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Fieldbus or Field Bus

For a detailed analysis we recommend you visit Wikipedia, but for a summary that will help the encoder user who is new to fieldbus:

Fieldbus is an industrial computer network for real time distributed control. It connects programmable logic controllers (PLCs) to the components which actually do the work of automation: sensors, actuators, lights, switches, valves, contactors, encoders etc. The object of a fieldbus is to enable the PLC to "instruct" the output device to change status (i.e move, return, on/off etc) and to "receive" information from the same or other devices. In early days of PLCs this was done with many cables (multi core wiring, cable looms and harnesses etc) each directly "hard wired". A fieldbus system replaces the mass of cables with typically just two, generally as far a a slave station. So we can call a fieldbus system a digital, two way, serial, two wire system.

All fieldbus systems share the major advantage over the old hardwire systems: cable elimination, speedier commissioning, and fewer fault opportunies all combine to reduce both captial expenditure and operational expenses in automation systems. These systems can be found in the automotive industy, in process control and in general automation.

Major manufactures of automation PLCs started to develop "in-house" bus systems limited to their own products in the 1980s but the need was for any manufacturer of I/O devices (Input/Output: sensors, actuators etc) to be able to connect in. From this came the "open" fieldbus systems which we know today.

An "open" fieldbus is one which is open to any manufacturer to build into its own product, should they so choose. Certification bodies control standards, approvals and marking for fieldbus. The drive was to standardise in some way (think Betamax and VHS, or BlueRay and HD-DVD) but no clear winner ever emerged. Eventually an IEC standard IEC61158 was developed, but allowing many protocols (which each must follow certain structures), but each incopmpatible with the other. Below are the popular fieldbus protocols supported by The Encoder Company and Hengslter.

What Next?

Industrial automation and control communication works in a hierachy. At the top is the computer, little more than a rugged PC (industrial PC or server) which "oversees" the entire system. In the middle are PLCs which operate real time programs to monitor and instruct the devices that actually do the work of automation (the I/O, the sensors, actuators, encooders etc) often called "slaves". Finally we have the those I/O devices themselves. In the 1990 - 2008 period the "top" PCs communicated between themselves and the middle layer via EtherNet (as our home PC peripherals do, using USB), whilst the middle layer communciated with its I/O or "slaves" using a two-wire fieldbus. This is an encoder help page, so please excuse some generalisations!. Recent trend is for major changes. At the bottom end of the industrial & process automation we see simple connection bus systems such as AS-Interface removing the need for ANY final wiring of simple I/O device. AS-I networks currently "interface up" to the higher level fieldbus networks shown on the right, but can generally work with ANY or stand-alone. At the same time Industrial Ethernet technology is moving "down" the hierachy - for example PROFINET & EtherCat. One projection is that these ethernet sytems (real time & very fast) will soon talk directly to many AS-i type masters connecting straight to individual I/O, thus rendering the 1980s born fieldbus protocols to eventual redundancy. If that's too dramatic a view and all you need is the right encoder, these summaries may help!

 

CANbus (Controller Area Network). Originally developed in 1988 by Intel Corporation and Robert Bosch GmbH for automotive applications, it has grown into an "open" protocol in general automation environments (also CANopen).

INTERBUS was originally developed by Phoenix Contact GmbH in 1987, specifically to communicate with distributed I/O modules. It has grown to be an "open" protocol to IEC61158 and has been integrated into PROFINET I/O

DeviceNet is based on CAN and was orginally developed by the American Company Allen-Bradley (now owned by Rockwell Automation) See also the ODVA, Open DeviceNet Vendors Association based in North America. DeviceNet has an added "connection bus" protocol similar to AS-Interface. More frequently found in North America than Europe.

PROFIBUS (Process Fieldbus) was first promoted by the German department of education & research BMBF, who brought together 21 companies to devise a master project called "field bus". A major contributor was Siemens who focus strongly on this protocol, but it is wrong to consider PROFIBUS as a product of Siemens. Orginally PROFIBUS FMS (Fieldbus Message Specification) was superceded by PROFIBUS DP (Decentralised Periphery). Also embraces PROFIBUS PA (Process Automation) for explosion-hazadous areas. PROFIBUS is generally considered market leader with over 20 million devices in use (2007).

PROFINET. Ethernet based bus - not considered here, but which will be in more common usage in the forseeable future. Will talk directly to I/O slaves.

SUCONET is developed from a proprietry fieldbus system of Klockner-Moeller - also SUCOM-A and SUCONET-K. Found mainly in subsidiary products (I/O slaves etc) that are are supplied specificially for use with Moeller networks.

Whats the difference between the fieldbus protocols?

On this encoder focussed website we can only refer you the visitor to other sources, for you to delve in to the depth of technical detail that each individual needs. The protocols vary in terms of architecture ("the physical layer"), meaning types of cable and connections, topography (also "the physical layer) such as loops, branches and overal length, and speed by which data is exchanged. Different protocols allow for varying numbers of "slaves" or I/O on any given network, and for how these communicate (say cyclically or by priority). One thing is certain, they are all incompatible and there is the risk you may invest in a particular protocol which does not thrive in the market. In our opinion first choice should go to the fieldbus which offers the greatest choice of suppliers (and producers) choosing to service it.