Sunday, January 8, 2012

Difference between Modbus and Profibus

There are some different of Modbus and Profibus which are sometimes called "Fieldbus". It is a common name used to tell any network industry that links the field devices. As a generic explanation it was fashionable at one point, but looks to have fallen out of use recently.

It is also occasionally applied as the simple form of a particular protocol called Foundation Fieldbus. It is only another protocol such as Profibus or Modbus, but it has attributes which are planned specifically for industrial processes.

It was also used as the short form for a failed effort of IEC at standardizing the networks industries. That standard is much forgotten now, it was said that the proprietary of protocols could define itself as standard.

There are 2 different communications protocols. Profibus has convinced protocol characteristics that allow certain versions of it run in multi-master mode using RS-485, while Modbus can be operate at only single master. Though, Modbus can run on Ethernet, include the multiple masters. While Profibus can't operate on Ethernet, but for Profinet it can operate on Ethernet, but Profinet is not same with Profibus.


Profibus is a real sensor or actor of the fieldbus. It can work at different speed of transmission from 1.2 kbps - 12Mbps and is highly optimized for the speed. A fieldbus is a term of technical for serial buses which is used for communication industry, such as USB (Universal Serial Bus), but it is integrated industrial devices and not devices of multimedia.

The fieldbus define as data communication for industry with serial bus systems is the main of the I/O system and for every distributed control. The systems of serial bus designed for real time communication data are defined as fieldbuses. Fieldbuses are networks industries to determine real time communication data between masters of bus, distributed sensors and actors. They are constructed for low transmission volumes and low latency. The fieldbuses packet sizes are in common in the range of 1 until 200 to 300 bytes. The speed of transmission can be selected in a wide range from 12 Mbps down to 1200 bps. This transmission speed flexibility is a great advantage against, for example, Ethernet based networks. The useable length of segment depends on the speed of transmission, media of transmission, and the individual fieldbus topology.
Normally, high speed of transmission reduces the segment length of bus oriented fieldbuses to a 50 to 100meters; low speed of transmission lets a segment length in the range of some kilometers. Fieldbuses with a ring topology such as EtherCAT or INTERBUS are operating frequently with a physical expansion in the kilometers range due to the signal refreshing is done by the I/O modules of fieldbus. The transmission Medias of fiber optic allow in common extended segment lengths and increased immunity of noise.

In other hand, Profibus was OK in its day, but its specialized characteristics tied it directly to RS-485, which is going to obsolete even though there are still lots of used in industries. The utilization of Profibus is still increasing, specialized sensor/actor fieldbus such Profibus have their market in the automation industry now and future.

Modbus is still going grow because its simplicity allows it develops to adjust to the Ethernet without major change. Modbus currently one of the main Ethernet protocols applied in automation. The older versions of serial like RS-232 and RS-485 are still around as well.

There is a group of fieldbus systems standardizes by the IEC 61158. Each of the group is open specified, standardizes, and are not proprietary protocols. The process to get the standard of IEC 61158 is not easy. There is no such simple look like.


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