PLC interview questions
Question : What is a PLC?
Answer: Programmable Logic Controllers are industrially hardened micro computers, designed to replace electro-mechanical relays, used to control machines and processes.
Question : What are the main advantages of PLC?
Answer:
1.Modular design
2. Increased reliability
3. Lower cost
4. Compact size
5. Ease of programming
6. Rugged construction and designed to withstand vibrations, temperature, humidity and noise
7. Ability to communicate with computer
Question : What are the major areas of application of PLC?
Answer:
1.Sequence control, timing, counting, and data calculation
2. Batch or continuous process control
3. Precise position/motion control
4. Open loop or feedback control, process data acquisition and display
Question : What are the advantages of PLC over Relays?
Answer:
1.They have to be hard-wired to perform a specific function
2. No hard-wire requirement
3. When the system requirement change, the relay wiring has to be changed or modified
4. Can be easily changed or expanded through program
5. Higher power consumption
6. Lower power consumption
Question : Explain the classification of PLC.
Answer:
There are five major classification of PLC based on number of inputs and outputs, cost, functionality and physical size.
They are
i. Nano : 16 I/O pins
ii. Micro : 32 I/O pins
iii. Nano or micro PLC could used on applications such as elevators, car washes or mixing machines.
iv. Small : 128 I/O pins,2Kbytes memory, capable of simple to advanced level of machine control
v. Medium: 2048 I/O pins, 32Kbytes memory, used for process control applications
vi. Large: 8192 I/O pins, 750 Kbytes memory, mostly used to control individual production process or entire plants like paper and pulp, chemical and automotive and power plants.
Question : Explain the functions of PLC-input and output modules
Answer
- Input interface modules accept signals from the machine or process devices (120Vac) and convert them into signals (5Vdc) that can be used by the controller.
- Output interface modules convert controller signals (5Vdc) in to external signals (120Vdc) used to control machine or process.
Question : List out the programming languages in PLC.
Answer:
a. Ladder Logic Diagram
b. Sequential Function Charts
c. Instruction List
d. Function Block
e. Structured Text Diagram
Question: What is Ladder Logic Diagram?
Answer:
Ladder Logic Diagram is a symbolic language used in PLC. This diagram consist of series of symbols interconnected by lines to indicate the flow of current through the various devices. In ladder logic diagram the sides of the ladder forms the power sources and current flows through the various –logic input devices that form the rungs of the ladder.
Question: List the conditions for drawing the ladder logic.
Answer:
a. Contacts may be always inserted in the upper left.
b. Coils must be inserted at the end of the rung.
c. All contact must run horizontally.
d. The number of contacts per network are limited
e. Only one outout may be connected to a group of contacts
f. Flow must be from left to right.
g. The contact must be nested.
h. Contact progression should be straight cross.
Question : Explain On-Delay and Off-Delay Timer instruction in PLC
Answer:
a) The on delay timer operates such that when the rung containing the timer is true, the timer tiomed out period commences. At the end of the timer time out period an output is made active.
b) The Off delay timer operation will keep the output energized for time period after the rung cotaining the timer has gone false.
Question: What is PLC SCAN ?
Answer:
During each operating cycle the processor reads all the inputs , takes the values and energizes or de-energizes the output according to the user program. This process is known as PLC Scan .
A single PLC scan consists of the I/O scan and the program scan.