Profibus was designed in the 1990s to meet all industrial communication needs for both factory and process automation (Figure 1). 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.

How does Profibus work?

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.

The general bus scan would happen as shown in Figure 2. 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.

Profibus Classification

        It’s classified into Profibus bus FMS field bus message specification and Profibus bus DP decentralized peripherals Profibus bus FMS is a peer to peer messaging format, and it is used for communication of data between Profibus masters Profibus DP is a protocol made for communication between Profibus masters and their remote i/o slaves.

        In later 1993 for the Profibus classification, one more communication type is added, that is Profibus PA Process Automation is used to monitor the equipment value of the process control system. In Process automation applications.


Profibus-DP – Optimized for high speed and inexpensive hookup.  DP version is designed especially for communication between automation control systems and distributed I/O at the device level. Can be used to replace parallel signal transmission with 24 V or 0 to 20 mA. This is the only card that will be supported in this specification document.

Profibus-PA – Designed especially for process automation. It permits sensors and actuators to be connected on one common bus line even in intrinsically safe areas. Permits data communication and power over the bus using 2-wire technology according to the international standard IEC 1158-2. Will not be implemented in this spec because only I/O will be controlled.

Profibus-FMS – General-purpose solution for communication tasks at the cell level. Powerful FMS services open up a wide range of applications and provide great flexibility. Can also be used for extensive and complex communication tasks. Will not be implemented in this spec because CODE provides much of the same function.


connection Architecture