The 1-Hour Modbus RS485 Primer
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The 1-Hour Modbus RS485 Primer

Learn How to Interconnect Modbus Devices on an RS485 Network in just 1 HOUR!
4.4 (262 ratings)
Instead of using a simple lifetime average, Udemy calculates a course's star rating by considering a number of different factors such as the number of ratings, the age of ratings, and the likelihood of fraudulent ratings.
561 students enrolled
Created by Emile Ackbarali
Last updated 7/2016
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  • 1 hour on-demand video
  • 18 Supplemental Resources
  • Full lifetime access
  • Access on mobile and TV
  • Certificate of Completion
What Will I Learn?
  • Understand the difference between Modbus and RS485
  • Modbus Protocol theory
  • RS485 theory
  • Modbus memory mapping
  • Modbus network communication
  • The essential device information required for Modbus connection
  • The steps involved in the connection of devices via Modbus
View Curriculum
  • Working knowledge of the process control devices in a typical Process Automation or SCADA system

The Modbus protocol is the oldest and still by far the most popular automation protocol in the field of process automation and SCADA. Knowing how to create Modbus based networks that run on the RS485 serial communication standard is essential for any electrical technician and engineer working in these fields. Being able to integrate devices from different manufacturers is a skill that is in demand and will ultimately make you more valuable and marketable in the industry.

This course gives you the theory behind the Modbus Protocol as well as RS485. It then goes on to show you how they work together to create a Modbus RS485 network. 

After completing this course, you will be able to integrate devices from the same manufacturers and different manufacturers, that are Modbus compliant, to form a complete seamless network.

Who is the target audience?
  • Electrical Engineers
  • Electrical Technicians
  • Electrical Tech or Engineering Students
  • Process Control Technicians
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Curriculum For This Course
1 Lecture 01:44

The Modbus protocol is the widest used fieldbus protocol in the automation industry. Almost every piece of intelligent device supports the Modbus protocol, which means that a thorough working knowledge of it is essential to knowing how to interconnect devices from the same and different manufacturers together.

Our course goal is simple: To give you the knowledge that will allow you to interconnect devices from various manufacturers using the Modbus protocol over an RS485 network.

We strongly feel that this skill will make you very valuable and marketable in the field of SCADA and Process Automation.

Preview 01:44
A Brief History
3 Lectures 06:55

Many have used the Modbus protocol for years without understanding the reason for it's existence. In this lecture we look at exactly why and how the Modbus protocol was created.

Preview 03:25

The precursor to USB was RS232. Besides being used in the computer industry, RS232 found many uses in the automation industry. Efforts to improve on RS232 for automation purposes lead to RS485. In this lecture we will look at:

The original use of RS232
Drawbacks of RS232 for automation
RS485 standard solves RS232 drawbacks
RS485 advantageous properties
Preview 01:57

Because the Modbus protocol is frequently implemented using an RS485 network, many believe that they depend on one another. This is simply not true. Both RS485 and Modbus can stand on their own in various applications. In this lecture we look at:

Modbus and RS485 key misconception
Modbus Protocol vs. RS485 Electrical Standard
Modbus used over multiple types of media
Why they are frequently implemented together
Preview 01:33
Course Problem to be Solved
1 Lecture 02:30

Learning new concepts is always easier and much more effective when we can apply that concepts to a particular problem situation.

That is what we are going to do in this lecture.

In fact, from this point onward, we will refer to this problem at the end of each section of the remainder of this course, each time solving more and more of it until we completely solve it at the end.

The problem involves the interconnection of three PLC's via Modbus RS485 for the purpose of transferring data from two of them into a single one. The latter will then be responsible for transmitting the data wirelessly to a central point.

Preview 02:30
The Modbus Protocol Memory Map
8 Lectures 17:32

When the Modbus protocol was developed, the specification implemented two different modes of operation: Modbus RTU and Modbus ASCII. In this lecture we will cover:

Modbus RTU vs. Modbus ASCII
Advantages of RTU over ASCII
Very high prevalence of Modbus RTU over ASCII
Modbus RTU vs. ASCII

Devices that support the Modbus protocol have three things in common. They are the:

1. Microprocessor or Central Processing Unit
2. Modbus Memory
3. Communication Interface
Modbus Device Components

Before we get into the specific way that Modbus memory is arranged, there are a few basic concepts about electronic memory in general that we must cover in this lecture. Lecture outline:

Memory divided into Memory Blocks
Memory Blocks have different sizes
Memory Address vs. Memory Value
PLC Memory block example to illustrate
Memory Address vs. Value

Memory blocks can come in different sizes. They are designed in different sizes mostly according to their purpose within the device in which they are used. We look closely at the various sizes and uses of Memory blocks. Lecture outline:

Memory block size measured in Bits
Bit = binary digit, binary counting system
1-bit to 32-bit sizes
Typical uses of different sizes
Illustration of use through an example
Memory Block Sizes

Within every Modbus compliant device there will be a portion of the memory that will be dedicated to Modbus. This portion is known as the Modbus memory area or Modbus memory map. Lecture outline:

Modbus memory divided into 4 areas
Coils, Inputs, Input and Holding Registers
Memory area ranges and sizes
Purposes of each memory area
How the areas are linked to device I/O
The Modbus Memory Map

Modbus memory is usually linked to the device I/O. However, more sophisticated devices will use memory for calculations and storage of cumulative or calculated values. This lecture looks at an example of this and explains why this evolution has taken place.

Modbus Memory Map Exceptions

Many of the devices that are Modbus compliant that you will come across will most likely have some form of direct connection to physical parameters in the real world.

It could be anything, motor speed, pressure, temperature, switch or breaker position. These devices would most likely be tied to some form of sensor or actuator that would be interacting with the real world.

Now even though the Modbus memory areas of coils, inputs, input registers and holding registers would be the same from device to device, the usage of the individual memory blocks will most likely be different.

And this is where the device documentation will come in.

Determining I/O Mapping to Memory

Now that we have covered Modbus memory mapping, we will look again now at our design challenge from Lecture 5 and see how Modbus memory applies to it and how we can start on our way toward the solution.

Application to our Design Problem
Modbus Protocol Network Messaging
5 Lectures 12:54

In the previous section we learned all about how Modbus memory is arranged in devices that are Modbus compliant. In this lecture we will start a look at how the actual Modbus communication takes place among devices. Lecture outline:

Request - Respond system
Master/Slave architecture
Single Master - Multiple Slave system
Modbus master permanence
Messaging Method

The Modbus Unit ID is a unique identifier given to every Modbus complaint device that is connected to a Modbus network. This Unit ID is essential to the proper operation of the network. Lecture outline:

Modbus Unit ID purpose
Role in network communication
Role in master/slave system
Illustration through example network
Modbus Unit ID

The Modbus master device on the network is the one that sends request messages to the slave devices on the network. All request commands, however, are not the same.

In this lecture we are going to look at the major types of request commands that a modbus master device can send to a slave.

Lecture outline:

Read Coil Status
Read Input Status
Read Holding Registers
Read Input Registers
Force Single Coil
Preset Single Register
Modbus Request Commands

In this lecture we look at the request commands in detail and explore the concept of block reads. Block reads is the particular way in which Modbus reads data. Lecture outline:

Data read in consecutive blocks only
Master start block and number of blocks
Illustration of block reads - Example
Modbus Block Reads

Now that we have covered Modbus messaging, we will look now again at our design challenge from Lecture 5 and see how Modbus messaging applies here and carry the solution still further toward being completely solved.

Application to our Design Problem
The RS485 Serial Communication Standard
3 Lectures 06:22

RS485 is not a protocol but rather a Serial Communication Standard. In this lecture we will look at the characteristics of RS485 and it's physical connection arrangement. Lecture outline:

RS485 port details
Multi-drop connection arrangement
Two wire interconnection style
Physical Connection

Every RS485 communications port has configuration parameters associated with it. In this lecture we take a look at these common communication parameters and their meanings. Lecture outline:

Baud Rate
Number of data bits
Number of stop bits
Parity error checking
Port Settings

Now that we have covered the RS485 standard, we will look now again at our design challenge from Lecture 5 and see how RS485 communication applies here.

This will bring us to the complete solution to our Modbus RS485 network problem.

Application to our Design Problem
In Conclusion
1 Lecture 00:48
About the Instructor
Emile Ackbarali
4.5 Average rating
1,028 Reviews
2,505 Students
13 Courses
Software Developer, Systems Integrator and Entrepreneur

I have been working in the fields of Software Development, Software Systems Integration and Process Automation since 1996. In 2003, I stepped out on my own and started working for myself providing software development and systems integration services to small and medium businesses.

I am also associated with a great company called Mora Systems Limited that under the leadership of it's Managing Director has produced innovative and outstanding products and services in the fields of GIS and Cellular based SCADA.

Teaching has always been a passion for me. I have taught at the tertiary level for many years on the subjects of programming, control systems as well as systems integration. In the last few years I found myself incorporating more and more videos into the curriculum with amazing results.

When I heard of uDemy I just could not help but sign up. With teaching in a classroom, you can reach only so many students, but with uDemy you can reach the world! I plan to publish and promote as many courses as I possibly can on the uDemy platform.