RFProfibus vs. Modbus: Key Differences Explained
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This article compares Profibus and Modbus, highlighting the key differences between these two popular communication protocols used in automation. We’ve previously discussed common interface standards like RS232, RS422, and RS485. Profibus and Modbus can be seen as enhancements to these earlier standards.
These protocols enable real-time communication between bus masters, distributed sensors, and actuators. They are particularly suitable for applications with low transmission volumes and minimal latency requirements.
It’s important to note that Modbus is primarily a protocol that defines the frame structure for messages transmitted over serial lines. RS485, on the other hand, defines electrical specifications. Modbus can utilize various electrical standards, including RS485. Therefore, Modbus communication can occur over RS232, RS422, RS485, radio, microwave, satellite links, and more.
| Feature | Profibus | Modbus |
|---|---|---|
| Mode of operation | Operates in multi-master mode using RS485. | Operates in single-master mode using ASCII/RTI. |
| Use of Ethernet | Cannot operate directly on Ethernet. Profinet, a related but distinct protocol, is used for Ethernet-based applications. | Can run on Ethernet using TCP/IP protocols. |
| Physical medium | Twisted pair, fiber optic cable. | Modbus doesn’t define a specific physical medium. The message structure can be used over any physical medium. However, Modbus RS485 is a common implementation. |
| Speed of transmission | Operates at varying speeds, from 1.2 Kbps to 12 Mbps. | Modbus RS485 typically supports transmission speeds up to 115 Kbps or 19.2 Kbps. |
| Distance coverage | Up to 32 stations can use Profibus for distances up to 1200 meters. 2 stations can communicate at 500 kbps for distances up to 400 meters. Lower speeds support longer distances. | Modbus RS485 cable should ideally be less than 700 meters in length. |
| Number of devices | Up to 126 devices (masters and slaves) can be connected to a single Profibus network. | A Modbus master can theoretically support 247 slave devices. However, due to the limitations of the RS485 protocol, only 32 devices (including the master) can be connected directly on the main cable. Repeaters are used to support more than 32 devices. |
| Future use | While standalone Profibus might become less common, specialized sensor/actuator fieldbus devices are increasing their market share in automation. | Its simplicity has made it a popular Ethernet protocol in automation. RS232, RS422, and RS485 remain in use, with devices employing addresses on the bus. |
| Application | A robust protocol suitable for automating entire plants. Ideal for scenarios with many points and diverse vendor equipment to be automated. | A simple protocol well-suited for interfacing with a single controller in a point-to-point configuration. Often used for smaller plant automation projects. |
