Automation Committee

Understanding SCADA Communication Protocols

Module: Basic Concepts

Host:James MondaleWayne Wilson

Reduce background noisePlease mute your phones

Gain working knowledge of communication SCADA Protocols

Learn the basics layers and architectures

Examine various architectures SCADA Protocols

Learn about common Protocol errors that can occur in your SCADA system

Introduction

In telecommunication,• a communication protocol is a system of rules that

allow two or more entities of a communications system to transmit information via any kind of variation of a physical quantity.

• Protocols may be implemented by hardware, software, or a combination of both.

• In SCADA it includes the collecting of the information via;• RTU (remote terminal unit), • PLC's (Programmable Logic Controllers) • IED's (Intelligent electronic devices)

Transferring it back to the central site, carrying out any necessary analysis and control and then displaying that information on a number of operator screens or displays.

DNP3

PROTOCOLS are available for many types of SCADA systems

• Industrial Controls

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DNP3

PROTOCOLS are available for many types of SCADA systems

✓• Process Automation

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DNP3

PROTOCOLS are available for many types of SCADA systems

• Power Systems

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Protocol Layer OverviewThe Open Systems interconnection model (OSI) model is used to define how data communication occurs on computer networks.

Fiber Optic

Scada systems in water & wastewater commonly useModbus or DNP3 .

Modbus and DNP3 are both byte-oriented protocols. Modbus is an application layer protocol, while DNP contains Application and Data Link Layers, with a pseudo-transport layer.

Both protocols are widely used over a variety of physical layers, including RS-232, RS- 422, RS-485, and TCP/IP.

The term “Modbus” typically refers to one of three related protocols: Modbus ASCII, Modbus RTU, or Modbus TCP/IP

Multi Drop network

Message Query initiates from Master

Master can broadcast to all slaves

or a single one

Multi Drop network

Slaves returns a response to only messages addressed to them individuallyBut do not respond to broadcast messages.

Modbus Slaves do not initiate messages on their own

DNP3

DNP3 Slaves can report by exception

Difference between Modbus and DNP3 Communication Protocols

Let’s use Modbus as an example to demonstrate how protocols work

Physical Media

Data Model

Service request from master

Protocol Data Unit (PDU)

Fields in PDU are broken down into Bytes

4 Basic Data types

Slave Address Function

Code

Data Field

CRCError

Check

Function Code field determines read or write and in which field

Slave memory registers

Typical problems

• Hardware problems

• Mis-wired cabling

• Faulty Cable connectors

• Corrosion on connector ends

• Cables not “snapped” into socket

• Poor signals

• Network collisions• Duplicate IP Addressing• Overwriting registers• Polling loop problems

• Software (protocol ) related problems

Questions?

Thank You!The End