Why Baud Rate is Important in Modbus Network?

Modbus is a protocol that is used widely in industrial automation. Modbus has two types of communication – RTU and ASCII. Modbus RTU is used mostly everywhere due to its simplicity in coding and configuration.

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Modbus is a serial communication, which is used due to it’s easy working and also allowing complex communications to be solved. In the Modbus RTU, one parameter that must be taken special care is the baud rate. In this post, we will see the concept of baud rate and understand why it is important for use.

What is the baud rate in Modbus?

Why Baud Rate is Important in Modbus Network

Modbus has the following parameters to set –

address
data bit
stop bit
parity
baud rate

All these five parameters must be set properly for correct modbus communication. Let us see what the baud rate is now.

Baud rate is a measure of communication speed in Modbus protocol. Data is transmitted in the form of bits, and that speed of communication per second is called the bit rate. Now, some bits make one symbol or a group. How much such symbols are communicated in one second, defines the baud rate.

The Baud rate specifies how fast the symbols (data) will be transmitted or received between two Modbus devices. It is also defined as the number of signal changes per second. In contrast, the bit rate is defined as the number of bits transmitted or received in a second.

So, for example, if a symbol carries three bits, then the bit rate will be thrice the baud rate. Do not get confused between both the terms and the major study area that is used in the configuration is the baud rate and not the bit rate.

For example, if the baud rate is 19200, then it means 19200 symbols can be communicated maximum in a second.

Baud rate should be the same in both the master and slave devices. It is not like one device will be set to 19200 and another at 9600. Otherwise, communication will not happen.

How to set the baud rate for Modbus?

Every manufacturer, when giving a Modbus device, will specify a default value in its operating manual. This simplifies the task firsthand. It gives an impression as to how their device is capable of performing. Then, once you have studied that, you can try to manipulate the baud rate.

As baud rate is directly related to speed, the factors to look for changing this parameter are – the distance between two devices, the total number of slave devices connected with the master, noise level, and the amount of data being transmitted. If the distance is large between devices, the baud rate should be kept to a low level.

Why Baud Rate is Important?

If the rate is high, then the device will try to throw symbols at a fast rate and there are chances that it will not reach the destined device in such a short time. If there are a large number of slave devices (a maximum of 255 can be connected to a master in Modbus), try to baud rate to a low value. This will ensure that data will reach each device effectively.

Otherwise, data will skip and not reach some devices smoothly. If the nearby EMI (electromagnetic interference) noise is higher and affects the signal performance, then too the baud rate should be kept low. And at last, if the data is bulk and large, then too the baud rate should be kept low. This ensures that each and every data reaches the device in a proper time.

This all shows that a high baud rate is preferable for short distances, a short number of total slave devices, low noise interference, or a low amount of data. One main reason for this is that Modbus is serial communication and data is transmitted serially, meaning each data should reach the device in line.

Standard Baud Rates

The standard baud rate values are –

9600
19200
38400
57600
76800
115200.

The two widely used rates in Modbus are 9600 and 19200. So, to determine the best baud rate, you must look at both the factors – speed and data reliability. Anyone cannot be tampered with, otherwise, communication will not happen properly. Both factors should be considered for proper optimization of data.

In this way, we saw how the baud rate is important in Modbus.

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