PLC Engineer Interview Questions

 

21^st October, 2023.

In this post, we will see an overview of interview questions generally asked for a PLC automation engineer.

Industrial automation is a very trending subject in the field of instrumentation, electrical and electronics engineering. Mostly, all the processes in an industry or commercial complex are done automatically nowadays with the help of industrial automation technologies. In this, one of the most basically and generally used techniques is PLC programming. It is the base of any automation level. So, many candidates who apply for jobs in PLC programming face some questions during interview which they had not expected. In this post, we will have a look at some general and tricky types of questions asked during interview for PLC programming and automation.

·       Difference between PC and PLC:

Both are programmable controllers, but you cannot connect hardware and field IO’s to the PC. For that, PLC specially is designed to do the job. It will read the inputs, execute the logic inside and control the outputs accordingly. Basically, you cannot interface real time IO’s with the PC, but this can be done with the PLC.

·       PLC Brands:

Some of the mostly used PLC brands are Siemens, Rockwell, Schneider, Mitsubishi, Delta, Fuji, GE Fanuc, Yaskawa, Honeywell, Omron, ABB, and Yokogawa.

·       Types of analog inputs in PLC:

The types of analog inputs in PLC are 0-10V DC, 4-20 mA, RTD, PT100, PT1000, NTC, and Thermocouples.

·       Power supply used for PLC:

The typical used power supplies for PLC systems are 24V DC, 230V AC, 110V AC, and 24V AC.

·       PLC backplane:

PLC is usually mounted on a rack system. Some of the racks are designed to carry both the data and power supply to expansion modules. They are called backplanes. You can use an expansion module without giving power supply to it, as it will take the supply from the rack directly. It is basically a common bus shared by all the modules in a PLC system.

·       Difference between fixed PLC and modular PLC:

Fixed PLC is also called as compact PLC. It means all the units of a PLC – power supply and IO units are in-built in one set. In a modular PLC, there are separate sets of power supply and IO units. They are not combined in a single unit.

·       Types of PLC IO connections:

It comes in three types – local, remote and distributed. In local IO, the IO’s are present in the same CPU set either embedded or as expansion modules. In remote IO, the IO’s are present in some other electrical panel at a far away location and their data is shared with main PLC through a communication adapter. Distributed IO is same as remote IO; the difference being that a CPU is present in place of communication adapter. This allows you to write small programs in it. This helps in reducing the memory consumption of main CPU.

·       What is sinking and sourcing in PLC?

It is a concept related to potential supply with the PLC. If a positive supply is given to the common of an IO terminal, then the current will flow from supply to the PLC and then to the field. This is called sourcing. And if a negative supply is given to the common of an IO terminal, then the current will flow from the supply to the field and then to PLC. This is called as sinking.

·       Why is 4-20 mA signal preferred over 0-10 V signal?

As voltage is involved in 0-10 V signal, there comes chances for voltage drop at larger distances due to the resistance offered. This issue does not occur in 4-20 mA signals. Due to this, there is no signal drop in current signals and you get proper readings in PLC.

·       Difference between 2-wire, 3-wire and 4-wire transmitters:

In 2-wire transmitter, power and signal are transmitted through the same wire. One carries positive supply to the field and the return wire carries both the negative supply and data to the PLC. In 3-wire transmitter, there are two separate wires for power supply. But, the third wire carries data and to mingle it with PLC, you have to connect 0V supply to the PLC common, which completes the circuit. In 4-wire transmitter, you have 4 separate wires for power supply and data; two each for that.

·       Types of PLC inputs and outputs:

PLC inputs are of two types – DC voltage and AC voltage. PLC outputs are of three types – relay (AC or DC voltage), transistor (DC voltage) and triac (AC voltage).

·       How does a PLC control a motor?

 A motor is turned on or off by digital PLC output, and it’s speed is controlled by analog PLC output.

·       Is it necessary to update PLC firmware?

Yes, because latest PLC firmware always updates it with recent features related to programming, security and communication. This increases the lifetime of the PLC in a better way.

·       Communication protocols used in PLC:

The mostly used types of protocols in PLC are Modbus RTU, Modbus TCPIP, Ethernet IP, Can Open, Profibus, and Profinet.

·       What is a watchdog timer in PLC?

A watchdog timer is something which continuously checks for completed scan time in PLC and if the maximum scan time elapses and still the scan is not over, then this timer turns on. After this, the PLC goes to stop mode and stays in this safe state. The programmer has to intervene and check the fault, and then put in run mode again.

·       What is PLC hot swapping?

Hot swapping is a feature to replace any module online, means when the plant is running and there is no need to stop the plant.

·       Difference between DINT data type and REAL data type:

DINT stands for double integer. You get a combination of 2 raised to 32 values, either signed or unsigned. But you cannot write decimals in it. For writing decimals, you have to use a real variable which allows for writing floating values and is also equally for a larger data range.

·       Difference between warm restart and cold restart in PLC:

In warm restart, the PLC is restarted without turning it off and starting the process from the last retained values. In cold restart, the PLC is restarted by turning it off and starting the process from the default values.

·       Can two PLC’s communicate with each other?

Yes, one PLC must be configured as master and the other PLC must be configured as slave. This setting is available in communication tabs of PLC program. There, you can set them accordingly and write the logic inside.

·       How to configure encoders in PLC?

For using encoders, you need to use high speed counters in PLC. As encoders cannot be used in a normal counter due to it’s speed, high speed counters are used which allows different types of configurations in it for usage.

I have covered some general and most commonly asked questions during PLC programming interviews. I have also not attempted to cover all the questions, as it can vary from case to case and there is no end to it. Once you are familiar with PLC programming and it’s usage, you can easily tackle all types of questions.

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.