Exploring Industrial Communication Protocols

Summary

Industrial automation—the control of machinery and processes by autonomous systems using technologies like robotics and computer software—is one of the major forces driving Industry 4.0. This automation is driven by high-speed communications between devices and software. For the wide variety of industrial automation devices to communicate, a standard communication protocol must be employed. Essentially, industrial protocols are communications protocols that ensure connectivity between machines, devices and systems as part of an industrial network.

Initially, industrial communications were based on serial connections. Many of the serial-based protocols—generally called Fieldbus networks—are still used today, such as Modbus and Profibus. However, in the past decade or so, industrial automation has shifted away from serial toward Ethernet communications, which results in protocols like EtherNet/IP and Profinet. Though to a much lesser extent, wireless communi­cations like WLAN and Bluetooth are also being deployed in industrial automation applications. Broadly, the major industrial automation communication protocols can be divided into industrial Ethernet, Fieldbus and wireless.

Industrial Ethernet

Among the three major industrial network categories, industrial Ethernet is the larg­est and the fastest growing. According to the annual HMS Industrial network market shares report, industrial Ethernet applica­tions account for 71% of the global market of newly installed nodes in factory automation; this represents a 7% growth from 2024. There are two main reasons for this: Ethernet connections have higher speeds than serial fieldbus and wire­less connections, and Ethernet cables can be effectively extended to much longer distanc­es than either serial connections or wireless connections.

Among the industrial Ethernet proto­cols, there are several that stand out.

PROFINET
According to the same HMS report, PROF­INET (Process Field Network)—and the next entry—EtherNet/IP—are tied for the most used industrial network protocols, with each holding an 18% market share. PROFINET is developed and maintained by PROFIBUS & PROFINET International (PI), a non-profit trade organization. In general, PROFINET is employed more often across Europe than in the U.S.
 
PROFINET is an open Ethernet-based protocol that grew out of and is a direct extension of PI’s experience with the seri­al-based PROFIBUS protocol. Because of the close relationship between the two proto­cols, PROFIBUS devices can integrate seam­lessly into a PROFINET system.
 
The PROFINET protocol defines the communication between components, like alarms, diagnostics, functional safety and additional information. Standard Ethernet ca­bles connect PROFINET components within a network, which allows other Ethernet proto­cols to coexist within the same infrastructure. Alongside PROFINET, user can employ other Ethernet-based protocols like SNMP, MQTT or HTTP to complement the network.
 
EtherNet/IP
EtherNet/IP is open-source and is man­aged and developed by the Open DeviceNet Vendors Association (ODVA), a tax-exempt business league that includes a wide variety of hardware and software manufacturers. Whereas PROFINET is used mostly in Eu­rope, EtherNet/IP is used primarily in the U.S.
 
EtherNet/IP combines the Common Industrial Protocol (CIP) with standard Ether­net technology. CIP is a media-independent, object-oriented communication protocol de­signed for industrial automation applications and based on the producer-consumer com­munication model. The CIP protocol incor­porates a comprehensive stack of messages and services that support the integration of industrial automation applications with en­terprise Ethernet networks and the Internet, facilitation of data exchange between net­work components, monitoring and control, synchronization and network management.
 
One of the major benefits that EtherNet/ IP offers is the ability to connect legacy devic­es. EtherNet/IP’s structure allows the ability to connect devices from any OEM to the net­work, thereby enabling access to the machine data platform. This eliminates the need for proprietary protocols that require expensive investment into a single vendor’s product.
 
EtherCAT
Ethernet for Control Au­tomation Technology (EtherCAT) is an open-source Ethernet protocol. EtherCAT was initially released by Beckhoff Automation in 2003. Then, in 2004, Beckhoff donated the rights to the EtherCAT Technology Group.
 
EtherCAT has a few unique features and characteristics. The protocol uses a distribut­ed clock mechanism. Each device in the Eth­erCAT network synchronizes its internal clock to a reference clock. This timing feature allows precise and synchronized commu­nications across all devices on the network. EtherCAT uses a master-slave structure and transmits data throughout the network in a novel way. The master is the only device that is allowed to transmit data, but each slave device can read, modify and add data as the original transmission is passed through the network. This eliminates the need for data to traverse the entire network loop, which re­duces communication latency and enables real-time control and response.
 
Fieldbus
According to the HMS report, Fieldbus net­works account for 22% of the global market of newly installed nodes in factory automation. That represents a 2% decrease in 2024. While Fieldbus networks have been declining in deployments since 2018, there are still many use cases where a Fieldbus protocol may be more applicable than an Ethernet protocol:
 
Legacy systems: If using existing automation systems or devices that use Fieldbus protocols, it can be more practical and cost-effective to continue using Fieldbus to maintain compatibili­ty with those systems. Replacing all the devices or retrofitting them to use Eth­ernet-based protocols can be a complex and expensive undertaking.
 
Limited bandwidth: If a system has low bandwidth requirements, such as sim­ple input/output (I/O) control or moni­toring applications, Fieldbus protocols are sufficient. Ethernet-based protocols may be unnecessary if the network bandwidth is underutilized.
 
Deterministic behavior: Fieldbus protocols offer inherently deterministic behavior. The timing and sequence of data exchange can be highly predict­able and consistent. This is particularly important for applications that require precise control and synchronization such as motion control or distributed control systems. While Ethernet net­works are fast, they are not determinis­tic. The speed of Ethernet can overcome some determinism concerns, but not all of them.
 
Modbus
Modbus, ini­tially devel­oped in 1979, is the oldest protocol in this list. Traditionally, Modbus was/is implemented using RS-232, RS-422 or RS-485 serial con­nections. There are several different Modbus protocols, but far and away the most com­mon is Modbus Remote Terminal Unit (RTU).
 
Modbus RTU makes use of a compact, binary representation of the data for proto­col communication. The RTU format follows the commands/data with a cyclic redundan­cy check checksum as an error check mech­anism to ensure data reliability. A Modbus RTU message must be transmitted contin­uously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods.
 
Like every protocol on this list, Modbus has some major benefits and some significant drawbacks. To start with the positives, Modbus is very easy to implement. As it is open source, and has been around since 1979, there are innu­merable pages of documentation, use cases and best practices available. Finally, imple­menting Modbus is incredibly cost-effective.
 
Modbus also has some drawbacks. First, RS-232, RS-422 or RS-485 serial connections have significantly slower transfer rates than Ethernet connections. Modbus supports a limited set of data types that are primarily focused on numerical values and discrete inputs/outputs. Handling more complex data structures, such as strings or arrays, may require additional encoding or custom implementation. Finally, as Modbus is a basic protocol, it lacks advanced features like built-in diagnostics, redundancy mechanisms or extensive error handling.
 
PROFIBUS
PROFIBUS (Process Field Bus) is the most widely deployed Fieldbus protocol. PROFIB­US—like its cousin PROFINET—is developed and maintained by PROFIBUS & PROFINET International (PI), a non-profit trade organi­zation. In general, PROFIBUS is employed more often across Europe than in the U.S.
 
The general principle of PROFIBUS is collecting multiple inputs and outputs from the field into a local I/O device and then transferring the data through just one cable to the master. This approach saves costs by the omission of additional hardware and cabling. Also, it saves engineering time as it streamlines network installation, mainte­nance and troubleshooting.
 
There are benefits and drawbacks to using PROFIBUS as a protocol. In terms of benefits, PROFIBUS can sup­port much higher transfer speeds than Modbus. PROFIBUS deployments are known for robustness and reliability and can withstand harsh industrial environments. PROFIBUS is a very robust protocol that was designed to automate entire plants. It works extremely well in mul­tivendor applications, with modems and has detailed diagnostics.
 
Deploying a PROFIBUS network is very complex and requires specialized knowledge and experience. While the protocol is faster than Modbus and other serial networks, it is still significantly slower than Ethernet-based networks.
 

Wrapping up

Ultimately, the choice of industrial commu­nication protocol depends on the require­ments and constraints of the application. For simple networks with deterministic control requirements, or networks with a small, fixed number of devices, a Fieldbus protocol will often be the best choice. For large networks or networks that are projected to grow, or networks that require high-speed data transfers, an Ethernet-based protocol is likely more suitable.

_{This feature originally appeared in the May 2025 edition of Automation.com Monthly.}