- Draw system architecture showing PLC, SCADA/MMI, networking devices communication protocol details like IP address etc.
- Draw detailed block diagram of any field parameter like temp, pressure reading coming to PLC can be shown on SCADA/MMI.
- Explain 2 wire & 4 wire wiring concept.
- Draw block diagram of close loop control. Difference open and close.
- Draw server client architecture with example.
- Name different types of timers used in PLC.
- Subroutine, instruction types,
- Explain PID?
- PID Cascade mode, primary secondary
- Ramp, feed forward
- Name communication protocols and basic parameters required to establish communication between two nodes.
- What is difference between signal ground and power ground?
- Explain redundancy of controller.
- Master slave concept in Modbus
- Modbus addressing
- What is CRC? Parity?
- Thermocouple –RTD difference
- Room temp resistance for RTD.
- IP Address classes.
- Scan time of PLC.
- Why 4-20 ma not 0-20 ma?
- 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.
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.
4mA ===> ‘0’ signal (minimum value) i.e “Live zero” circuit is uncut.
0mA ===> i.e “dead zero” circuit is dead or cut.
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
Current signal can travel longer (100m approx.) without any losses.
Also,limiting the power of signals( 4mA - 20mA) in the transmitter is better for 'intrinsically safe design’
- 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.
- 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).
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 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.
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…
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
***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.