# Interview Question for Automation,DCS,PLC Engineers **URL:** \ **Category:** Industrial Automation\ **Created:** [December 31, 2014, 5:13am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215 "2014-12-31T05:13:46Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![denizen](https://forumautomation.com/letter_avatar_proxy/v4/letter/d/848f3c/32.png) [@denizen](https://forumautomation.com/u/denizen)\ **Post date:** [December 31, 2014, 5:13am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/1 "2014-12-31T05:13:47Z")
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 ---
**Author:** ![manishdprasad04](https://forumautomation.com/letter_avatar_proxy/v4/letter/m/8e8cbc/32.png) [@manishdprasad04](https://forumautomation.com/u/manishdprasad04)\ **Post date:** [January 25, 2015, 7:41am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/2 "2015-01-25T07:41:54Z")
> [@denizen](#): > > Explain 2 wire & 4 wire wiring concept. 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. ---
**Author:** ![manishdprasad04](https://forumautomation.com/letter_avatar_proxy/v4/letter/m/8e8cbc/32.png) [@manishdprasad04](https://forumautomation.com/u/manishdprasad04)\ **Post date:** [January 25, 2015, 8:06am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/3 "2015-01-25T08:06:39Z")
> [@denizen](#): > > Name different types of timers used in 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. ---
**Author:** ![manishdprasad04](https://forumautomation.com/letter_avatar_proxy/v4/letter/m/8e8cbc/32.png) [@manishdprasad04](https://forumautomation.com/u/manishdprasad04)\ **Post date:** [January 25, 2015, 10:10am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/4 "2015-01-25T10:10:19Z")
> [@denizen](#): > > Why 4-20 ma not 0-20 ma 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’** ---
**Author:** ![GlennAvans](https://forumautomation.com/letter_avatar_proxy/v4/letter/g/dc4da7/32.png) [@GlennAvans](https://forumautomation.com/u/GlennAvans)\ **Post date:** [March 16, 2015, 9:17am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/5 "2015-03-16T09:17:19Z")
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. ---
**Author:** ![GlennAvans](https://forumautomation.com/letter_avatar_proxy/v4/letter/g/dc4da7/32.png) [@GlennAvans](https://forumautomation.com/u/GlennAvans)\ **Post date:** [March 16, 2015, 9:27am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/6 "2015-03-16T09:27:10Z")
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). ---
**Author:** ![Tommy\_Van\_de\_Pol](https://forumautomation.com/letter_avatar_proxy/v4/letter/t/e19adc/32.png) [@Tommy\_Van\_de\_Pol](https://forumautomation.com/u/Tommy_Van_de_Pol)\ **Post date:** [March 20, 2015, 1:49pm UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/7 "2015-03-20T13:49:25Z")
IP Address classes. you can find information about the ip adres classes in the link below: > **[Address Classes](https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-2000-server/cc940018(v=technet.10)?redirectedfrom=MSDN)** ---
**Author:** ![instrumen](https://forumautomation.com/letter_avatar_proxy/v4/letter/i/65b543/32.png) [@instrumen](https://forumautomation.com/u/instrumen)\ **Post date:** [March 21, 2015, 2:27pm UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/8 "2015-03-21T14:27:16Z")
What Is SIL? SIL stands for Safety Integrity Level.A SIL is a measure of safety system performance.it 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. ---
**Author:** ![sirigina](https://forumautomation.com/letter_avatar_proxy/v4/letter/s/5f8ce5/32.png) [@sirigina](https://forumautomation.com/u/sirigina)\ **Post date:** [March 22, 2015, 2:06am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/9 "2015-03-22T02:06:40Z")
using 2 -wire current loop only, when their is a power supply failure how can use instrument with out bedding ---
**Author:** ![vijaypatel](https://forumautomation.com/letter_avatar_proxy/v4/letter/v/9fc29f/32.png) [@vijaypatel](https://forumautomation.com/u/vijaypatel)\ **Post date:** [October 16, 2015, 4:51am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/10 "2015-10-16T04:51:30Z")
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… ---
**Author:** ![rensklijn](https://forumautomation.com/letter_avatar_proxy/v4/letter/r/c89c15/32.png) [@rensklijn](https://forumautomation.com/u/rensklijn)\ **Post date:** [June 3, 2016, 8:04am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/11 "2016-06-03T08:04:19Z")
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. ---
**Author:** ![Dhinesh\_Junaa](https://forumautomation.com/user_avatar/forumautomation.com/dhinesh_junaa/32/505_2.png) [@Dhinesh\_Junaa](https://forumautomation.com/u/Dhinesh_Junaa)\ **Post date:** [October 21, 2017, 11:54am UTC](https://forumautomation.com/t/interview-question-for-automation-dcs-plc-engineers/215/12 "2017-10-21T11:54:04Z")
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.