circuit for 

the START-STOP with self-latch operation of a motor (see Section 3.1.1).

As mentioned above, the programming device for a PLC can be a PC or a specially designed 

digital device. In the second case, the programming device or programmer is not just a simple 

keyboard, but includes a liquid crystal display, a memory, a compiler, and various communica-

tion ports to communicate with the PLC or other peripheral devices. These units are necessary 

in a programming device, since the programming is not performed directly in the PLC. For the 

programming procedure, initially the program is developed in the programming device and after-

wards it is translated into a machine language that is stored initially in the programmer’s memory 

and then transferred to the PLC’s memory, provided that there is a communication link between 

the programmer and the PLC. From the programmer, it is usually feasible to monitor the opera-

tion of the PLC in order to detect the status of the PLC’s internal elements and to perform various 

diagnostic tests.

I1

Q1

I2

Q1

ANOT  I1

AND  I2

OR 

Q1

OUT  Q1

Ladder diagram

Instruction list

Logic diagram

AND

OR

Q1

I2

I1