4-20 mA, Fieldbus, Wireless Technology 4-20 mA, Fieldbus, or Wireless Technology — There Are Pros and Cons Either Way for Transmitting Field Data
The WirelessHart and ISA100 wireless standards provide facility operators with two additional communications solutions for transmitting field data. Many users are therefore asking which wired or wireless protocol represents the future-oriented technology that is best suited for efficient and safe production.
Field signals can be connected to the process control system (PCS) via fieldbus, wireless, or conventional 4-20 mA technology. The choice of a technology must take into account the application and its particular requirements. Using wireless communication to connect field devices to the process control system is currently a topic of discussion in many media outlets and at conferences and trade shows.
However, wireless transmission is not a new technology. As far back as the 1990s, data was being transmitted to computer systems wirelessly. Even back then, services such as private mobile radio and trunked radio systems made it possible to bridge distances of more than one kilometer.
In the last several years, GHz-frequency bands have been allocated for such applications, leading to the development of a number of proprietary wireless systems such as Trusted Wireless by Phoenix Contact. WirelessHart and ISA100 provide facility operators with further vendor-neutral wireless standards from which to choose.
Process Control Technology Determines the Fieldbus System
Various fieldbus systems are also available for connecting field devices directly to the process control system. Foundation Fieldbus FF-H1 and Profibus PA have established themselves as process engineering standards (Figure 1). The physics involved in both standards is the same: two-wire technology for the operating voltage with modulated data.
However, they differ with respect to data communication, shielding design, and bus termination. The required installation materials, cable, and connection technology are likewise identical, and may thus be used for both fieldbus systems. The advent of modularized active fieldbus distributors allows them to be expanded while in operation or replaced if faults occur.
The maximum current to the field device can be adjusted according to consumption, thus allowing the fieldbus to operate more efficiently and safely and also increasing facility availability (Figure 2).
Since process control engineering field devices and interface components are not bus-compatible, the user must opt for a fieldbus system. The selection is often made on the basis of the process control system, as the various PCS vendors are currently capable of integrating both the Foundation Fieldbus FF-H1 and the Profibus PA.
However, they typically acquire more expertise with a system for which they provide better support in terms of tools, functionality, and integration. The process control system therefore determines the choice of the bus system.