NE 74 has been revisedfrom 2016-12-07
The document differs from NE 74 edition 2003-01-15 beside editiorial adjustments as follows:
- Title modified
- Document updated completely
- Requirements on Ethernet Fieldbus Systems added (see section 6.8)
Abstract NE 74 ‘Fieldbus Requirements’
A field bus is defined as the hardware and software of a communication system which provides for a continuous digital exchange of information between decentralised sensors and actuators (field devices) and controller within an overall industrial control system (ICS). In general, there is a clear spatial separation between sensors and actuators on the one hand and automation components on the other hand. In the case of process control plants, for instance, automation components are accommodated in a control room or switch room, while sensors and actuators are placed directly in the field, i.e. mounted on devices and machines. They generally receive their energy supply by way of the communication system.
At present, the controller communicates with the field equipment through individual lines. This technology with its standardised power and voltage signals (e.g. 4 - 20 mA, NAMUR signal for initiators, 24-V binary signal etc.) is increasingly pushed to its limits if increasing requirements regarding the scope of communication and the accuracy and safety of transmission are to be met.
Thus, field buses are expected to provide better and more comprehensive possibilities of communication with equipment in the field which makes it possible to transmit status signals for maintenance and diagnosis, parametrisation data and the like in addition to the actual instrument readings and controller outputs.
The increasing communication requirements can be fulfilled by using „intelligent“ field devices (scales, analysis equipment, „smart“ transmitters), which enable partially configured and partially parametrised information processing within the equipment, delivering better quality of measurement signals and reducing subsequent signal processing in the controller of the industrial control system (ICS).
In contrast to the standardised analogue signal (4 - 20 mA), digital transmission of signals makes it possible to transmit signals with a higher resolution and to control the accuracy of the data transmitted. As a matter of principle, digital transmission does not compromise accuracy.
If the additional status signals available were to be transmitted by conventional means, the enormous amount of cabling already required today would assume unrealistic proportions. Migration to a field bus (or rather a field bus system, to be precise) will present the only option to solve this problem.