Introduction to Industrial Automation

7.4   Basic Programming Instructions

It should be made clear from the beginning that what it is called “software for PLC program-

ming” is a complex software environment with many general operations offered to the user, such 

as the downloading/uploading of an automation program to/from the PLC, online monitoring of 

the program execution, run-stop controlling of the PLC via the software environment, hardware 

selection and configuration, and many others, as well as writing an automation program. Under 

the term “programming of a PLC” two main tasks are involved:

 

◾

The configuration and the general setup of the PLC, which includes the establishment of 

communication between programmer device and PLC

 

◾

The programming of the PLC functional logic operation

In many cases, engineers are only focused on the second part of pure programming, while 

ignoring the overall configuration of the PLC, a factor that is a key issue to the success of the 

overall automation. Most PLC manufacturers are offering a joint software package for program-

ming PLCs of different types and technical features (at least for a family of PLCs). Furthermore, 

as has been presented, many PLCs offer a series of hardware configurations (I/O modules, CPU, 

memory, etc.) that require careful selection. The automation program that is going to be loaded 

to the PLC is hardware agnostic, which means that the software is not aware about the kind of 

available hardware which cannot be detected automatically. Thus, for the proper execution of an 

automation program, it should be stated as precisely as it can be for the exact type of available 

hardware. In particular, it should state the type of CPU, the kind and number of I/O modules, 

the type of communication processor if it exists. Also, it should be able to recognize or select the 

I/O addresses. This work is performed by the user within the user-friendly software environment 

of the PLC and is called “configuration of the PLC hardware”. The remainder of this chapter will 

focus on the second task of basic programming of the automation logic included in the automa-

tion program.

The software environment of a PLC, regardless of the manufacturer, includes a large number 

of programming instructions; however, since the main objective is the comprehension of the way 

that PLC programming works, which does not extend to all possible PLC instructions, what fol-

lows will be a proper set of instructions from the three basic languages, namely Boolean, FBD, and 

LAD. With these instructions, listed in Table 7.1, the automation engineer will be able to handle 

most of the needs of industrial applications.

In the first three columns of Table 7.1, the syntax of each instruction is provided for the 

three programming languages: Boolean, FBD, and LAD, respectively. In the places where there 

is an empty cell, this indicates that the specific instruction does not exist in the corresponding 

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