Modbus is the most pervasive communications protocol in industrial and building automation and the most commonly available means of connecting automated electronic devices. Why did that happen? Why did Modbus have such an impact on the Industrial Automation industry that it survives to this day as one of the leading industrial networks of the 21st century? There are several reasons.
Modbus is an open standard. Modicon, the creators of Modbus, did not keep the standard proprietary. They released it as a non-proprietary standard and welcomed developers, even competitors, to implement it. They rightly assumed that it would be best for everyone, including them, if Modbus became successful in the marketplace. Because of this thinking, Modbus became the first widely accepted fieldbus standard. In a short time, hundreds of vendors implemented the Modbus messaging system in their devices and Modbus became the de facto standard for industrial communication networks.
Standard transports. The transport layer for Modbus RTU commands is also simple to understand. It uses RS485, a differential communication standard which supports up to 32 nodes in a multi-dropped bus configuration. RS485 provided noise immunity superior to the RS232 electrical standard.
Modbus implements a very simple data representation. Modbus is very easy to understand. Its primary purpose is simply to move data between an RTU Master device (a Client in Modbus TCP) and an RTU Slave device (a Server in Modbus TCP). There are only two kinds of data to move, registers and coils. Registers are 16-bit unsigned integers. Coils are single bits.
Modbus implements an uncomplicated Request–Response command structure. A Modbus Master requests or sends data to a Slave and the Slave responds. There are simple commands to read a register, read a coil, write a register, and write a coil.
Complete simplicity was extremely important when the protocol was released. Remember that microprocessor technology when Modbus was created was not only brand new, but extremely limited. Programmers often had as little as 64 bytes to manage. That’s 64 b-y-t-e-s, not 64K or 64Meg. Developing an automation app with just 64 bytes of RAM meant that every single byte was treasured. There just weren’t any bytes to frivolously waste.
Modbus fitted well in that world. It required little code space, often as little as 1K. RAM varied with the size of your data space. Simple automation devices with little bits of data—imagine a photo eye—could be implemented with hardly any RAM space. These devices could now, for the first time, send their data to a control system as part of a daisy-chained 485 network, avoiding hardwired point-to-point communications.
Another reason Modbus was so successful was the fact that it could be so readily understood by non-programmers. Engineers who built glue machines, meters, measuring devices, and such could easily understand the concept of coils/registers and the simple commands to read and write them.
The simplicity of Modbus has been both a blessing and a curse over the years. The simplicity has led to an incredible amount of activity and propagation of Modbus into many different industries around the world. There is probably no product category in the last thirty years that hasn’t had an offering with Modbus.
Modbus has gained wide market acceptance wherever Industrial Automation Systems (IAS) or Building Management Systems (BMS) need to communicate with other devices. In fact, Modbus is probably the most implemented automation protocol of all time.
Real Time Automation has a wealth of Modbus expertise and many gateway devices that move Modbus data around the factory floor. Click on Modbus Gateways to see our entire line of Modbus Gateway products.
At RTA we’re saving the world from inaccessible Modbus data!
John S Rinaldi
John Rinaldi is Chief Strategist, Business Development Manager and CEO of Real Time Automation (RTA). After escaping from Marquette University with a degree in Electrical Engineering, John worked in various jobs in the Automation Industry before once again fleeing back into the comfortable halls of academia. At the University of Connecticut, he once again talked his way into a degree, this time in Computer Science (MS CS). John is a recognized expert in industrial networks and the author of six books: Modbus: The Everyman’s Guide to Modbus, OPC UA – Unified Architecture: The Everyman’s Guide to OPC UA, EtherNet/IP: The Everyman’s Guide to EtherNet/IP, The Smart Product Manager’s Guide to Industrial Automation Connectivity, The Smart Product Manager’s Guide to Connectivity in the Packaging Industry, and his latest, The Everyman’s Guide to Properly Architecting Ethernet/IP Networks.
I am John S. Rinaldi, Chief Strategist, Business Development Manager, and CEO of Real Time Automation (RTA). With a background in Electrical Engineering from Marquette University and a Master's degree in Computer Science from the University of Connecticut, I have dedicated my career to the field of industrial networks. I am a recognized expert in this domain, having authored six books, including "Modbus: The Everyman’s Guide to Modbus."
The enduring impact of Modbus on the Industrial Automation industry can be attributed to several key factors, reflecting a depth of knowledge in the evolution and significance of this protocol.
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Open Standard Approach:
- Modbus, developed by Modicon, stands out as an open standard. Unlike some proprietary protocols, Modbus was released as a non-proprietary standard. This decision facilitated widespread adoption, as developers and even competitors were invited to implement it. This open approach contributed to Modbus becoming the first widely accepted fieldbus standard.
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Standard Transports:
- Modbus employs RS485, a differential communication standard, as its transport layer for Modbus RTU commands. RS485 supports up to 32 nodes in a multi-dropped bus configuration, providing superior noise immunity compared to the RS232 standard. This choice of transport layer enhanced the reliability of Modbus communication in industrial settings.
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Simple Data Representation:
- The simplicity of Modbus extends to its data representation. It primarily involves moving data between an RTU Master and RTU Slave using only two types of data: registers (16-bit unsigned integers) and coils (single bits). This straightforward approach enhances ease of use and understanding.
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Uncomplicated Request-Response Structure:
- Modbus employs a simple Request–Response command structure. A Master requests or sends data to a Slave, and the Slave responds. This simplicity was crucial when Modbus was introduced, considering the limitations of microprocessor technology at that time. The protocol's efficiency and effectiveness were particularly relevant given the constraints of early automation app development.
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Memory and Resource Efficiency:
- In an era of limited microprocessor technology, Modbus was designed to be resource-efficient. With as little as 1K of code space and varying RAM sizes, Modbus accommodated the constraints of early automation devices, allowing them to send data over a daisy-chained 485 network, eliminating the need for hardwired point-to-point communications.
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User-Friendly for Non-Programmers:
- Modbus was designed to be easily understood by non-programmers. Engineers in various industries could grasp the concepts of coils/registers and the simple commands for reading and writing them. This user-friendly nature contributed to the widespread adoption of Modbus across different sectors.
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Wide Market Acceptance:
- Over the years, the simplicity of Modbus has proven to be both a blessing and a curse. While its simplicity led to widespread adoption in various industries worldwide, it also meant that Modbus became a default choice, sometimes limiting the use of more advanced protocols. Nevertheless, Modbus remains one of the most implemented automation protocols, especially in Industrial Automation Systems (IAS) and Building Management Systems (BMS).
In conclusion, the enduring success of Modbus in the 21st century can be attributed to its open standard approach, robust transport layer, simple data representation, uncomplicated command structure, memory efficiency, user-friendly design, and wide market acceptance. These factors collectively position Modbus as a leading industrial network, and at Real Time Automation, we continue to leverage our Modbus expertise to facilitate seamless data communication in the industrial landscape.
