Modbus TCP/IP Keeps it Simple

Nov. 28, 2023
How the universal internet TCP/IP transport protocol is used for the Modbus messaging structure.

“Not being overcomplicated is Modbus TCP/IP’s biggest draw, not only for PLCs, but especially for PC-based applications,” said Jason Haldeman, senior product specialist for I/O and networking at Phoenix Contact USA.

This simplicity is rooted in how Modbus TCP/IP (often shortened to Modbus TCP) is designed. At its core is the widely used the Modbus messaging structure developed by Modicon in 1979 for establishing client-server communications among intelligent devices.

“Modbus TCP is easy to implement because it is based on a very simple message structure containing an IP address, function code and data registers,” explained Haldeman. “The registers can be in 16-bit words or single bits, and the function code indicates whether you want to read or write to single or multiple registers. This makes it easier to implement and produce software drivers for many programming environments.”

In Modbus TCP/IP, this vendor-neutral data structure is then “wrapped” with TCP/IP (transmission control protocol and internet protocol), the universal transport protocol for communications on the internet. The layered set of protocols that make up TCP/IP has proved to be a reliable mechanism for transmitting data among machines over Ethernet, which has become the de facto standard for the physical medium underlying both enterprise and industrial networks.

Among the benefits of combining an Ethernet physical infrastructure with the universal TCP/IP networking protocol and the vendor-neutral Modbus data structure are simplicity and lower costs in both development and hardware. For example, buying and installing expensive, special chipsets is unnecessary. Users can connect devices with standard PC Ethernet cards.

Then, those who have a Modbus driver and who understand Ethernet and TCP/IP sockets can have a driver running and talking to a PC in a few hours, according to the Modbus Organization Inc., the member-based association that manages Modbus TCP/IP. Standard Ethernet also frees users from being tied to a particular vendor and gives them access to a transmission medium that has steadily improved in performance—from 10 Mb to 100 Mb, and now to 1 Gb.

Other benefits of Modbus TCP/IP include openness, compatibility with the Modbus installed base and the ability to support the Industrial Internet of Things (IIoT) and other emerging technologies—benefits that evolved from the original serial technology, which existed well before Ethernet had proven itself in industrial networks.

A steady evolution

The predecessor to Modbus TCP/IP version was Modbus RTU (remote terminal unit), which was “developed to run on standard 232 and 485 networks,” continued Haldeman. “It provided a deterministic and easy-to-implement network using the standard serial port already on your computer or control devices.”

Because Modbus, at its core, is merely an application protocol for organizing and interpreting data, it can work over various transport media and with different transport protocols. Consequently, various versions of Modbus—both serial and Ethernet—could develop over the years, including more than one serial version. Although Modbus RTU is the most common version, another serial version called Modbus ASCII relies on ASCII characters, rather than the binary form used by Modbus RTU.

Modbus’s use of standard serial ports and cables quickly caught the attention of third-party automation vendors, who then developed products and custom drivers for their software platforms. As a consequence, Modbus quickly became entrenched as an industrial networking standard. Today, the number of Modbus nodes exceeds 7 million in North America and Europe, according to the Modbus Organization.

As Ethernet advanced and “moved from a simple 10-Mbit coaxial network to the powerhouse it is today, it took over the front-office world and inched its way onto the plant floor, initially only at the top levels of machinery,” explained Haldeman. “But it kept proving itself to be robust enough to compete against the serial protocols. Ethernet has provided a fast, flexible, and open backbone to handle data communications down to the system’s peripherals.”

This led the automation industry to adjust or develop networking protocols and standards to take advantage of the continuing evolution of Ethernet. To prepare Modbus for the twenty-first century, Schneider Electric developed Modbus TCP/IP in 1999, about five years after the automation vendor acquired Modicon.

“We wanted to bring the advantages of Modbus serial, such as simplicity and openness, to Ethernet and IP technologies,” said Todd Snide, consulting engineer at Schneider Electric. “And we wanted to improve the speed, reliability, and security necessary to handle complex use cases.”

Making connections

Schneider Electric designed a protocol that would both preserve the simplicity of Modbus and reap the benefits of Ethernet. “They encapsulated Modbus RTU data unmodified into a TCP/IP packet, providing a pathway for simple conversion between RTU and TCP communications,” said Haldeman.

“Modbus TCP/IP is simply a Modbus RTU protocol with a TCP interface,” noted Chris Kiris, product portfolio manager at RS Americas Inc. “Modbus TCP/IP is mainly used in industrial automation where communications among devices, such as PLCs, HMIs and sensors is paramount.”

Not only does the TCP/IP wrapper allow connecting devices that support TCP/IP sockets to an industrial network, but it also streamlines connecting existing Modus serial devices. For these existing devices, many gateway devices are available to convert from the serial physical layer to Ethernet and to convert the Modbus protocol to Modbus TCP/IP.

Because TCP/IP is the transport protocol of the Internet, the TCP/IP wrapper also opens Ethernet-based industrial networks to communications over both the Internet and most corporate intranets. This means Modbus TCP/IP supports transmitting data from factory automation over a corporate intranet to production applications, such as manufacturing execution systems (MESs). And, because Modbus TCP/IP works over the Internet, data from a Modbus device can be accessed anywhere in the world to perform tasks such as routine maintenance, troubleshooting and optimization.

Soon after creating Modbus TCP/IP, Schneider Electric recognized that the protocol would need to be an open one if it was to remain strong and relevant enough for other automation vendors to work it into their own control strategies. Consequently, Schneider Electric worked with other automation vendors and users to found the Modbus Organization in April 2004 as an independent user-community organization that manages the protocol’s development.

Phoenix Contact was among the automation vendors that saw the benefits of the protocol. “So, we developed control, I/O and gateway products utilizing both Modbus RTU and TCP,” said Haldeman. “Our line of controllers using our PLCnext Technology is capable of controlling and being controlled through Modbus, along with many other protocols.

Future capable

Since the release of Modbus TCP/IP, most of its advancements have not been in the protocol itself, but in the surrounding Ethernet technologies. “The protocol is just the words spoken on Ethernet,” explained Haldeman. “It’s the Ethernet backbone that has been showing true advancement with faster speeds, advanced redundancy technologies and security features.”

An example is switching technology. “Although Ethernet is not an inherently deterministic network, today’s switch technologies allow it to perform very well as the physical layer for most industrial communications protocols,” said Haldeman.

To show the potential of high-speed Ethernet switches on high-performance intranets, the Modbus Organization points to a test conducted by Schneider Electric. As many as 4,000 I/O bases were scanned per second, each I/O base having as many as sixteen 12-bit analog I/O or 32 discrete I/O. Four bases could be updated in one millisecond.

These results were obtained with the automation vendor’s Momentum Ethernet PLC with Ethernet I/O, which had an 80186 CPU running at 50 MHz and an effective computing power of 3 MIPS. Performance should only improve over time as greater CPU power becomes increasingly more cost-effective to deploy.

Besides the deployment of high-speed Ethernet switches, another development has been gateways that serve as intermediaries between PLCs and other devices to support IIoT and Industry 4.0 applications. “In the early 2000s, modern Modbus IIoT gateways were developed to streamline client-server communications that incorporated the cloud,” said Gary Guess, product portfolio manager at RS.

Another improvement added to Modbus TCP/IP through the transport layer is security. Because the Modbus protocol itself does not specify defenses against unauthorized commands or interception of data, security needs to be added at the transport layer, which is what Modbus/TCP Security does.

“Modbus/TCP Security uses the standard Transport Layer Security (TLS) to provide end-to-end authentication and authorization to Modbus TCP,” said Snide. He stresses that this updated version retains the advantages of Modbus TCP/IP—including its simplicity, reliability, and scalability.

Yet another notable milestone for Modbus TCP/IP has been its proliferation into other Ethernet networks. “For example, an EtherNet/IP originator can access data from a Modbus TCP target device thanks to the integration of Modbus TCP into EtherNet/IP protocol,” Snide said.

In addition to being integrated into other Ethernet-based protocols, Modbus TCP/IP can also coexist with others. For example, it will also work alongside the new time-sensitive networking (TSN) bridging standards (IEEE 802) that have been causing excitement in the automation industry for a few years now. The TSN standards being developed and promulgated by the Institute of Electrical and Electronics Engineers promise to allow multiple industrial Ethernet protocols to coexist and run on the same physical wire. said Snide added, “Because Modbus TCP is based on standard IP and Ethernet technologies, it can work on any Ethernet network, including those supporting TSN.”

He also noted that Modbus TCP/IP also supports “ascending technologies such as OPC UA FX to meet advance use cases such as Industry 4.0, IT/OT convergence and interoperability between different distributed components,” he said. “We will continue to support Modbus TCP where it makes sense.”

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