Interview question based on PLC data communication

What are the basic three communication levels of functionality?

Device level: This level involves various sensor and actuator devices of machines and processes.

Control Level: This level would be the networks industrial controllers are on. This level may include controllers such as PLCs and robot controllers.

Information Level: The information level is a plantwide network typically composed of the company’s business networks and computers.

What is a repeater?

Repeaters are devices that are used to amplify a signal to its original strength because signals get distorted when they are transmitted through long distance cable.

What are Device bus network and Process bus network?

Device bus networks interface with low-level information devices such as pushbuttons and limit switches that primarily transmit data relating to ON/OFF state of device and its operational status.

Process bus networks connect with high-level information devices such as smart process valves and flowmeters, which are typically used in process control applications.

What does a communication protocol means?

A protocol is a set of rules that two or more devices must follow if they are to communicate with each other. A network protocol defines how data is arranged and coded for transmission on a network.

What is gateway?

Gateways make communication possible between different architectures and protocols. They repackage and convert data going from one network to another network so that the one can understand the other’s application data.

What are Half-duplex and Full-duplex communication system?

A half-duplex system provides for communication in both directions, but only one direction at a time. Used in master- slave communication.

Full-duplex transmission allows the transmission of data in both directions simultaneously and can be used for peer-to-peer communications

Name some serial communication standards?

RS-232, RS-422 & RS-485

What are data highways?

Data network used to connect a PLC network with another PLC network.

Name some data highway network?

Data highway plus (DH+)


What is modbus?

Modbus is a communicaion protocol developed by modicon. it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves.

What is DeviceNet & ControlNet?

DeviceNet is an open device-level network. It is relatively low speed but effi cient at handling the short messages to and from I/O modules. ControlNet is positioned one level above DeviceNet. It uses the Common Industrial Protocol (CIP) to combine the functionality of an I/O network and a peer-to-peer communication.

What is Profibus DP?

Profibus DP is an open, international fieldbus communication standard that supports both analog and discrete signals. The physical media are defined via the RS-485 or fiber optic transmission technologies.