Mastering Modbus RS485 Network Communication
- 3.5 hours on-demand video
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- Understand the difference between Modbus and RS485
- Modbus Protocol theory
- RS485 theory
- Modbus Messaging Structure and Configurations
- Creation of a virtual software based Modbus network
- Troubleshoot any Modbus network
- 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. Two freeware applications are then used to create a Modbus network right on your PC so you can see the communication in action.
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.
- Electrical Engineers
- Electrical Technicians
- Electrical Tech or Engineering Students
- Process Control Technicians
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.
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. Lecture outline:
Distributed Control Systems - expensive to purchase, operate and maintain
The advent of the PLC from Modicon Corporation
The need for PLC interconnection - the birth of Modbus
Modbus as a simple and open fieldbus
The rapid adoption and spread of Modbus
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
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
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.
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
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
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 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
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
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.
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
Single Master - Multiple Slave system
Modbus master permanence
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
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.
Read Coil Status
Read Input Status
Read Holding Registers
Read Input Registers
Force Single Coil
Preset Single Register
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