PLC (Programmable logic controller) Interview questions

1.What is PLC?

PLC are solid state members of the computer family using integrated circuit instead of electromechanical devices to implement control functions.

2.What are the two basic sections of PLC? CPU The Input/Output interface system

3.What are the three major components of CPU? Processor Memory system System power supply

4.Explain logic functions of “AND” and “OR” gate?

5.Explain logic functions of “NOT” gate?

6.Explain timer instructions in ladder logic programming ?

7.Explain counter instructions in ladder logic plc programming ?

8.Differentiate SCADA and PLC?

9.Common PLC programming languages ? Ladder logic Functional Block diagram Sequential Function Chart

10.How to select a PLC (Selection Criteria ) ?

Cost of Hardware and Software Reliability Flexibility Scalability
Ease of database configuration Graphics development Interlocks and batch processing Integration of high level application

11.Leading PLC brands?

  1. American

Allen Bradley Modicon Texas Instruments General Electric

  1. European

Siemens Festo Telemechanique


Toshiba Omron Fanuc Mistubishi