The 1-Hour Modbus RS485 Primer

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.

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

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.

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.

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.

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

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.

Lecture outline:

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

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.

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

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

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.