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PLC Important Questions and Answers

This post covers the PLC important questions and answers which are discussed in an interview for automation engineer or PLC engineer.

PLC Important Questions

PLC Important Questions and Answers

What is PLC ?

An industrial  grade computer that is capable of being programmed to perform control functions.

Identify four tasks in addition to relay switching operations that PLCs are capable of performing.

Performing other applications such as

  • timing,
  • counting,
  • calculating,
  • comparing, and
  • the processing of analog signals.

List six distinct advantages that PLCs offer over conventional relay-based control systems.

Increased Reliability

Once a program has been written and tested, it can be easily downloaded  to other PLCs. Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error

The program takes the place of much of the external wiring that would normally be required for control of a process. Hardwiring, though still required to connect field devices, is less intensive. PLCs also offer the reliability associated with solid-state components.

More Flexibility

It is easier to create and change a pro-gram in a PLC than to wire and rewire a circuit. With a PLC the relationships between the inputs and outputs are determined by the user program instead of the manner in which they are interconnected.

Original equipment manufacturers can provide system updates by simply sending out a new program. End users can modify the program in the field, or if desired, security can be provided by hardware features such as key locks and by software passwords.

Lower Cost

PLCs were originally designed to re-place relay control logic, and the cost savings have been so significant that relay control is becoming obsolete except for power applications.

Generally, if an application has more than about a half-dozen control relays, it will probably be less expensive to install a PLC.

Communications Capability

A PLC can communicate with other controllers or computer equipment to perform such functions as supervisory control, data gathering, monitoring devices and process parameters, and download and upload of programs. User program PLC All the logic is contained in the PLC’s memory.

Relationships between the inputs and outputs are determined by the user program. Contactor, Light, Solenoid are Outputs. Inputs are Pushbutton, Limit switch, Sensor.

Faster Response Time

PLCs are designed for high-speed and real-time applications. The programmable controller operates in real time, which means that an event taking place in the field will result in the execution of an operation or output.

Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick-response capability.

Easier to Troubleshoot

PLCs have resident diagnostics and override functions that allow users to easily trace and correct software and hardware problems. To find and fix problems, users can display the  control program on a monitor and watch it in real time as it executes.

Explain the differences between open and proprietary PLC architecture?

An open architecture design allows the system to be connected easily to devices and programs made by other manufacturers. Open architectures use off-the-shelf components that conform to approved standards.

A system with a closed architecture is one whose design is proprietary, making it more difficult to connect to other systems. Most PLC systems are in fact proprietary, so you must be sure that any generic hardware or software you may use is compatible with your particular PLC.

Also, although the principal concepts are the same in all methods of programming, there might be slight differences in addressing, memory allocation, retrieval, and data handling for different models. Consequently, PLC programs cannot be interchanged among different PLC manufacturers.

State two ways in which I/O is incorporated into the PLC.

There are two ways in which I/Os (Inputs/Outputs) are incorporated into the PLC: fixed and modular.

Fixed I/O

Fixed I/O is typical of small PLCs that come in one package with no separate, removable units. The processor and I/O are packaged together, and the I/O terminals will have a fixed number of connections built in for inputs and outputs.

The main advantage of this type of packaging is lower cost. The number of available I/O points varies and usually can be expanded by buying additional units of fixed I/O.

One disadvantage of fixed I/O is its lack of flexibility; you are limited in what you can get in the quantities and types dictated by the packaging. Also, for some models, if any part in the unit fails, the whole unit has to be replaced.

Modular I/O

Modular I/O is divided by compartments into which separate modules can be plugged. This feature greatly increases your options and the unit’s flexibility. You can choose from the modules available from the manufacturer and mix them any way you desire.

The basic modular controller consists of a rack, power supply, processor module (CPU), input/output (I/O modules), and an operator interface for programming and monitoring.

Describe how the I/O modules connect to the processor in a modular-type PLC configuration.

The modules plug into a rack. When a module is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack.

The PLC processor is also connected to the backplane and can communicate with all the modules in the rack.

Explain the main function of each of the following major components of a PLC:

  • Processor module (CPU)
  • I/O modules
  • Programming device
  • Power supply module

The processor(CPU) is the “brain” of the PLC. A typical processor usually consists of a micro-processor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing the results of the logical operations performed by the microprocessor.

The I/O system forms the interface by which field devices are connected to the controller.The purpose of this interface is to condition the various signals received from or sent to external field devices.

Input devices such as pushbuttons, limit switches, and sensors are hardwired to the input terminals. Output devices such as small motors, motor starters, solenoid valves, and indicator lights are hardwired to the output terminals.

A programming device is used to enter the desired program into the memory of the processor. The program can be entered using relay ladder logic, which is one of the most popular programming languages. Instead of words, ladder logic programming language uses graphic symbols that show their intended outcome. A program in ladder logic is similar to a schematic for a relay control circuit.

The power supply : DC power to other modules that plug into the rack. For large PLC systems, this power supply does not normally supply power to the field devices. With larger systems, power to field devices is provided by external alternating current (AC) or direct current (DC) supplies. For some small micro PLC systems, the power supply may be used to power field devices.

What are the two most common types of PLC programming devices?

  1. Hand-held programming devices and
  2. personal computer (PC)

Explain the terms program and programming languages they apply to a PLC.

A program is a user-developed series of instructions that directs the PLC to execute actions. A programming language provides rules for combining the instructions so that they produce the desired actions.

What is the standard programming language used with PLCs?

Relay ladder logic (RLL) is the standard programming language used with PLCs. Its origin is based on electromechanical relay control. The relay ladder logic program graphically represents rungs of contacts, coils, and special instruction blocks.

RLL was originally designed for easy use and un-derstanding for its users and has been modified to keep up with the increasing demands of industry’s control needs.

Compare the method by which the process control operation is changed in a relay-based system to the method used for a PLC-based system.

If a relay system were used, it would require some re-wiring of the circuit to achieve the desired change. However, if a PLC system were used, no rewiring would be necessary. The inputs and outputs are still the same. All that is required is to change the PLC ladder logic program.

Compare the PLC and PC with regard to:

  1. Physical hardware differences
  2. Operating environment
  3. Method of programming
  4. Execution of program

unlike PCs, the PLC is designed to operate in the industrial environment with wide ranges of ambient temperature and humidity. A well-designed industrial PLC installation, is not usually affected by the electrical noise inherent in most industrial locations.

PLC is programmed in relay ladder logic or other easily learned languages. The PLC comes with its program language built into its memory and has no permanently attached keyboard, CD drive, or monitor. Instead, PLCs come equipped with terminals for input and output field devices as well as communication ports.

Computers are complex computing machines capable of executing several programs or tasks simultaneously and in any order. Most PLCs, on the other hand, execute a single program in an orderly and sequential fashion from first to last instruction.

A personal computer (PC) can be made to operate as a programmable logic controller if you provide some way for the computer to receive information from devices such as pushbuttons or switches. You also need a program to process the inputs and decide the means of turning load devices off and on.

What two categories of software written and run on PCs are used in conjunction with PLCs?

PLC software that allows the user to program and document gives the user the tools to write a PLC program using ladder logic or another programming language and document or explain the program in as much detail as is necessary.

PLC software that allows the user to monitor and control the process is also called a human interface (HMI).It enables the user to view a process or a graphical representation of a process on a monitor, determine how the system is running, trend values, and receive alarm conditions. Many operator interfaces do not use PLC software. PLCs can be integrated with HMIs but the same software does not program both devices.

What is a programmable automation controller (PAC)?

Most recently automation manufacturers have responded to the increased requirements of industrial control systems by blending the advantages of PLC-style control with that of PC-based systems. Such a device has been termed a programmable automation controller, or PAC (Programmable automation controllers) combine PLC ruggedness with PC functionality.

Using PACs, you can build advanced systems incorporating software capabilities such as advanced control, communication, data logging, and signal processing with rugged hardware per-forming logic, motion, process control, and vision.

List four criteria by which PLCs are categorized.

  • functionality,
  • number of inputs and outputs,
  • cost, and
  • physical size

Compare the single-ended, multitask, and control management types of PLC applications.

A single-ended or stand-alone PLC application involves one PLC

controlling one process. This would be a stand-alone unit and would not be used for communicating with other computers or PLCs. The size and sophistication of the process being controlled are obvious factors in determining which PLC to select. The applications could dictate a large processor, but usually this category requires a small PLC.

A multitask PLC application involves one PLC controlling several processes. Adequate I/O capacity is a significant factor in this type of installation. In addition, if the PLC would be a subsystem of a larger process and would have to communicate with a central PLC or computer, provisions for a data communications network are also required.

A control management PLC application involves one PLC controlling several others . This kind of application requires a large PLC processor designed to communicate with other PLCs and possibly with a computer. The control management PLC supervises several PLCs by downloading programs that tell the other PLCs what has to be done. It must be capable of connection to all the PLCs so that by proper addressing it can communicate with any one it wishes to.

What is the memory capacity, expressed in bits, for a PLC that uses 16-bit words and has an 8 K word capacity?

memory capacity is maximum value of memory on PLC for stores datam instruction, and control program.

16 * 8 * 1024 = 131072 bits

List five factors affecting the memory size needed for a particular PLC installation.

  1. Number of I/O points used
  2. Size of control program
  3. Data-collecting requirements
  4. Supervisory functions required
  5. Future expansion

What does the instruction set for a particular PLC refer to?

The instruction set for a particular PLC lists the different types of instructions supported. Typically, this ranges from 15 instructions on smaller units up to 100 instructions on larger, more powerful units.

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