What are the differences between Modbus and Profinet protocols?

I. Polling Mechanism

Modbus's most criticized feature is its polling mechanism. What is polling? Simply put, the master station queries each slave station one by one, and the slave stations passively respond. This is manageable with a master station supporting 30-50 slave stations, but with 100-200, the latency becomes unbearable. For example, in some projects with over 300 meters polled using Modbus RTU, reading all the data took four to five seconds, causing the client to frown and ultimately requiring the addition of several more master stations for load balancing.

II. Profinet

Profinet is different. It uses a producer-consumer model, actively sending data as it changes, instead of passively waiting for polling. Furthermore, Profinet supports IRT synchronization. Some photovoltaic projects require slicing accuracy of 0.01 millimeters, which Modbus couldn't handle due to latency. Profinet IRT was ultimately adopted, providing very high multi-axis synchronization accuracy. However, IRT mode is indeed more difficult to debug, requiring time synchronization and consideration of network latency, making it much more complicated than the Modbus system. III. Cost Issues

Another issue is cost. Chip prices have risen significantly in the past two years, making Profinet modules considerably more expensive. The same 100 I/O points might cost around 5,000 RMB with Modbus, but would start at least at 20,000 RMB with Profinet. This only covers hardware costs, not training costs and debugging time. Therefore, for projects with tight budgets, clients often recommend using Modbus as a temporary solution and upgrading later.

How to Choose Between Modbus and Profinet:

First, consider your real-time requirements. This is the most crucial factor. For simple data transmission such as monitoring temperature or displaying output, Modbus is perfectly adequate. However, for servo control and multi-axis linkage, Modbus's latency will be unacceptable. Generally, this applies: for control cycles longer than 10 milliseconds, try Modbus; for cycles shorter than 10 milliseconds, go for Profinet.

Second, consider your budget. Many projects are straightforward once you see the budget report. For projects with a budget under 100,000, forget about Profinet; for budgets between 100,000 and 300,000, consider using some Profinet depending on the situation; for budgets over 300,000, consider using Profinet entirely.

Third, consider your equipment ecosystem. If you've already purchased a lot of Siemens equipment, choose Profinet directly. If you're using various off-brand equipment, Modbus is more convenient. Modbus has significantly better compatibility. In some extreme cases, a single project might involve six different brands of PLCs and instruments, all relying on Modbus for unified communication. Switching to Profinet would require extensive compatibility testing alone.

Fourth, consider maintenance capabilities. While Profinet is powerful, troubleshooting can be difficult. In some factories, the client doesn't have anyone familiar with Profinet, requiring the vendor to be called in every time a problem arises, incurring significant travel expenses. Eventually, the client preferred adding multiple Modbus gateways rather than dealing with Profinet. Therefore, if engineers are unfamiliar with Profinet, it's advisable to choose this option with caution.