Interview Question for Automation,DCS,PLC Engineers

  1. Draw system architecture showing PLC, SCADA/MMI, networking devices communication protocol details like IP address etc.
  2. Draw detailed block diagram of any field parameter like temp, pressure reading coming to PLC can be shown on SCADA/MMI.
  3. Explain 2 wire & 4 wire wiring concept.
  4. Draw block diagram of close loop control. Difference open and close.
  5. Draw server client architecture with example.
  6. Name different types of timers used in PLC.
  7. Subroutine, instruction types,
  8. Explain PID?
  9. PID Cascade mode, primary secondary
  10. Ramp, feed forward
  11. Name communication protocols and basic parameters required to establish communication between two nodes.
  12. What is difference between signal ground and power ground?
  13. Explain redundancy of controller.
  14. Master slave concept in Modbus
  15. Modbus addressing
  16. What is CRC? Parity?
  17. Thermocouple –RTD difference
  18. Room temp resistance for RTD.
  19. IP Address classes.
  20. Scan time of PLC.
  21. Why 4-20 ma not 0-20 ma?
  22. Bump less transfer

2 Wire —> Loop Power ( 1 Wire for power and other for signal transmit) 4 Wire —> Self Power ( 2 Wire for saperate Power Ssupply and other pair for signal transmit). In 2 wire if 1 wire demaged or disconnected, instrument goes in bed at DCS or PLC.

In plc ,timers may be On delay timer,Off delay timer,Retentive or accumulative timer & Pulse timer. The basis of every time is ON DELAY timer. using only On delay we can arrange various other types of times.

Its a very good question in any interview.I will ans this according to my knowledge,

above all . . .it is a standard signal specified by ISA. Then, 1)Two concept “Live zero” & “dead zero” have to be considered . In order to sense ,if the circuit is without cuts or, in other words ,if the 0 signal from the field corresponds to actual 0 or the signal is produced because of wire cut.

Therefore 4mA ===> ‘0’ signal (minimum value) i.e “Live zero” circuit is uncut. 0mA ===> i.e “dead zero” circuit is dead or cut.

  1. Most of the device use 1-5V , therefor By using simple 250 ohms is series with the current. the corresponding signal we require becomes 4mA - 20mA for 1-5V

  2. Current signal can travel longer (100m approx.) without any losses.

  3. Also,limiting the power of signals( 4mA - 20mA) in the transmitter is better for 'intrinsically safe design’

  1. Explain P&ID

P&ID stands for piping and instrumentation diagram. It is a diagram in the process industry which shows the installed equipment. It is a like a blue print of a system. Also it has a very logical numbering.

  1. Scan time of PLC.

If you mean the cycle time, it depends on how big your program is. it could be from 10-600ms (depends on manufacture).

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IP Address classes.

you can find information about the ip adres classes in the link below:

What Is SIL? SIL stands for Safety Integrity Level.A SIL is a measure of safety system has four levels.Higher the SIL level , lower the probability of failure for the safety systems.The cost and complexity also increases as SIL level increases.


using 2 -wire current loop only, when their is a power supply failure how can use instrument with out bedding

in 0-20mA signal, if there is break in sensor then plc can not detect the break because it will get 0 signal in off and even break situation…while that is not the case in the 4-20 mA signal…of there is break in sensor then plc will get 0 ampere so it decide that sensor is break while in off position it will get 4mA signal…

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Why 4-20 ma and not 0-20 ma?

When you use a 4-20 ma signal, the PLC will be able to detect a cable break, because it returns 0 ma. When you use a 0-20 ma signal, the PLC won’t be able to detect a cable break, because it returns 0 ma and the PLC will think there is nothing wrong with the signal.


Why 4 mA is used instead of 0 mA and why 20 mA is used? **If instrument is faulty, it can be easily identified. If the output is 0 mA it will be difficult to identify whether it is a faulty signal or a good signal. For error detection 4 mA is good. ***20 mA is used as maximum because the human heart can withstand up to 30 mA of current only. For safety reason 20 mA chosen as maximum value. ***Scaling of 4-20 mA is easy.