is a digital, two-way, multi-drop communication link among intelligent control devices that will replace the 4-20 mA standard in future.
First of all, fieldbus is digital. Although computers, programmable logic controllers (PLCs), and remote terminal units (RTUs) communicate with each other digitally, most end devices (e.g. valves, pressure transducers, switches, etc.) still use analog signals to communicate. For example, an analog value of 4 mA might correspond to a pressure of no flow while a value of 20 mA might correspond to a 1000 GPM flow value. With discrete devices, the presence of a signal might represent a "closed" or "alarm" condition while the absence of a signal might represent "open" or "normal".
Two-way communications means that a value can not only be read from the end device but it is now possible to write to the device. For example, the calibration constants associated with a particular
sensor can now be stored directly in the device itself and changed as needed.
The multi-drop capability of a fieldbus will perhaps result in the most immediate cost saving benefit for users. With analog devices, a separate cable needs to be run between the end device and the control system because only a single analog signal can be represented on the circuit. Modern distributed systems partially solve this problem by locating remote multiplexing devices out in the field. The ultimate solution, however, is to be able to connect a reasonable number of sensors all located in the same area to the same cable. This is exactly what fieldbus allows.
Finally, fieldbus will replace the 4-20 mA standard, although this will not happen overnight. There are millions of instruments in the world using this standard right now, which does in fact have some advantages. It is simple and well understood. Devices from different suppliers using the 4-20 mA standard can easily operate together (ie. interoperate).