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.
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