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Modbus is a serial communication protocol that was developed by Modicon. It was first published in 1979 for use with the company’s programmable logic controllers (PLCs). In basic terms, it is one of the methods used for information transmission over serial lines between electronic devices. The Modbus Master is the device that requests the information while Modbus Slaves supply the information. In a typical Modbus network, there will be one Master and as many as 247 Slaves, with every Slave Address being unique, from 1 to 247.
A Brief History Of Modbus
Before we get into the particulars regarding the use of Modbus, let’s take a look at the brief history of this platform. In the early days of industrial automation, factories required an approach for the first Programmable Logic Controllers (PLCs) to communicate with each other. The first PLC was developed in 1968 by Modicon, as per General Motors’ request. After this started to showcase effectiveness at General Motors, the device was called Modicon 084, and mass production began. This resulted in several new competitors entering the market toward the end of the 1970s, for example, Allen-Bradley was one of the main competitors that started to create such solutions, and PLCs were highly common in factories. However, information was still not being communicated via the PLCs, and there was an arms race to find out who would be the first to innovate their technology so that communication could take place between the devices. Around the same time, Modicon created Modbus while Allen-Bradley released Data Highway. However, many viewed Modbus as the grandfather of the industrial communication protocol field. When it comes to connecting automated electronic devices in industrial settings, this is the
most common method used. When Modbus was released by Modicon, the company did so without their standard licensing protocol being kept. This enabled any developer to incorporate Modbus into their products as it is a universal standard that many devices can support. It is also very easy and straight foward to understand and set up, even by those who do not have an engineering background.
Modbus is an open protocol platform, meaning, manufacturers can build their equipment free of charge without needing to pay royalties. It has become a standard communications protocol in the sector, and it is a common means for connecting industrial electronic devices. Manufacturers across a number of industries use Modbus. It is typically utilized for the purpose of transmitting signals from control and instrumentation devices back to the chief controller or data gathering system. For instance, this could be a system that measures humidity and temperature, and then communicates the results back to a computer.
Modbus is generally transmitted over serial lines between devices. The most straightforward setup involves a single serial cable that is connected to serial ports on two devices; a Slave and a Master. The data will be sent as a series of zeroes and ones called bits. Each bit is sent as a voltage. Zeroes are sent as positive voltages, and ones are sent as negatives. The bits are sent very high quality. A standard transmission speed is typically 9600 baud (bits per second).
The Modbus Organization has managed the development and any needed updates to Modbus protocols in an association of users and advocates of the technology ever since Schneider Electric bequeathed the rights to them in 2004. It is an independent, non-profit organization that requires membership to join. Their mission is to build the infrastructure needed for full adoption of Modbus across multiple industries by sharing information and educating users on the best practices for successful implementation.
Several protocols for Modbus exist and are constantly evolving through experimentation and innovation from users within the Modbus Organization. Three are most commonly used, with Modbus TCP being the protocol that the Modbus Organization favors in its development and universal implementation objectives.
- Modbus RTU
Modbus RTU is a type of serial Modbus. This transmission mode encodes messages as bytes that are sent one after the other without any spaces between them – though a 3-½-character space inside acts as a delimiter between messages. For each eight-bit byte: one start bit, eight data bits, one bit for parity, and one stop bit are sent, for a total of 11 bits per byte. Every Modbus RTU message is ended with an accompanying error checksum in what is called a cyclic redundancy check (CRC).
- Modbus ASCII
The Modbus ASCII protocol initiates every message with a colon “:” character. An ASCII carriage return/line feed (CR/LF) character ends every message transmission. This provides variability in what is being communicated with more spacing in between the bytes, giving it the ability to transmit through certain modems. ASCII characters are used to communicate this data. For each eight–bit byte: one start bit, seven data bits, one bit for parity, and one stop bit are sent, for a total of 10 bits. Modbus ASCII messages are ended with an error checksum called a longitudinal redundancy check (LRC).
There are pros and cons to be weighed when using either of these two types – Modbus RTU and Modbus ASCII – as Modbus ASCII is much simpler to read by just looking at the message, but the RTU messages are much smaller, allowing for an enormous amount of data to be communicated in the same amount of time. Modbus RTU is used most commonly.
- Modbus TCP
Modbus TCP is different in that it does not require a checksum, but is basically the RTU protocol running on Ethernet and a TCP interface. The TCP port 502 is reserved for Modbus, while the new Modbus/TCP Security uses Port 802. Much more on this can be found at Modbus.org.
Modbus Protocols and IoT
The Modbus Protocols may be older than most people reading this article, but its simplicity and efficiency at powering many sectors of the manufacturing industry is still transitioning along with new advancements and innovations like the Internet of Things. Programmable Logic Controllers (PLC), Industrial Control Systems (ICS), Distributed Control Systems (DCS), and/or Variable Frequency Drivers (VFD) are all frequently seen in factories around the world. They perform actions and transmit data in real-time to manufacture many of the common household items you possess, from vehicles to breakfast cereals, and it is all communicated with Modbus Protocols. Essentially, Modbus, being by now so pervasive and adopted into millions of devices – and not just on the factory floor, but in millions of products from absolute encoders to zero-point thermometers – has already been doing what the Internet of Things (IoT) and IIoT (Industrial Internet of Things) is only now doing. The difference between them is that IoT is faster, more efficient, and more accurate at communicating information, which leads to more productivity, more cost-effectiveness, and more profits.
But this does not mean that Modbus is heading to the trash heap as IoT takes over – not for a while, anyway. Its wide adoption across so many industries all over the world cannot be dispensed with so quickly, or so easily. There are now several software companies providing solutions for Modbus and IoT integration, but only one of them has the experience and reliability to liberate your data with unparalleled connectivity, Open Automation Software.
Open Automation Software Modbus IoT Data ConnectorThe OAS Modbus IoT Data Connector provides a direct connection to Modbus slave devices and the ability to host data to Modbus masters with the built in Modbus Driver Interface. The Modbus Driver Interface supports communications over Ethernet and Serial interfaces with Modbus TCP, Modbus RTU, and Modbus ASCII protocols.
With Modbus Automation Software from OAS, businesses will find the easiest process for adding Modbus communications in Real-Time Automation on the market – with one click of a button! Three of the most widely used Modbus Protocols can then be seamlessly implemented – Modbus TCP, Modbus RTU, and Modbus ASCII protocol – onto both Ethernet and Serial Port interfaces with the Modbus Master connection. Modbus TCP over Ethernet and Modbus RTU and ASCII over Serial Ports with the Modbus Slave connection.
After a quick and easy setup, the communication speeds you experience while running multiple Ethernet connections are at the fastest possible, with update rates averaging 15 milliseconds per packet. Data from anywhere, to anywhere, are sent in real-time with direct access to Modbus compatible devices. A PC and standard internet connection are all that is required to host live Modbus data. Remote data logging is also made highly secure, and backup devices can be optionally defined with Automated Communications Failover.
The ease of access for Modbus IoT Data Connector isn’t limited to only a few operating systems, either. Supported systems include Windows PC/Server, Windows Embedded, Windows IoT, Linux Server, Virtual Machines (Win/Linux), Raspberry Pi 4, and Docker Containers (Win/Linux).
The OAS Platform: Full ConnectivityOpen Automation Software began 28 years ago with the philosophy that user interfaces and database archiving software should be open, far-reaching, and provide easy implementation and cost-effectiveness for consumers. An Open Architecture plan allows unparalleled customization with open accessibility to third party reporting systems, .NET applications, web interfaces, open standards like OPC UA and MQTT, and even remote Microsoft Excel workbooks.
The nature of the OAS Platform empowers you with flexibility. Purchasing the Universal Data Connector in combination with connectors for devices, databases, applications, IoT services and more, will give you the most powerful and versatile framework for enterprise systems integration. The built-in compatibility with other developer tools and APIs give you an unlimited amount of options for customization and automation.
The full range of products include:Universal Data Connector
The core of the OAS Platform, the UDC is a Windows based program allowing it to be implemented in several different ways. Enable data connectivity, server-to-server networking, and more.
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