Modbus FAQ: About the Protocol

What is Modbus ® protocol?

Modbus Protocol is a messaging structure developed by Modicon in 1979. It is used to establish client-server communication between intelligent devices. It is a de facto standard, truly open and the most widely used network protocol in the industrial manufacturing environment. It has been implemented by hundreds of vendors on thousands of different devices to transfer discrete/analog I/O and register data between control devices. It's a lingua franca or common denominator between different manufacturers. One report called it the "de facto standard in multi-vendor integration". Industry analysts have reported over 7 million Modbus nodes in North America and Europe alone.

Where Modbus ® is used?

Modbus is used in multiple client-server applications to monitor and program devices; to communicate between intelligent devices and sensors and instruments; to monitor field devices using PCs and HMIs. Modbus is also an ideal protocol for RTU applications where wireless communication is required. For this reason, it is used in innumerable gas and oil and substation applications. But Modbus is not only an industrial protocol. Building, infrastructure, transportation and energy applications also make use of its benefits.

What is Modbus TCP/IP protocol?

TCP/IP is the common transport protocol of the Internet and is actually a set of layered protocols, providing a reliable data transport mechanism between machines. Ethernet has become the de facto standard of corporate enterprise systems, so it comes as no surprise that it has also become the de facto standard for factory networking. Ethernet is not a new technology. It has matured to the point that the cost of implementing this network solution has been dropping to where its cost is commensurate with those of today's field-buses.

Using Ethernet TCP/IP in the factory allows true integration with the corporate intranet and MES systems that support the factory. To move Modbus into the 21st century, an open Modbus TCP/IP specification was developed in 1999. The protocol specification and implementation guide are available for download (www.modbus.org/specs).

Combining a versatile, scaleable, and ubiquitous physical network (Ethernet) with a universal networking standard (TCP/IP) and a vendor-neutral data representation, Modbus gives a truly open, accessible network for exchange of process data. It is simple to implement for any device that supports TCP/IP sockets.

Where is Modbus TCP/IP used?

Modbus TCP/IP has become ubiquitous because of its openness, simplicity, low-cost development, and minimum hardware required to support it. There are several hundred Modbus TCP/IP devices available in the market - more being developed each year. It is used to exchange information between devices, monitor, and program them. It is also used to manage distributed I/Os, being the preferred protocol by the manufacturers of this type of devices.

Why should I use Modbus TCP/IP?

When it comes to choosing a network for your device, Modbus TCP/IP offers several significant advantages:

» Simplicity: Modbus TCP/IP simply takes the Modbus instruction set and wraps TCP/IP around it. If you already have a Modbus driver and you understand Ethernet and TCP/IP sockets, you can have a driver up and running and talking to a PC in a few hours. Development costs are exceptionally low. Minimum hardware is required, and development is easy under any operating system.
» Standard Ethernet: There are no exotic chipsets required and you can use standard PC Ethernet cards to talk to your newly implemented device. As the cost of Ethernet falls, you benefit from the price reduction of the hardware, and as the performance improves from 10 to 100 Mb and soon to 1 Gb, your technology moves with it, protecting your investment. You are no longer tied to one vendor for support, but benefit from the thousands of developers out there who are making Ethernet and the Internet the networking tools of the future. This effort has been complemented opportunely with the assignment of the well-known Ethernet port 502 for the Modbus TCP/IP protocol.
» Open: The Modbus protocol was transferred from Schneider Electric to the Modbus Organization in April 2004, signaling a commitment to openness. The specification is available free of charge for download, and there are no subsequent licensing fees required for using Modbus or Modbus TCP/IP protocols. Additional sample code, implementation examples, and diagnostics are available on the Modbus TCP toolkit, a free benefit to Modbus Organization members and available for purchase by nonmembers
» Availability of many devices: Interoperability among different vendors' devices and compatibility with a large installed base of Modbus-compatible devices makes Modbus an excellent choice.

How do I implement a Modbus TCP/IP device?

To implement a Modbus ® TCP/IP device, download the Modbus TCP/IP Protocol specification and the Modbus TCP/IP implementation guide from the website. You also need to understand the basics of implementing a TCP driver. To test your driver, you will need a minimum of two devices: a server and a client to exchange information. This could be two PCs talking to each other using standard Ethernet cards, or a PC talking to a sensor or device with an embedded microcomputer.

Can I use Modbus TCP/IP over the Internet?

Modbus TCP/IP is an Internet protocol. The fact that TCP/IP is the transport protocol of the Internet automatically means that Modbus TCP/IP can be used over the Internet. It was designed to reach this goal. In practical terms, this means that a Modbus TCP/IP device installed in Europe can be addressed over the Internet from anywhere in the world. The implications for an equipment vendor or an end-user are endless. Performing maintenance and repair on remote devices using a PC and browser reduces support costs and improves customer service. Logging onto a plant's control system from home allows the maintenance engineer to maximize his plant's uptime and reduces time in the field. Managing geographically distributed systems becomes easy using commercially available internet/intranet technologies.

Can existing Modbus devices communicate over Modbus TCP/IP?

Since Modbus TCP/IP is simply Modbus protocol with a TCP wrapper, it is very simple for existing Modbus devices to communicate over Modbus TCP/IP. A gateway device is required to convert from the current physical layer (RS232, RS485 or others) to Ethernet and to convert Modbus protocol to Modbus TCP/IP. Such a gateway device could be implemented using a PC. Commercial products to do this are available from several different manufactures. The Modbus device database can help you identify gateways and other Modbus devices.

What sort of performance can I expect from a Modbus TCP/IP system?

The performance depends on the network and the hardware. If you are running Modbus TCP/IP over the Internet, you won't get better than typical Internet response times. However, when communicating for debug and maintenance purposes, this may be perfectly adequate and save you from having to catch a plane or go to site on a Sunday morning!

For a high-performance intranet with high-speed Ethernet switches guaranteeing performance, the situation is completely different. In theory Modbus TCP/IP carries data at up to 250/(250+70+70) or about 60 percent efficiency when transferring registers in bulk. Since 10BaseT Ethernet carries about 1.25 Mbps raw, the theoretical throughput is 1.25M/2 * 60% = 360000 registers per second and the 100BaseT speed is 10 times greater.

This assumes that you are using devices that can service Ethernet as fast as the available bandwidth. Practical tests carried out by Schneider Electric using a MOMENTUM™ Ethernet PLC with Ethernet I/O demonstrated that up to 4000 I/O bases could be scanned per second, each I/O base having up to 16 12-bit analog I/O or 32 discrete I/O. Four bases could be updated in one millisecond. While this is below the theoretical limit calculated above, remember that the tested device was running with only a 80186 CPU running at 50 MHz with an effective computing power of 3 MIPS (compared to the 700 MIPS of a 500 MHz Pentium). Nevertheless, these results are faster than the proprietary I/O scan methods used to date. As low-end CPUs get less expensive, Momentum-type devices will chase the theoretical limit, although they'll never reach it because the limit will continue to be pushed further away with 1 Gigabit Ethernet, 10 Gigabit Ethernet, etc. Contrast this with other field-buses, which are inherently stuck at one speed.