PLC Interview Basics

PLC Interview Basics

What Is PLC?

PLC means Programmable Logic Controller. It is a class of industrially hardened devices that provides hardware interface for input sensors and output control element. The field I/p include element like limit switches, sensors, push button and the final control elements like actuator, solenoid/control valves, drives, hooters etc.

PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.



Applications Of PLC?

PLC can be used in almost all industrial application solutions right from small machine to large manufacturing plants. Even it caters applications of redundant systems at critical process plants.

Explain The Role Of PLC In Automation ?

PLC plays most important role in automation. All the monitoring as well as the control actions are taken by PLCs. PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.

Explain The Role Of CPU ?

This component act as a brain of the system.

CPU consist of Arithmetic Logic Unit, Program memory, Process image memory, Internal timers and counters, flags It receives information from I/P device, makes decisions depending upon the information and logic written and sends information through the O/P devices.

The CPU’s are distinguished with following features:

  • Memory capacity
  • Instruction set supported
  • communication option
  • time required to execute the control program.

Explain The Role Of Power Supply In PLC System ?

Power supply provides system power requirement to processor, I/O and communication modules.

Typically the power supply has input voltage 120 V – 230 V AC or 24 V DC and back plane output current 2 A to 5 A at 5 V DC

Explain The Role Of Rack Or Chassis In PLC System ?

  • A hardware assembly, which houses the processor, communication and I/O modules.
  • It does following functions.
  • Power distribution
  • Containment of I/O modules
  • Communication path between I/O module and CPU
  • The chassis are available in different slots in various PLC systems. Additional chassis can be connected using chassis interconnecting cable.

What Is Role Of I/O Modules?

Electronic plug in units used for interfacing the i/p and o/p device in the machine or process to be controlled.

I/P module receives data from i/p devices (Pushbutton, Switches, Transmitters) and send it to processor. The O/P module receives data from processor and send it to output device (Relay, Valves).

Digital/Discrete :- Sends and Receives On/Off signal

Analog :- Sends and receives variable input or output signals

Explain The Role Of EEPROM Memory Module?

This module is inserted into processor system for maintaining a copy of project (PLC program). This is helpful in case of memory corruption or Extended power loss.

Difference Between Fixed And Modular PLC?

  • In non modular PLCs the processor will have inbuilt power supply and I/Os in one unit.
  • The modular PLC, will have separate slots for components like Power supply, I/O modules. You can select the I/Os or power supply as per the need.

What Are The Types Of I/Os?

Local – These are the I/Os placed in the PLC main rack containing CPU. These I/Os are connected to CPU through backplane.

Distributed/Remote – These are the I/O placed at remote location from the main rack containing the CPU.

These I/O’s are to be connected on communication bus like control net, device net or FIP I/O.

What Is Meaning Of Resolution In I/O Cards In PLCs ?

It is the minimum change in i/p parameter which can sensed by the i/p card. As far as Digital I/O is concerned it takes only one bit for operation. In case of analog input the resolution determines how much bits are used for input or output. For example a 12 bit resolution card means the input will come as 0 to 4095 count (2^12). For 16 bit data the counts will be from 0 – 65536(2^16). More the resolution the data will be more accurate

What Is An Analog Input Module ?

An I/O module that contains circuits that convert analog input signals to digital values that can be manipulated by the processor. The signals for pressure, flow, level, temperature transmitters are connected to this module. Typically the input signal is 4-20 mA, 0-10 V

What Is Analog Output Module ?

An I/O module that contains circuits that output an analog dc signal proportional to a digital value transferred to the module from the processor. By implication, these analog outputs are usually direct (i.e., a data table value directly controls the analog signal value).

What Is Meaning Of Universal Analog Input Card?

Normally there are different cards for different signals. But in universal input card the same channels can be configured for RTD, Thermocouple, Current or voltage input.

Give Examples Of I/P And Output Connected To PLCs?

  • Digital I/P (Pushbutton, Switches )
  • Analog I/P (Temp, Pressure, Flow, Level)
  • Digital O/P (Solenoids valves, Contactors)
  • Analog O/P (Control Valves, Speed)

Explain Source And Sink Concept ?

Sinking Source type modules gives out the current to the field digital devices while sink type modules draw current when the device is in high state

Sinking – When active the output allows the current to flow to a common ground.

Sourcing – When active, current flows from a supply, through the output device and to ground.

What Is Forcing Of I/O ?

  • Forcing the I/Os means making the desired status of I/O in PLCs irrespective of its status coming from the field.
  • In certain cases when there is problem in receiving field input /output, we can force the i/Os so that the logic takes desired state.

What Is Meaning Of Scan Time In PLC?

Scan time is the Time required to read the I/P, Process the logic and update the output in one cycle.

What Is Typical Scan Time In PLCs ? What Affects Scan Time ?

Typically it is less than 10 ms. It depends on the complexity of logic, PID algorithm etc.

How To Program PLCs ?

Every PLC manufacturer have their own software for programming the PLC. For example Siemens uses Simantic S7 Manager, Allen Bradley uses RS Logix and Modicon uses PLC pro programming software.

The programming language used is

  • Ladder Logic (LD),
  • Statement List (STL),
  • Functional Block Diagram (FBD),
  • Sequential Function Chart (SFC),
  • Instruction List (IL) etc

What Is Ladder Diagram ?

This is a programming language, which expresses a program as a series of “coils” and “contacts”, simulating the operation of electromechanical relays. The resultant program is the equivalent of an equation, which is executed continuously in a combinatorial manner. The advantage of this language is the familiarity many electricians have with the simple operation of relays. Disadvantages include the complexity of large, cross-connected programs, and the difficulty of expressing such non-binary functions as motion control and analog I/O.

What Is Redundancy ?

The capacity to switch from primary equipment to standby equipment automatically without affecting the process under control. Redundancy means provision for standby module. In case of failure of one module is running process, the standby module takes over. Hot redundancy means the changeover of control from active processor to standby processor in less than 1 scan time.

What Is The Need Of Redundancy?

In critical processes, it is important to run the plant without failure. In such case it is important to have redundancy so that even if one system fails the redundant system can take care without affecting plant.

Explain The Types Of Redundancy?

CPU redundancy: In case of CPU failure the standby CPU takes care of the plant

Power Supply redundancy: In case the power supply fails the standby power supply takes control of the situation.

Communication : Multiple communication channels are provided to take care of communication failure

I/O Redundancy : Multiple I/O channels are provided to take care of input or output failure.

What Are Components Of Redundant PLC System ?

Typical component on Schneider Redundant PLCs

The backplane used is either 4 slot o6 6 slot with

  • Power Supply
  • Controller with built-in Modbus Plus and Modbus ports
  • Optional dual cable Modbus Plus
  • Optional fiber optic Modbus Plus
  • CHS Hot Standby module
  • Dual cable Remote I/O Head

The master and Standby configuration must be identical