Modbus based interview questions

plc
instrumentation

#1

What is Modbus Protocol?

Modbus is a serial communication protocol is developed by MODICON inc. Modbus devices use master-slave communication technique in which only one device(master) can initiate the transaction and other devices respond to the master request.

What software support Modbus and Modbus/TCP protocol?

Most of SCADA and HMI softwares.

What is Mobus TCP/IP?

MODBUS/TCP is a communication protocol designed to allow industrial equipment such as Programmable Logic Controllers, computers, operator panels, motors, sensors, and other types of physical input/output devices to communicate over a network.

What is TCP/IP stands for in Modbus TCP/IP?

TCP : Transmission Control Protocol
IP: Internet Protocol

What is the difference between Modbus and Modbus TCP/IP?

Modbus TCP/IP is an advanced version of Modbus to the industrial automation. Modbus is serial communication protocol which supports RTU and ASCII code message framing.

Modbus defines how the data is structured for transmission, RTU and TCP/IP wrap the structured data for specific methods of transmission.

Modbus TCP uses the same RTU message framing technique but uses the Ethernet protocol for communication. TCP/IP supports both the Ethernet and the Internet protocol.

The benefit of using Modbus TCP/IP is using the existing Ethernet network equipment that is widely and available and cost-effective.

How can existing communication Modbus device can communicate over Modbus TCP/IP?

Gateway devices are used to between Modbus and Modbus TCP/IP to make the communication possible.

What are the benefits of using Modbus and Modbus TCP/IP?

  • Open source, no royalty

  • Easy to use

  • Easily integrated variant devices

  • Low development cost

  • Wide knowledge case

What is Modbus message framing?

A message frame is used to mark the beginning and ending point of a message allowing the receiving device to determine which device is being addressed and to know when the message is completed.

The method differs for ASCII & RTU modes.

What is the basic difference between ASCII mode and RTU mode?

Both are message framing techniques.

ASCII mode sends message within a double colon character, which means the characters within the colon is single message

An RTU message mode keeps a character time interval between message. When a particular amount of time interval is received at the slave, a slave understands the message is complete.

What are LRC and CRC?

They are Frame checking methods which are used to check the errors in a message.

LRC (Longitudinal Redundancy Check) In the ASCII transmission mode, the character frame includes an LRC field as the last field preceding the CRLF characters. This field contains two ASCII characters that represent the result of a longitudinal redundancy calculation for all the fields except the starting colon character and ending CR LF pair of characters.

CRC RTU Mode message frames include an error checking method that is based on a Cyclical
Redundancy Check (CRC). The error-checking field of a message frame contains a 16-bit value
(two 8-bit bytes) that contains the result of a Cyclical Redundancy Check (CRC) calculation
performed on the message contents.