Industrial Communication Protocols
A Guide to PROFINET, PROFIBUS, EtherNet/IP, Modbus and OPC UA
Industrial automation depends on equipment being able to communicate.
PLCs need to exchange information with remote I/O, Variable Speed Drives, HMIs, SCADA systems, instrumentation, other PLCs and increasingly higher-level data systems.
The communication protocols used to make those connections have changed significantly as industrial automation has moved from traditional fieldbus networks towards Industrial Ethernet.
However, older protocols have not simply disappeared. Many manufacturing facilities contain several generations of automation equipment operating simultaneously.
An industrial environment might therefore contain PROFIBUS equipment installed decades ago, PROFINET controlling newer machinery, Modbus communicating with instrumentation and OPC UA providing information to supervisory or data systems.
Understanding what each protocol does, and where it belongs within the automation architecture, is important when designing, integrating or modernising industrial control systems.
What Is an Industrial Communication Protocol?
An industrial communication protocol defines how automation devices exchange information.
Devices may include:
PLCs.
Remote I/O.
Variable Speed Drives.
HMIs.
SCADA systems.
Instrumentation.
Motor control equipment.
Industrial computers.
Other controllers.
Higher-level systems.
The protocol establishes how information is structured and transmitted so that different devices can interpret it correctly.
Industrial communications also need to operate reliably within environments where availability and predictable control behaviour are important.
The appropriate protocol therefore depends on the devices being connected, the information being exchanged and the wider automation architecture.
PROFINET
PROFINET is an Industrial Ethernet communication technology widely used within modern industrial automation.
It is particularly common within the Siemens automation ecosystem and is frequently used to connect:
Siemens PLCs.
Remote I/O.
Variable Speed Drives.
HMIs.
Industrial devices.
Other automation equipment.
PROFINET allows industrial devices to communicate over Ethernet-based infrastructure while supporting the requirements of real-time automation.
A typical modern Siemens control system might use a SIMATIC PLC communicating with distributed I/O, drives and other equipment over PROFINET.
PROFINET is particularly relevant when designing new Siemens-based automation or modernising older PROFIBUS installations.
PROFIBUS
ROFIBUS is an established fieldbus technology that remains widely installed across industrial facilities.
Before Industrial Ethernet became dominant, PROFIBUS was commonly used to connect PLCs with distributed automation equipment.
Existing networks may include:
Remote I/O.
Variable Speed Drives.
Instrumentation.
Motor control equipment.
Other field devices.
Many PROFIBUS networks continue to operate reliably.
The challenge often arises when ageing equipment needs replacing or when newer automation needs to communicate with the existing fieldbus environment.
A modernisation project therefore does not necessarily require the immediate removal of every PROFIBUS device. Existing infrastructure can be assessed to determine whether it should remain, be integrated or gradually migrate towards newer technology.
PROFINET vs PROFIBUS
Despite their similar names, PROFINET and PROFIBUS are different communication technologies.
PROFIBUS is a traditional fieldbus technology.
PROFINET is based on Industrial Ethernet.
PROFINET can provide greater network flexibility and integration with modern automation architectures, while PROFIBUS remains important because of its enormous installed base.
For a completely new Siemens automation system, PROFINET will commonly be the more relevant technology.
For an existing factory, however, the decision is more complicated.
If a reliable PROFIBUS network already connects large amounts of equipment, replacing everything simply to move to PROFINET may not be necessary.
Migration can instead be phased alongside PLC, drive, remote I/O and wider automation modernisation.
EtherNet/IP
EtherNet/IP is an Industrial Ethernet protocol widely associated with Rockwell Automation and Allen-Bradley control systems.
It is commonly used to connect:
Allen-Bradley PLCs.
Remote I/O.
Variable Speed Drives.
HMIs.
Motor control equipment.
Industrial devices.
Other automation systems.
EtherNet/IP uses standard Ethernet technology combined with the Common Industrial Protocol, commonly referred to as CIP.
It is particularly common within North American manufacturing environments and machinery developed around the Rockwell Automation ecosystem.
UK manufacturers may therefore encounter EtherNet/IP extensively within imported machinery or facilities operating Allen-Bradley as an automation standard.
Modbus RTU
Modbus is one of the longest-established communication protocols used within industrial automation.
Modbus RTU typically operates over serial communications such as RS-485.
It remains common within industrial equipment including:
Instrumentation.
Energy meters.
Variable Speed Drives.
Controllers.
Sensors.
Other field devices.
One reason Modbus remains widely used is its relative simplicity and broad support across equipment from different manufacturers.
However, successful integration requires an understanding of how the device structures its data.
Engineers may need to determine which registers contain the required information, how values are formatted and how communication faults should be handled within the PLC.
Modbus TCP
Modbus TCP takes the Modbus communication model and uses Ethernet-based networking.
It is commonly used where PLCs or supervisory systems need to communicate with industrial equipment supporting Modbus over Ethernet.
Typical applications may include:
PLC communications.
Instrumentation.
Energy monitoring.
Variable Speed Drives.
Remote I/O.
Industrial gateways.
Third-party equipment.
The distinction is therefore important:
Modbus RTU generally uses serial communication.
Modbus TCP operates over Ethernet.
Both can remain useful when integrating equipment from different manufacturers because of the widespread availability of Modbus interfaces.
OPC UA
OPC UA occupies a somewhat different position from protocols primarily used for direct field-level control.
It is increasingly used to exchange structured automation information between control systems, supervisory platforms and higher-level applications.
OPC UA can potentially connect:
PLCs.
SCADA systems.
Industrial computers.
Historian systems.
Data platforms.
Manufacturing applications.
Other software systems.
Rather than simply exchanging individual raw values, OPC UA can provide a structured way of representing industrial information.
This makes it particularly useful where information needs to move beyond the immediate machine control environment.
However, it should not automatically be treated as a replacement for every field-level industrial protocol.
The appropriate technology depends on what systems are communicating and what the communication is expected to achieve.
Communication Between Different PLC Manufacturers
Industrial facilities frequently contain PLCs from multiple manufacturers.
For example, a production environment might contain:
Siemens.
Allen-Bradley.
Mitsubishi.
Omron.
Schneider.
These systems may need to exchange machine status, production information, commands or other data.
Cross-platform communication can potentially be achieved using supported protocols, communication modules or industrial gateways.
The appropriate solution depends on:
PLC hardware.
Available communication interfaces.
Existing networks.
Required data.
Communication speed.
System criticality.
Existing architecture.
The objective should be to establish a reliable and maintainable interface rather than simply finding any technical method that allows the two controllers to communicate.
Industrial Communication Gateways and Legacy Integration
Older automation equipment may use communication protocols that are not directly supported by newer systems.
Replacing otherwise functional machinery solely because of communication incompatibility may not always be necessary.
Industrial gateways can sometimes provide an interface between different technologies.
Applications may include connecting:
Serial equipment to Ethernet networks.
Modbus devices to other automation platforms.
Legacy PLCs to newer supervisory systems.
Older fieldbus equipment to modern control environments.
Gateways can be extremely useful, but they also introduce another component into the control architecture.
Engineers should therefore consider long-term supportability, diagnostics and what happens if the gateway fails.
In some cases a gateway is an appropriate long-term solution. In others, it may be better used as part of a phased modernisation strategy.
Choosing the Right Industrial Communication Protocol
There is no single industrial protocol that is appropriate for every application.
The decision should begin with the systems that need to communicate.
Consider:
Existing automation:
Which PLCs, drives, instrumentation and networks are already installed?
Manufacturer ecosystem:
Some technologies are particularly common within specific automation environments.
Device compatibility:
What protocols are supported by the equipment being connected?
Control requirements:
Is the communication being used for real-time equipment control or simply transferring information?
Existing infrastructure:
Can the current industrial network support the new equipment?
Lifecycle:
Is the protocol associated with equipment approaching obsolescence?
Diagnostics:
Can engineering teams effectively troubleshoot communication problems?
Future integration:
Will information eventually need to reach SCADA, historians or higher-level systems?
For existing industrial facilities, the answer may involve several protocols rather than attempting to force every device onto one network.
Why Understanding Industrial Communications Matters
Industrial communication problems can be difficult to diagnose because the physical equipment may appear healthy while information is no longer moving correctly between systems.
A PLC may be operating correctly while communication with remote I/O has failed. A drive may be functional but unavailable to the PLC. A SCADA system may stop receiving information even though the underlying process continues to operate.
Effective industrial networking therefore requires an understanding of the complete automation environment.
Stratos works across PLC programming, industrial networking, SCADA, HMI, control panels and automation integration, allowing communication problems to be investigated in the context of the systems on both sides of the network.
This is particularly important when integrating equipment from different manufacturers or modernising facilities containing several generations of industrial communication technology.
Need Help with Industrial Communications?
Industrial networks often contain several generations of equipment and multiple communication technologies.
Whether you're integrating a new PLC with existing machinery, troubleshooting intermittent communications or planning the migration of a legacy industrial network, understanding the complete automation architecture is essential.
Stratos provides industrial networking, PLC integration, SCADA integration and automation modernisation, helping manufacturers connect new and existing control systems reliably.
Speak to our engineers about your industrial communication or automation integration requirements.
Frequently Asked Questions
What are the most common industrial communication protocols?
Common industrial automation protocols include PROFINET, PROFIBUS, EtherNet/IP, Modbus TCP, Modbus RTU and OPC UA. The protocols encountered depend heavily on the PLC manufacturers, equipment and age of the automation environment.
What is the difference between PROFINET and PROFIBUS?
PROFIBUS is a traditional industrial fieldbus, while PROFINET is an Industrial Ethernet technology. Both are widely associated with Siemens automation, although PROFINET is more commonly used within newer Ethernet-based control systems.
What is the difference between Modbus TCP and Modbus RTU?
Modbus RTU typically communicates over serial connections such as RS-485, while Modbus TCP operates over Ethernet. Both use the Modbus communication model but rely on different network technologies.
What is OPC UA used for?
OPC UA is commonly used to exchange structured industrial information between PLCs, SCADA, historians, industrial computers and higher-level software systems. It is particularly useful where automation data needs to move beyond individual machine control.
Can PLCs from different manufacturers communicate?
Often, yes. PLCs from different manufacturers may communicate through mutually supported protocols, communication modules or industrial gateways. The appropriate method depends on the controllers, networks and information that needs to be exchanged.