We live in a multi-protocol world – and this will likely not change anytime soon. Different protocols work better in different applications.
In this article, we will provide an overview of both protocols and discuss their key strengths and applications. Comparing the two, we’ll see that both protocols have their own particular strengths. We’ll also discuss which one works best in which applications – although there is some overlap in what each can do. What’s more, they can complement each other in joint applications.
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Introduction to Modbus
Modbus is the “granddaddy” of industrial communication protocols. It was originally designed in the mid-1970s by Modicon as a way to link intelligent devices with PLCs using a simple master/slave concept. “Simple” is a key descriptor for Modbus – and also its biggest strength. It is easy to implement and easy to use.
When it was first introduced, it was a proprietary protocol that only Modicon could use. However, it was later published royalty-free so that anyone could use it. Finally, Modicon made it an open protocol. When it was published, a number of companies started using it, creating different interpretations and modifications of the original specification. As a result, there are now quite a few variations in the field.
The specification document is fewer than 100 pages in length, which is a good indication of the protocol’s low level of complexity. In comparison, Profibus’ specification document is thousands of pages long. The term “Modbus” typically refers to one of three related protocols: Modbus ASCII, Modbus RTU, or Modbus TCP/IP:
- Modbus ASCII was the first Modbus and is a serial protocol, typically running on either the RS-232 or RS485 physical layer. All slaves are polled on demand by the master, and there is only one master. The message frame can be up to 252 bytes in length, and up to 247 addresses are possible. The message frame and function codes are very simple.
- Modbus RTU is really just a small variation on the Modbus ASCII protocol. The only difference is in the encoding of the data. ASCII encodes the message in ASCII characters, while RTU uses bytes, thus increasing the protocol’s throughput. In general, RTU is more popular, particularly in new installations.
- Modbus TCP/IP was added much later. One simple way of thinking about Modbus TCP/IP is to picture it as simply encapsulating a Modbus RTU packet within a TCP/IP packet. There is a bit more to it than that, but this is essentially what Modbus did. As a result, Modbus TCP/IP is also very simple to implement. The trade-off is that, because it uses TCP/IP protocol for all messages, it is slow compared to other Ethernet industrial protocols – but still fast enough for monitoring applications.
Introduction to Profibus
If Modbus is the “granddaddy” of protocols, then Profibus is the young athlete – lean and fast. Profibus was designed in the 1990s to meet all industrial communication needs for both factory and process automation. As with Modbus, there are a number of terms associated with this protocol: Profibus DP, Profibus PA, Profisafe, Profidrive, and Profinet.
One way of visualizing how these terms fit together is to think of Profibus as a book with many chapters. The book would be called Profibus DP (Decentralized Peripheral). The book’s chapters would be called Profibus PA (Process Automation), Profisafe for safety application, and Profidrive for high-speed drive applications. In addition, there would be a second book by the same authors called Profinet, with many chapters, including Profisafe and Profidrive.
Profibus is also a master-slave type protocol like Modbus but with an additional token ring protocol to allow for multiple masters. Also, unlike Modbus, all devices go through a startup sequence during which they “join” the network. Each slave maintains a failsafe timer. If the master does not talk to it within a certain time limit, the slave goes into a safe state; the master must then go through the startup sequence again before further data exchange can occur. This, in combination with a watchdog timer in the master, ensures that all communication occurs every bus cycle with a certain time value.
PROFIBUS vs Modbus
Modbus is a very simple, easy to use, modem-friendly protocol. However, there is a fair amount of variation in the protocol itself and in its physical layer definition, which creates problems in multi-vendor applications. 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.
When connecting a controller to one smart device in a point-to-point configuration, or if there is only one remote site, Modbus is an easy solution. For situations where there are more points, where different vendors are involved, or where there is a hazardous environment, Profibus is a better solution.
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