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Ladder And Functional Block Diagramming Add in library

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Describe about the Ladder and Functional Block Diagramming?



Programmable Logic Controller is a the part of industrial environment, which is used for facilitating automation in factory assembly lines. It is basically a digital computer which is used to control the machines of factory assembly lines and lighting fixtures.

PLC programs should be documented to track the process of PLC. If there are any error in the program then it is used to track back to know the area where the problem occurs. The documentation of PLC is important, and the steps to be followed to document PLC are:

The documentation would be in the form of flow chart to understand the process graphically, with a proper explanation the program step-by-step.

All the inputs and outputs are to be listed out which helps the programmer to understand how the functions are working.

All the step-by-step programs are to be documented so that the function of tracking back operations becomes easier.

PLCs have positively impacted process control and automated systems.Automation is a technology which is concerned about performing various process with the help of programmed commands, the commands being used to execute the proper instructions. However the process of automating a systems results in a more complex as well as sophisticated system. It represents both levels of capability as well as performance to surpass the ability of human beings to achieve the same activity. Using PLCs, the process control system provides both accuracy and stability in the production process. Many operational tasks which are complex are to be solved by using PLC. In an automated system, PLC becomes the central part of the process control system. At the time of running more complex processes, more PLCs are to be connected with the central computer. Now-days, PLCs are to be used to control and automate the job in single machine to increase the automation of manufacturing or testing process.

The limits or disadvantages of using PLCs in process control and automated systems are the lack of standardization. This causes much confusion if the PLC used for an application is replaced by one from a different manufacturer or if a PLC programmer is replaced by a person with a different understanding of PLC programming

Advantages of Flow chart in PLC:

It is simple to understand and also describes a process graphically, so that the process which is being used is understood by everyone on the plant.

Flow chart will explain the program step-by-step.

The time required to design a control system should be decreased by using flowchart programming.

Disadvantages of Flow chart in PLC:

As the symbols of flow chart cannot be typed so creation of flow chart will be a problem.

At the time when the program logic is complicated then the flow chart also becomes complex.

Advantages of Truth table in PLC:

The truth table in PLC describes what actually the circuit should do.

In the truth table, the activities of the circuits are described properly, such that one can understand the exact logic following which the circuits operates.

Using truth table, theapplications of the logic gates used to design the circuit can easily be understood.


Disadvantages of Truth table in PLC:

It is difficult to understand the logic gate when the circuit is complex.

Advantage of Logic function diagram to design in PLC:

Logic function diagram has developed a programming language which is used to represent a program by a graphical diagram based on thecircuit diagrams of relay logic hardware.

Disadvantage of Logic function diagram to design in PLC:

One of the disadvantages of designing the circuit is the lack of portability

Offline programming: It allows modification as well as development of programs without being connected to PLC processor.

Online programming: The ability to modify, monitor and create data as well as program sequences at the time when PLC processor is both online and/ or in run mode.

Advantages of Online Programming: It allows changes without stopping the immediate feedback as well as process.

Disadvantages of Online Programming: the entire circuit experiences various risk situations in case of any failure: damages may occur in the equipment , besides giving rise to  safety problems.

Advantages of Offline Programming: It is safer as the machine gets stopped in case of a failure and affected personnel will be able to detect the issues when a new code is to be tested.

Disadvantage of Offline Programming: It causes error as offline program contains untested code.

The methods be used to debug the PLC program are the following:

Static Analysis: It analyzes the software to gain some information about it. It gains accesses to the actual instructions that the software has been executing. The limitation of static analysis is that it requires access to the source code and it is unable to find the issues which are related to operational deployment environment.

Dynamic Analysis: It is used to test the software: in order to identify whether it functions properly under various hypothetical situation. The advantage of such system is that it needs to test a system in an operating condition to perform the test:  it does not require any source code.

The limitation of such an analysis is that it is not able to test the actual instructions which are being executed.

Correction-based Debugging: It is used to obtain the accuracy while considering the desired output responses at the observation point. It is a cost effective method. The limitation is that it consumes a lot of time to debug the PLC programs (Sulflow and Drechsler, 2015).

A programming panel is used at the time when a program is to be transferred from PC to PLC After transferring the program, an option is to be given to compare this program. PC is evolved to include the functionality of PLC. PC is a robust controller which allows operating in harsh environment. The comparison between the programs is to be done based on some major areas such as operation, serviceability, security, safety, robustness and hardware integration. PC can accomplish the same reliability as PLC. Safety is major concern for the entire user.The comparison is important to identify which is best PC or PLC.



Bolton, W. (2015). Ladder and Functional Block Diagramming. [online] Available at: [Accessed 27 Jun. 2015].

Lipson, P. and Zalm, G. (2015). [online] Available at: [Accessed 27 Jun. 2015].

Programmable Logic Controller. (2015). 1st ed. [ebook] Available at: [Accessed 27 Jun. 2015].

Rockwell, (2015). Programmable Logic Controller. [online] Available at: https://file:///C:/Users/AVERI/Downloads/pet10882OLCSampleChapterconstrained72.pdf [Accessed 27 Jun. 2015].

Sulflow, A. and Drechsler, R. (2015). Automatic Fault Localization for Programmable Logic Controllers?. 1st ed. [ebook] Available at: [Accessed 27 Jun. 2015].

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