PLC / SCADA / DCS – Interview Questions & Answers

1. What is a PLC?

A Programmable Logic Controller used to control machines, processes, and field devices.

2. Difference Between PLC, SCADA, and DCS

Feature	PLC	SCADA	DCS
Purpose	Control devices	Monitoring & Control	Process automation
Architecture	Centralized	Central server + RTU/PLC	Distributed
Application	Machines	Plant monitoring	Continuous process

3. PLC Scan Cycle

Read Inputs
Execute Logic
Update Outputs
Self Diagnostics

4. PLC Programming Languages (IEC-61131-3)

Ladder Logic (LD)
Function Block Diagram (FBD)
Structured Text (ST)
Instruction List (IL)
Sequential Function Chart (SFC)

5. What is Ladder Logic?

Graphical programming method using contacts and coils to represent relay logic.

6. NO & NC Contacts

NO (Normally Open): Open in normal state.
NC (Normally Closed): Closed in normal state.

7. Latching vs Non-Latching Coil

Latching: Stays ON until RESET is triggered.
Non-Latching: ON only when rung is TRUE.

8. What is PID?

Proportional, Integral, Derivative control used for continuous process control.

9. What is HMI?

Screen used to interact with PLC (graphs, buttons, alarms).

10. What is SCADA?

Supervisory Control and Data Acquisition for real-time monitoring and control.

11. What is DCS?

Distributed Control System with multiple controllers spread across plant.

12. PLC I/O Types

DI – Digital Input
DO – Digital Output
AI – Analog Input
AO – Analog Output

13. What is 4–20 mA?

Standard analog signal used for noise immunity and live zero detection.

14. What is Modbus?

Modbus RTU (RS-485)
Modbus TCP (Ethernet)

15. What is Interlock?

Logic that stops machine during unsafe conditions.

16. What is Retentive Timer?

Keeps elapsed time even if input goes OFF.

17. What is Watchdog Timer?

Ensures PLC program scan is healthy.

PLC Ladder Logic Example

1. Start/Stop Motor (Ladder logic)

  Start PB     Motor healthy    Stop PB          Motor
|---] [-----------] [------------] / [------------( )-|

2. TON (On-Delay Timer)

 Sensor                     TON T1 (5s)
|---] [--------------------------(T1)-|

 T1.DN                          Motor
|---] [--------------------------( )--|

PLC Practice Test Questions

Explain PLC scan cycle.
Difference between DI and AI.
What is PID?
Draw latching logic for motor.
Difference between Modbus RTU & TCP.