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:1

Modbus ASCII was the first Modbus and is a serial protocol, typically running on either the RS-232 or RS-485 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, shown in Figures 1 and 1.1, 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 tradeoff 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.

How does Profibus work?
Profibus is also a master-slave type protocol like Modbus (see Figure 2) 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. The general bus scan would happen as shown in Figure 7. Master A receives the token, which gives it control of the bus. It will then exchange data with each of its slaves, and when complete, pass on the token to the next master (if there is one). The requirement for detailed diagnostics from each slave is also built into the protocol. During normal data exchange, a slave can alert the master that it has diagnostics, which the master will then read during the next bus scan.