Choosing the right communication protocol is critical for PLCs to connect reliably with SCADA, HMIs, and other devices. This guide breaks down 10 common protocols, their features, and typical industrial applications to help you make the right choice.
10 common PLC communication protocols
No.1 Ethernet/IP
Ethernet/IP is an open application layer protocol. It is intended for home and business purposes only. It is not designed to operate in industrial applications. It is an advanced version of standard Ethernet. The protocol was developed by Rockwell Automation.
No.2 Modbus
Modbus is a protocol based on master-slave technology for transferring information over serial lines or Ethernet. It is further categorized as Modbus RTU, Modbus ASCII, and Modbus TCPIP.
No.3 Profibus and Profinet
PROFIBUS is a classic serial fieldbus. PROFINET is a type of industrial Ethernet characterized by faster speeds, greater bandwidth, and larger messages. The PROFIBUS protocol is designed for high-speed communication with distributed I/O devices (remote I/O). The PROFIBUS system uses a bus master to poll RS485 serial buses with DP I/O devices that are distributed in a multipoint fashion. The PROFIBUS system uses a bus master to poll DP I/O devices distributed in multiple points on the RS485 serial bus.
No.4 EtherCAT
Originally developed by Beckhoff Automation, EtherCAT is a real-time Industrial Ethernet technology that focuses on short cycle times (<= 100 us). Each device reads the data in its frame and inserts the data into the frame as it travels downstream. The data stream can be as small as a few bits to kilobytes of information. If the controller requests information from only one device, the data stream will be shorter and faster compared to collecting information from all devices.
No.5 Optomux
Optomux is a serial network protocol that works on RS-232/RS-485 and works on the ASCII standard. It was developed by Opto22. Similar protocols to these standards are Interbus and DirectNet.
No.6 DF-1
DF-1 is an asynchronous, byte-oriented protocol used to communicate with RS-232-based Rockwell devices.
No.7 Interbus
Interbus is a serial network protocol that operates on RS-232/RS-485 and works on the RTU standard. Developed by Phoenix Contact.
No.8 HostLink
HostLink, this protocol is developed based on the ASCII protocol, which is typically used to communicate over RS232 or RS422.
No.9 Data Highway (DH+)
It is a protocol developed by Rockwell Automation that uses transformers to couple differential signals; this means that the transmitter and receiving station do not need to be at the same ground potential. Works with the concept of differential signaling. It uses two wires for data transmission, and the data is represented by the voltage difference between the two wires. Here, data is transmitted through differential voltage, and noise is easily removed in both wires. It uses half-duplex transmission for communication. This is a very old protocol used only for Rockwell PLCs. Today, it is obsolete and rarely used. It is suitable for token-passing protocols and uses relay lines with drops.
No.10 CAN
CAN protocol is an application layer protocol that uses peer-to-peer messages to communicate with various devices. It is a multi-master slave communication system and has an object dictionary that contains all the functions of a device. It is a practical communication method with standard communication objects for real-time data (PDO), configuration data (SDO), timestamps, synchronization messages, emergency messages, startup messages, NMT messages, and error control messages, as well as other data. In a hardware configuration, termination at the end device is required to establish communication over the entire link.
Quick Comparison of Common PLC Communication Protocols
The following table provides a quick overview of the most common PLC communication protocols, including their communication type, relative speed, and typical industrial applications. While actual performance depends on the implementation and network design, this comparison can help you quickly identify which protocol is best suited for a particular automation project.
| Protocol | Communication Type | Relative Speed | Typical Application |
|---|---|---|---|
| Ethernet/IP | Industrial Ethernet | High | Rockwell Automation Systems |
| Modbus RTU | Serial | Low | PLC-to-Device Communication |
| Modbus TCP | Ethernet | Medium | SCADA and Industrial Networking |
| Profibus | Serial Fieldbus | Medium | Legacy Factory Automation |
| Profinet | Industrial Ethernet | High | Real-Time Factory Automation |
| EtherCAT | Real-Time Ethernet | Very High | Motion Control and Robotics |
| Optomux | Serial (ASCII) | Low | Remote I/O Communication |
| DF1 | Serial | Low | Allen-Bradley PLC Communication |
| Interbus | Fieldbus | Medium | Distributed Industrial Networks |
| HostLink | Serial | Low | Omron PLC Communication |
| Data Highway Plus (DH+) | Proprietary Network | Medium | Legacy Rockwell Systems |
| CAN | Fieldbus | Medium | Automotive and Embedded Control |
As shown in the table above, Ethernet-based protocols such as EtherCAT, Profinet, and Ethernet/IP dominate modern industrial automation, while Modbus, Profibus, and other serial protocols continue to play an important role in legacy systems and cost-sensitive applications.
PLC Communication Protocol Integration Challenges
While understanding individual PLC communication protocols is important, many automation projects involve multiple protocols operating within the same system. Integrating devices from different manufacturers often requires protocol conversion, data collection, and centralized management. As a result, industrial gateways have become a practical solution for connecting PLCs, HMIs, SCADA systems, and cloud platforms across different communication standards.
For example, IOTRouter EG Series Edge Gateways support docking various mainstream PLC brands, including but not limited to SIEMENS, MELSEC, OMRON, LSis, AllenBrandly, Beckhoff, Inovance, Keyence, Panasonic, Delta, XinJE, MegMeet, Yaskawa, Fatek, Vigor, Fuji, and so on.
Summary
In practice, no single PLC communication protocol fits every scenario. Modbus and Profibus remain reliable for legacy systems, EtherCAT and Profinet excel in high-speed automation, and CAN is ideal for distributed or automotive networks. In real-world projects, the challenge is rarely choosing one protocol—it’s ensuring that devices, systems, and vendors can communicate effectively across multiple protocols. Taking the time to understand each protocol’s strengths and limitations and to plan for interoperability is key to building a stable, efficient, and future-proof automation system.
