Fieldbus

Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, standardized as IEC 61158. Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, standardized as IEC 61158. A complex automated industrial system — such as manufacturing assembly line — usually needs a distributed control system—an organized hierarchy of controller systems—to function. In this hierarchy, there is usually a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non-time-critical communications system (e.g. Ethernet). At the bottom of the control chain is the fieldbus that links the PLCs to the components that actually do the work, such as sensors, actuators, electric motors, console lights, switches, valves and contactors. Fieldbus is an industrial network system for real-time distributed control. It is a way to connect instruments in a manufacturing plant. Fieldbus works on a network structure which typically allows daisy-chain, star, ring, branch, and tree network topologies. Previously, computers were connected using RS-232 (serial connections) by which only two devices could communicate. This would be the equivalent of the currently used 4–20 mA communication scheme which requires that each device have its own communication point at the controller level, while the fieldbus is the equivalent of the current LAN-type connections, which require only one communication point at the controller level and allow multiple (hundreds) of analog and digital points to be connected at the same time. This reduces both the length of the cable required and the number of cables required. Furthermore, since devices that communicate through fieldbus require a microprocessor, multiple points are typically provided by the same device. Some fieldbus devices now support control schemes such as PID control on the device side instead of forcing the controller to do the processing. Arguably the precursor field bus technology is HP-IB as described in IEEE 488 / 1975. 'It became known as the General Purpose Interface Bus (GPIB), and became a de facto standard for automated and industrial instrument control.' See IEEE-488 The oldest commonly used field bus technology is Bitbus. Bitbus was created by Intel Corporation to enhance use of Multibus systems in industrial systems by separating slow i/o functions from faster memory access. In 1983, Intel created the 8044 Bitbus microcontroller by adding field bus firmware to its existing 8051 microcontroller. Bitbus uses EIA-485 at the physical layer, with two twisted pairs - one for data and the other for clocking and signals. Use of SDLC at the data link layer permits 250 nodes on one segment with a total distance of 13.2 km. Bitbus has one master node and multiple slaves, with slaves only responding to requests from the master. Bitbus does not define routing at the network layer. The 8044 permits only a relatively small data packet (13 bytes), but embeds an efficient set of RAC (remote access and control) tasks and the ability to develop custom RAC tasks. In 1990, the IEEE adopted Bitbus as the Microcontroller System Serial Control Bus (IEEE-1118). Today BITBUS is maintained by the BEUG - BITBUS European Users Group.