Programmable Logic Controller (PLC) and Supervisory Control And Data Acquisition (SCADA) Interview Question and Answers
Programmable Logic Controller (PLC) and Supervisory Control And Data Acquisition (SCADA) Interview Question and Answers
PLC Interview Questions
1. 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 elements. The field inputs include elements like limit switches, sensors, and push buttons, while outputs include actuators, solenoid/control valves, drives, and hooters. PLC senses the input through input modules, processes the logic through CPU and memory, and gives output through output modules.
2. Applications of PLC?
PLC can be used in almost all industrial applications, from small machines to large manufacturing plants. It also supports redundant systems for critical process applications.
3. Explain the role of PLC in automation.
PLC plays the most important role in automation. All the monitoring and control actions are taken by PLCs. It senses inputs, processes logic, and provides the required outputs for efficient process control.
4. Explain the role of CPU.
The CPU is the brain of the PLC system. It consists of Arithmetic Logic Unit (ALU), program memory, process image memory, internal timers, counters, and flags. It receives data from input devices, processes it, and sends control signals to output devices. The performance of a CPU is determined by memory capacity, instruction set, communication options, and execution time.
5. Explain the role of power supply in PLC system.
The power supply provides the necessary power for the processor, I/O, and communication modules. Typical input voltages are 120–230 V AC or 24 V DC, and the backplane output is around 5V DC, 2A–5A.
6. Explain the role of rack or chassis in PLC system.
A chassis is a hardware assembly that houses the processor, communication, and I/O modules. It provides power distribution, containment, and communication between modules. Multiple chassis can be connected using interconnecting cables.
7. What is the role of I/O modules?
I/O modules interface between field devices and the processor. Input modules receive data from field devices like switches and sensors, while output modules send control signals to actuators or relays. I/O modules are categorized as digital/discrete (on/off) and analog (variable signals).
8. Explain the role of EEPROM memory module.
The EEPROM memory module stores a copy of the PLC program, ensuring data retention in case of power failure or memory corruption.
9. Why is the 4–20 mA signal preferred over 0–10 V?
4–20 mA is preferred because it eliminates signal noise, maintains accuracy over long distances, and allows detection of wire breaks (since 4 mA is the live zero).
10. What is remote mount with MTA option?
MTA (Mass Termination Assembly) allows for easy remote connection in instruments like vortex flowmeters. MTA connectors, such as MTA-100 or MTA-156, simplify wiring.
11. How to write a program for a single push button using ladder logic?
Create a push button input, use a counter to track presses, compare the counter value, and energize or re-energize coils based on count conditions.
12. Difference between fixed and modular PLCs?
Fixed PLCs have built-in power supply and I/Os in one unit. Modular PLCs allow separate selection of power supply, CPU, and I/O modules as per requirements.
13. What are the types of I/Os?
Local I/O: Located in the same rack as CPU.
Distributed/Remote I/O: Located away from CPU and connected via communication networks like DeviceNet or ControlNet.
14. What is the meaning of resolution in I/O cards?
Resolution defines the smallest input change that can be detected by an I/O card. Example: 12-bit resolution = 0–4095 counts; 16-bit = 0–65536 counts.
15. What is an analog input module?
An analog input module converts analog signals (like 4–20 mA, 0–10 V) into digital data for the processor. Used with transmitters for pressure, flow, temperature, etc.
16. What is an analog output module?
An analog output module converts digital values from the CPU to proportional analog signals (e.g., 4–20 mA, 0–10 V) for controlling actuators.
17. What is a universal analog input card?
A universal card allows configuration of each channel for RTD, thermocouple, current, or voltage input without separate modules.
18. Give examples of inputs and outputs connected to PLCs.
Digital I/P – Pushbuttons, Switches
Analog I/P – Pressure, Temperature, Flow
Digital O/P – Solenoid valves, Contactors
Analog O/P – Control valves, VFD speed control
19. Explain source and sink concept.
Sinking: When active, output allows current flow to ground.
Sourcing: When active, current flows from supply through load to ground.
20. What is forcing of I/O?
Forcing allows overriding actual I/O status for testing or troubleshooting, setting I/O states manually regardless of field conditions.
21. What is meaning of scan time in PLC?
Scan time is the total time taken by the PLC to read inputs, execute the program, and update outputs once.
22. Typical scan time and factors affecting it?
Usually less than 10 ms. It depends on program size, logic complexity, and communication overhead.
23. How to program PLCs?
Manufacturers provide their software: Siemens – STEP 7, Allen Bradley – RSLogix, Modicon – Concept. Common languages: Ladder Logic (LD), Function Block (FBD), STL, SFC.
24. What is Ladder Diagram?
A graphical programming method using contacts and coils, resembling relay logic. It’s intuitive for electricians but complex for large programs.
25. What is redundancy?
Redundancy means having a backup module that automatically takes over when the main system fails. Hot redundancy ensures switchover within one scan cycle.
26. Why is redundancy needed?
Used in critical processes where continuous operation is mandatory, preventing shutdowns due to hardware or communication failure.
SCADA Interview Questions and Answers
29. What is SCADA?
SCADA stands for Supervisory Control and Data Acquisition. It is a software-based system used to monitor, control, and analyze industrial processes through communication with field devices such as PLCs and RTUs.
30. Explain the architecture of SCADA.
SCADA architecture typically includes:
• Field Devices (Sensors, Transmitters, Actuators)
• RTUs / PLCs – Interface between field devices and control room
• Communication Network – Ethernet, RS-485, Modbus, etc.
• SCADA Server – Collects, stores, and processes data
• HMI/Workstations – Displays data and allows operator control
31. What is the difference between PLC and SCADA?
| PLC | SCADA |
|---|---|
| Hardware-based control system | Software-based monitoring system |
| Executes control logic | Monitors and visualizes process data |
| Works in real time at field level | Supervises multiple PLCs/RTUs from a central location |
| No graphical interface | Provides graphical HMI for operators |
32. What are the main components of a SCADA system?
• Master Terminal Unit (MTU) / SCADA Server
• Remote Terminal Units (RTU) or PLCs
• Human Machine Interface (HMI)
• Communication network
• Database / Historian
• Engineering workstation
33. What is an HMI in SCADA?
HMI (Human Machine Interface) is the operator interface of a SCADA system used to monitor process variables, visualize alarms, and send control commands to field devices.
34. What is a historian?
A historian is a database within the SCADA system that stores time-stamped process data for trend analysis, reporting, and troubleshooting.
35. What is redundancy in SCADA?
Redundancy provides backup for servers, communication paths, or power sources to ensure continuous operation even if a primary system fails.
36. What are the communication protocols used in SCADA?
Common protocols: Modbus, Profibus, DNP3, OPC, Ethernet/IP, and IEC-60870-5-104.
37. What is OPC in SCADA?
OPC (OLE for Process Control) is a standard interface that allows communication between different devices and software systems from multiple vendors.
38. What are analog and digital tags in SCADA?
Analog Tags: Continuous signals like temperature or flow (e.g., 4-20 mA).
Digital Tags: Discrete on/off signals like start/stop or open/close.
39. What are alarms in SCADA?
Alarms notify operators of abnormal conditions such as high temperature, pressure deviations, or equipment faults, enabling timely corrective action.
40. What are trends in SCADA?
Trends are graphical plots of process variables over time. They help in analyzing performance, identifying patterns, and troubleshooting.
41. What is the purpose of data logging?
Data logging records process parameters continuously for analysis, report generation, and future reference.
42. What is the use of scripts in SCADA?
Scripts (often written in VBScript or C-like languages) automate repetitive tasks, calculations, or dynamic visual updates in SCADA systems.
43. What is the difference between RTU and PLC?
| RTU | PLC |
|---|---|
| Used in remote or unmanned locations | Used in local plant control |
| Lower speed, optimized for telemetry | High-speed logic execution |
| Includes built-in communication features | Communication via add-on modules |
44. What are some popular SCADA software packages?
Wonderware InTouch, Siemens WinCC, GE iFIX, Rockwell FactoryTalk View, Schneider Citect SCADA, and InduSoft Web Studio.
45. What is the function of the SCADA Server?
The SCADA Server acts as the central data hub. It gathers data from PLCs/RTUs, processes it, logs it, and distributes it to operator stations and historians.
46. What is meant by polling in SCADA?
Polling is the process where the SCADA master continuously queries RTUs or PLCs to read updated process data.
47. What are the types of redundancy in SCADA systems?
• Server Redundancy (Primary & Standby Servers)
• Communication Redundancy (Dual Networks)
• Power Redundancy (UPS / Dual Supplies)
48. What is the function of a tag database?
The tag database holds all process variable definitions (tag name, address, range, description, and alarm limits) used by the SCADA application.
49. What are typical network topologies used in SCADA?
Star, Ring, Bus, and Mesh topologies depending on redundancy and distance requirements.
50. What are the advantages of SCADA systems?
• Real-time process monitoring
• Centralized control
• Data logging and reporting
• Alarm management
• Improved efficiency and safety
• Remote access and analysis
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