Hello PILearner;

You can find many sites on the web discussing the basic differences betweenindustrial communication protocols. Here is a chart I like a lot, it resumes many properties of different fieldbuses in use iwith PLCs:

http://www.mtl.de/pdfs/news/open_fieldbus.pdf

It is of course not complete, many different protocols have been developped by many manufacturers in the history of industrial controls. Many companies make a nice living providing gateways between different protocols to satisfy the industry at a minor cost than developping communication protocols for all the different existing brands of controller.

Note that if Profibus is very much in demand and provided by many manufacturers, either natively (Siemens and Bosch,for example), it is not by far the only one used. Rockwell strands behind their own DeviceNet, Ethernet/IPand ControlNet buses, Beckhoff pushes EtherCat, Mitsubishi promotes CC Link, and Ethernet in different forms is being more and more used industrially (Modbus/TCP, Profinet, CC Link IE...). All of the shown protocolscan get the job done on the plantfloor (which is basically establishing a reliable exchange with field instruments, sensors, actuators and PLCs), with varying degrees of speed,complexity and stability.

"Smart" field instruments can be connected to a controlling PLC through HART, Foundation Fieldbus and Profibus-PA. FF and PA share a common standard, based on FISCO/FINCO (non-incendiary communication requirements), and now HART is associating itself with FF to share commonalities, so there is movement towards conformity in that area.

The oldest and mst common fieldbus in the industry is Modbus (RTU), developpend aroiund 1979 by Modicon. It is also an open communication protocol, which means the specifications are readily and freely available in the public domain (look the up on the web) and anyone is able to use them to create their own implementation of the protocol on their equipment. So many manufacturers of drives and sensors use it to generically exchange data with a controller, PLC or DCS, and so it is still much in use today.

Hope this helps,

Daniel Chartier